Adding upstream version 14.5.upstream/14.5upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

7 files changed, 6833 insertions, 0 deletions

diff --git a/src/backend/storage/buffer/Makefile b/src/backend/storage/buffer/Makefile
new file mode 100644
index 0000000..fd7c40d
--- /dev/null
+++ b/src/backend/storage/buffer/Makefile
@@ -0,0 +1,22 @@
+#-------------------------------------------------------------------------
+#
+# Makefile--
+#    Makefile for storage/buffer
+#
+# IDENTIFICATION
+#    src/backend/storage/buffer/Makefile
+#
+#-------------------------------------------------------------------------
+
+subdir = src/backend/storage/buffer
+top_builddir = ../../../..
+include $(top_builddir)/src/Makefile.global
+
+OBJS = \
+	buf_init.o \
+	buf_table.o \
+	bufmgr.o \
+	freelist.o \
+	localbuf.o
+
+include $(top_srcdir)/src/backend/common.mk
diff --git a/src/backend/storage/buffer/README b/src/backend/storage/buffer/README
new file mode 100644
index 0000000..a775276
--- /dev/null
+++ b/src/backend/storage/buffer/README
@@ -0,0 +1,276 @@
+src/backend/storage/buffer/README
+
+Notes About Shared Buffer Access Rules
+======================================
+
+There are two separate access control mechanisms for shared disk buffers:
+reference counts (a/k/a pin counts) and buffer content locks.  (Actually,
+there's a third level of access control: one must hold the appropriate kind
+of lock on a relation before one can legally access any page belonging to
+the relation.  Relation-level locks are not discussed here.)
+
+Pins: one must "hold a pin on" a buffer (increment its reference count)
+before being allowed to do anything at all with it.  An unpinned buffer is
+subject to being reclaimed and reused for a different page at any instant,
+so touching it is unsafe.  Normally a pin is acquired via ReadBuffer and
+released via ReleaseBuffer.  It is OK and indeed common for a single
+backend to pin a page more than once concurrently; the buffer manager
+handles this efficiently.  It is considered OK to hold a pin for long
+intervals --- for example, sequential scans hold a pin on the current page
+until done processing all the tuples on the page, which could be quite a
+while if the scan is the outer scan of a join.  Similarly, a btree index
+scan may hold a pin on the current index page.  This is OK because normal
+operations never wait for a page's pin count to drop to zero.  (Anything
+that might need to do such a wait is instead handled by waiting to obtain
+the relation-level lock, which is why you'd better hold one first.)  Pins
+may not be held across transaction boundaries, however.
+
+Buffer content locks: there are two kinds of buffer lock, shared and exclusive,
+which act just as you'd expect: multiple backends can hold shared locks on
+the same buffer, but an exclusive lock prevents anyone else from holding
+either shared or exclusive lock.  (These can alternatively be called READ
+and WRITE locks.)  These locks are intended to be short-term: they should not
+be held for long.  Buffer locks are acquired and released by LockBuffer().
+It will *not* work for a single backend to try to acquire multiple locks on
+the same buffer.  One must pin a buffer before trying to lock it.
+
+Buffer access rules:
+
+1. To scan a page for tuples, one must hold a pin and either shared or
+exclusive content lock.  To examine the commit status (XIDs and status bits)
+of a tuple in a shared buffer, one must likewise hold a pin and either shared
+or exclusive lock.
+
+2. Once one has determined that a tuple is interesting (visible to the
+current transaction) one may drop the content lock, yet continue to access
+the tuple's data for as long as one holds the buffer pin.  This is what is
+typically done by heap scans, since the tuple returned by heap_fetch
+contains a pointer to tuple data in the shared buffer.  Therefore the
+tuple cannot go away while the pin is held (see rule #5).  Its state could
+change, but that is assumed not to matter after the initial determination
+of visibility is made.
+
+3. To add a tuple or change the xmin/xmax fields of an existing tuple,
+one must hold a pin and an exclusive content lock on the containing buffer.
+This ensures that no one else might see a partially-updated state of the
+tuple while they are doing visibility checks.
+
+4. It is considered OK to update tuple commit status bits (ie, OR the
+values HEAP_XMIN_COMMITTED, HEAP_XMIN_INVALID, HEAP_XMAX_COMMITTED, or
+HEAP_XMAX_INVALID into t_infomask) while holding only a shared lock and
+pin on a buffer.  This is OK because another backend looking at the tuple
+at about the same time would OR the same bits into the field, so there
+is little or no risk of conflicting update; what's more, if there did
+manage to be a conflict it would merely mean that one bit-update would
+be lost and need to be done again later.  These four bits are only hints
+(they cache the results of transaction status lookups in pg_xact), so no
+great harm is done if they get reset to zero by conflicting updates.
+Note, however, that a tuple is frozen by setting both HEAP_XMIN_INVALID
+and HEAP_XMIN_COMMITTED; this is a critical update and accordingly requires
+an exclusive buffer lock (and it must also be WAL-logged).
+
+5. To physically remove a tuple or compact free space on a page, one
+must hold a pin and an exclusive lock, *and* observe while holding the
+exclusive lock that the buffer's shared reference count is one (ie,
+no other backend holds a pin).  If these conditions are met then no other
+backend can perform a page scan until the exclusive lock is dropped, and
+no other backend can be holding a reference to an existing tuple that it
+might expect to examine again.  Note that another backend might pin the
+buffer (increment the refcount) while one is performing the cleanup, but
+it won't be able to actually examine the page until it acquires shared
+or exclusive content lock.
+
+
+Obtaining the lock needed under rule #5 is done by the bufmgr routines
+LockBufferForCleanup() or ConditionalLockBufferForCleanup().  They first get
+an exclusive lock and then check to see if the shared pin count is currently
+1.  If not, ConditionalLockBufferForCleanup() releases the exclusive lock and
+then returns false, while LockBufferForCleanup() releases the exclusive lock
+(but not the caller's pin) and waits until signaled by another backend,
+whereupon it tries again.  The signal will occur when UnpinBuffer decrements
+the shared pin count to 1.  As indicated above, this operation might have to
+wait a good while before it acquires the lock, but that shouldn't matter much
+for concurrent VACUUM.  The current implementation only supports a single
+waiter for pin-count-1 on any particular shared buffer.  This is enough for
+VACUUM's use, since we don't allow multiple VACUUMs concurrently on a single
+relation anyway.  Anyone wishing to obtain a cleanup lock outside of recovery
+or a VACUUM must use the conditional variant of the function.
+
+
+Buffer Manager's Internal Locking
+---------------------------------
+
+Before PostgreSQL 8.1, all operations of the shared buffer manager itself
+were protected by a single system-wide lock, the BufMgrLock, which
+unsurprisingly proved to be a source of contention.  The new locking scheme
+avoids grabbing system-wide exclusive locks in common code paths.  It works
+like this:
+
+* There is a system-wide LWLock, the BufMappingLock, that notionally
+protects the mapping from buffer tags (page identifiers) to buffers.
+(Physically, it can be thought of as protecting the hash table maintained
+by buf_table.c.)  To look up whether a buffer exists for a tag, it is
+sufficient to obtain share lock on the BufMappingLock.  Note that one
+must pin the found buffer, if any, before releasing the BufMappingLock.
+To alter the page assignment of any buffer, one must hold exclusive lock
+on the BufMappingLock.  This lock must be held across adjusting the buffer's
+header fields and changing the buf_table hash table.  The only common
+operation that needs exclusive lock is reading in a page that was not
+in shared buffers already, which will require at least a kernel call
+and usually a wait for I/O, so it will be slow anyway.
+
+* As of PG 8.2, the BufMappingLock has been split into NUM_BUFFER_PARTITIONS
+separate locks, each guarding a portion of the buffer tag space.  This allows
+further reduction of contention in the normal code paths.  The partition
+that a particular buffer tag belongs to is determined from the low-order
+bits of the tag's hash value.  The rules stated above apply to each partition
+independently.  If it is necessary to lock more than one partition at a time,
+they must be locked in partition-number order to avoid risk of deadlock.
+
+* A separate system-wide spinlock, buffer_strategy_lock, provides mutual
+exclusion for operations that access the buffer free list or select
+buffers for replacement.  A spinlock is used here rather than a lightweight
+lock for efficiency; no other locks of any sort should be acquired while
+buffer_strategy_lock is held.  This is essential to allow buffer replacement
+to happen in multiple backends with reasonable concurrency.
+
+* Each buffer header contains a spinlock that must be taken when examining
+or changing fields of that buffer header.  This allows operations such as
+ReleaseBuffer to make local state changes without taking any system-wide
+lock.  We use a spinlock, not an LWLock, since there are no cases where
+the lock needs to be held for more than a few instructions.
+
+Note that a buffer header's spinlock does not control access to the data
+held within the buffer.  Each buffer header also contains an LWLock, the
+"buffer content lock", that *does* represent the right to access the data
+in the buffer.  It is used per the rules above.
+
+* The BM_IO_IN_PROGRESS flag acts as a kind of lock, used to wait for I/O on a
+buffer to complete (and in releases before 14, it was accompanied by a
+per-buffer LWLock).  The process doing a read or write sets the flag for the
+duration, and processes that need to wait for it to be cleared sleep on a
+condition variable.
+
+
+Normal Buffer Replacement Strategy
+----------------------------------
+
+There is a "free list" of buffers that are prime candidates for replacement.
+In particular, buffers that are completely free (contain no valid page) are
+always in this list.  We could also throw buffers into this list if we
+consider their pages unlikely to be needed soon; however, the current
+algorithm never does that.  The list is singly-linked using fields in the
+buffer headers; we maintain head and tail pointers in global variables.
+(Note: although the list links are in the buffer headers, they are
+considered to be protected by the buffer_strategy_lock, not the buffer-header
+spinlocks.)  To choose a victim buffer to recycle when there are no free
+buffers available, we use a simple clock-sweep algorithm, which avoids the
+need to take system-wide locks during common operations.  It works like
+this:
+
+Each buffer header contains a usage counter, which is incremented (up to a
+small limit value) whenever the buffer is pinned.  (This requires only the
+buffer header spinlock, which would have to be taken anyway to increment the
+buffer reference count, so it's nearly free.)
+
+The "clock hand" is a buffer index, nextVictimBuffer, that moves circularly
+through all the available buffers.  nextVictimBuffer is protected by the
+buffer_strategy_lock.
+
+The algorithm for a process that needs to obtain a victim buffer is:
+
+1. Obtain buffer_strategy_lock.
+
+2. If buffer free list is nonempty, remove its head buffer.  Release
+buffer_strategy_lock.  If the buffer is pinned or has a nonzero usage count,
+it cannot be used; ignore it go back to step 1.  Otherwise, pin the buffer,
+and return it.
+
+3. Otherwise, the buffer free list is empty.  Select the buffer pointed to by
+nextVictimBuffer, and circularly advance nextVictimBuffer for next time.
+Release buffer_strategy_lock.
+
+4. If the selected buffer is pinned or has a nonzero usage count, it cannot
+be used.  Decrement its usage count (if nonzero), reacquire
+buffer_strategy_lock, and return to step 3 to examine the next buffer.
+
+5. Pin the selected buffer, and return.
+
+(Note that if the selected buffer is dirty, we will have to write it out
+before we can recycle it; if someone else pins the buffer meanwhile we will
+have to give up and try another buffer.  This however is not a concern
+of the basic select-a-victim-buffer algorithm.)
+
+
+Buffer Ring Replacement Strategy
+---------------------------------
+
+When running a query that needs to access a large number of pages just once,
+such as VACUUM or a large sequential scan, a different strategy is used.
+A page that has been touched only by such a scan is unlikely to be needed
+again soon, so instead of running the normal clock sweep algorithm and
+blowing out the entire buffer cache, a small ring of buffers is allocated
+using the normal clock sweep algorithm and those buffers are reused for the
+whole scan.  This also implies that much of the write traffic caused by such
+a statement will be done by the backend itself and not pushed off onto other
+processes.
+
+For sequential scans, a 256KB ring is used. That's small enough to fit in L2
+cache, which makes transferring pages from OS cache to shared buffer cache
+efficient.  Even less would often be enough, but the ring must be big enough
+to accommodate all pages in the scan that are pinned concurrently.  256KB
+should also be enough to leave a small cache trail for other backends to
+join in a synchronized seq scan.  If a ring buffer is dirtied and its LSN
+updated, we would normally have to write and flush WAL before we could
+re-use the buffer; in this case we instead discard the buffer from the ring
+and (later) choose a replacement using the normal clock-sweep algorithm.
+Hence this strategy works best for scans that are read-only (or at worst
+update hint bits).  In a scan that modifies every page in the scan, like a
+bulk UPDATE or DELETE, the buffers in the ring will always be dirtied and
+the ring strategy effectively degrades to the normal strategy.
+
+VACUUM uses a 256KB ring like sequential scans, but dirty pages are not
+removed from the ring.  Instead, WAL is flushed if needed to allow reuse of
+the buffers.  Before introducing the buffer ring strategy in 8.3, VACUUM's
+buffers were sent to the freelist, which was effectively a buffer ring of 1
+buffer, resulting in excessive WAL flushing.  Allowing VACUUM to update
+256KB between WAL flushes should be more efficient.
+
+Bulk writes work similarly to VACUUM.  Currently this applies only to
+COPY IN and CREATE TABLE AS SELECT.  (Might it be interesting to make
+seqscan UPDATE and DELETE use the bulkwrite strategy?)  For bulk writes
+we use a ring size of 16MB (but not more than 1/8th of shared_buffers).
+Smaller sizes have been shown to result in the COPY blocking too often
+for WAL flushes.  While it's okay for a background vacuum to be slowed by
+doing its own WAL flushing, we'd prefer that COPY not be subject to that,
+so we let it use up a bit more of the buffer arena.
+
+
+Background Writer's Processing
+------------------------------
+
+The background writer is designed to write out pages that are likely to be
+recycled soon, thereby offloading the writing work from active backends.
+To do this, it scans forward circularly from the current position of
+nextVictimBuffer (which it does not change!), looking for buffers that are
+dirty and not pinned nor marked with a positive usage count.  It pins,
+writes, and releases any such buffer.
+
+If we can assume that reading nextVictimBuffer is an atomic action, then
+the writer doesn't even need to take buffer_strategy_lock in order to look
+for buffers to write; it needs only to spinlock each buffer header for long
+enough to check the dirtybit.  Even without that assumption, the writer
+only needs to take the lock long enough to read the variable value, not
+while scanning the buffers.  (This is a very substantial improvement in
+the contention cost of the writer compared to PG 8.0.)
+
+The background writer takes shared content lock on a buffer while writing it
+out (and anyone else who flushes buffer contents to disk must do so too).
+This ensures that the page image transferred to disk is reasonably consistent.
+We might miss a hint-bit update or two but that isn't a problem, for the same
+reasons mentioned under buffer access rules.
+
+As of 8.4, background writer starts during recovery mode when there is
+some form of potentially extended recovery to perform. It performs an
+identical service to normal processing, except that checkpoints it
+writes are technically restartpoints.
diff --git a/src/backend/storage/buffer/buf_init.c b/src/backend/storage/buffer/buf_init.c
new file mode 100644
index 0000000..a299be1
--- /dev/null
+++ b/src/backend/storage/buffer/buf_init.c
@@ -0,0 +1,181 @@
+/*-------------------------------------------------------------------------
+ *
+ * buf_init.c
+ *	  buffer manager initialization routines
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/storage/buffer/buf_init.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "storage/buf_internals.h"
+#include "storage/bufmgr.h"
+
+BufferDescPadded *BufferDescriptors;
+char	   *BufferBlocks;
+ConditionVariableMinimallyPadded *BufferIOCVArray;
+WritebackContext BackendWritebackContext;
+CkptSortItem *CkptBufferIds;
+
+
+/*
+ * Data Structures:
+ *		buffers live in a freelist and a lookup data structure.
+ *
+ *
+ * Buffer Lookup:
+ *		Two important notes.  First, the buffer has to be
+ *		available for lookup BEFORE an IO begins.  Otherwise
+ *		a second process trying to read the buffer will
+ *		allocate its own copy and the buffer pool will
+ *		become inconsistent.
+ *
+ * Buffer Replacement:
+ *		see freelist.c.  A buffer cannot be replaced while in
+ *		use either by data manager or during IO.
+ *
+ *
+ * Synchronization/Locking:
+ *
+ * IO_IN_PROGRESS -- this is a flag in the buffer descriptor.
+ *		It must be set when an IO is initiated and cleared at
+ *		the end of the IO.  It is there to make sure that one
+ *		process doesn't start to use a buffer while another is
+ *		faulting it in.  see WaitIO and related routines.
+ *
+ * refcount --	Counts the number of processes holding pins on a buffer.
+ *		A buffer is pinned during IO and immediately after a BufferAlloc().
+ *		Pins must be released before end of transaction.  For efficiency the
+ *		shared refcount isn't increased if an individual backend pins a buffer
+ *		multiple times. Check the PrivateRefCount infrastructure in bufmgr.c.
+ */
+
+
+/*
+ * Initialize shared buffer pool
+ *
+ * This is called once during shared-memory initialization (either in the
+ * postmaster, or in a standalone backend).
+ */
+void
+InitBufferPool(void)
+{
+	bool		foundBufs,
+				foundDescs,
+				foundIOCV,
+				foundBufCkpt;
+
+	/* Align descriptors to a cacheline boundary. */
+	BufferDescriptors = (BufferDescPadded *)
+		ShmemInitStruct("Buffer Descriptors",
+						NBuffers * sizeof(BufferDescPadded),
+						&foundDescs);
+
+	BufferBlocks = (char *)
+		ShmemInitStruct("Buffer Blocks",
+						NBuffers * (Size) BLCKSZ, &foundBufs);
+
+	/* Align condition variables to cacheline boundary. */
+	BufferIOCVArray = (ConditionVariableMinimallyPadded *)
+		ShmemInitStruct("Buffer IO Condition Variables",
+						NBuffers * sizeof(ConditionVariableMinimallyPadded),
+						&foundIOCV);
+
+	/*
+	 * The array used to sort to-be-checkpointed buffer ids is located in
+	 * shared memory, to avoid having to allocate significant amounts of
+	 * memory at runtime. As that'd be in the middle of a checkpoint, or when
+	 * the checkpointer is restarted, memory allocation failures would be
+	 * painful.
+	 */
+	CkptBufferIds = (CkptSortItem *)
+		ShmemInitStruct("Checkpoint BufferIds",
+						NBuffers * sizeof(CkptSortItem), &foundBufCkpt);
+
+	if (foundDescs || foundBufs || foundIOCV || foundBufCkpt)
+	{
+		/* should find all of these, or none of them */
+		Assert(foundDescs && foundBufs && foundIOCV && foundBufCkpt);
+		/* note: this path is only taken in EXEC_BACKEND case */
+	}
+	else
+	{
+		int			i;
+
+		/*
+		 * Initialize all the buffer headers.
+		 */
+		for (i = 0; i < NBuffers; i++)
+		{
+			BufferDesc *buf = GetBufferDescriptor(i);
+
+			CLEAR_BUFFERTAG(buf->tag);
+
+			pg_atomic_init_u32(&buf->state, 0);
+			buf->wait_backend_pid = 0;
+
+			buf->buf_id = i;
+
+			/*
+			 * Initially link all the buffers together as unused. Subsequent
+			 * management of this list is done by freelist.c.
+			 */
+			buf->freeNext = i + 1;
+
+			LWLockInitialize(BufferDescriptorGetContentLock(buf),
+							 LWTRANCHE_BUFFER_CONTENT);
+
+			ConditionVariableInit(BufferDescriptorGetIOCV(buf));
+		}
+
+		/* Correct last entry of linked list */
+		GetBufferDescriptor(NBuffers - 1)->freeNext = FREENEXT_END_OF_LIST;
+	}
+
+	/* Init other shared buffer-management stuff */
+	StrategyInitialize(!foundDescs);
+
+	/* Initialize per-backend file flush context */
+	WritebackContextInit(&BackendWritebackContext,
+						 &backend_flush_after);
+}
+
+/*
+ * BufferShmemSize
+ *
+ * compute the size of shared memory for the buffer pool including
+ * data pages, buffer descriptors, hash tables, etc.
+ */
+Size
+BufferShmemSize(void)
+{
+	Size		size = 0;
+
+	/* size of buffer descriptors */
+	size = add_size(size, mul_size(NBuffers, sizeof(BufferDescPadded)));
+	/* to allow aligning buffer descriptors */
+	size = add_size(size, PG_CACHE_LINE_SIZE);
+
+	/* size of data pages */
+	size = add_size(size, mul_size(NBuffers, BLCKSZ));
+
+	/* size of stuff controlled by freelist.c */
+	size = add_size(size, StrategyShmemSize());
+
+	/* size of I/O condition variables */
+	size = add_size(size, mul_size(NBuffers,
+								   sizeof(ConditionVariableMinimallyPadded)));
+	/* to allow aligning the above */
+	size = add_size(size, PG_CACHE_LINE_SIZE);
+
+	/* size of checkpoint sort array in bufmgr.c */
+	size = add_size(size, mul_size(NBuffers, sizeof(CkptSortItem)));
+
+	return size;
+}
diff --git a/src/backend/storage/buffer/buf_table.c b/src/backend/storage/buffer/buf_table.c
new file mode 100644
index 0000000..caa03ae
--- /dev/null
+++ b/src/backend/storage/buffer/buf_table.c
@@ -0,0 +1,162 @@
+/*-------------------------------------------------------------------------
+ *
+ * buf_table.c
+ *	  routines for mapping BufferTags to buffer indexes.
+ *
+ * Note: the routines in this file do no locking of their own.  The caller
+ * must hold a suitable lock on the appropriate BufMappingLock, as specified
+ * in the comments.  We can't do the locking inside these functions because
+ * in most cases the caller needs to adjust the buffer header contents
+ * before the lock is released (see notes in README).
+ *
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/storage/buffer/buf_table.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "storage/buf_internals.h"
+#include "storage/bufmgr.h"
+
+/* entry for buffer lookup hashtable */
+typedef struct
+{
+	BufferTag	key;			/* Tag of a disk page */
+	int			id;				/* Associated buffer ID */
+} BufferLookupEnt;
+
+static HTAB *SharedBufHash;
+
+
+/*
+ * Estimate space needed for mapping hashtable
+ *		size is the desired hash table size (possibly more than NBuffers)
+ */
+Size
+BufTableShmemSize(int size)
+{
+	return hash_estimate_size(size, sizeof(BufferLookupEnt));
+}
+
+/*
+ * Initialize shmem hash table for mapping buffers
+ *		size is the desired hash table size (possibly more than NBuffers)
+ */
+void
+InitBufTable(int size)
+{
+	HASHCTL		info;
+
+	/* assume no locking is needed yet */
+
+	/* BufferTag maps to Buffer */
+	info.keysize = sizeof(BufferTag);
+	info.entrysize = sizeof(BufferLookupEnt);
+	info.num_partitions = NUM_BUFFER_PARTITIONS;
+
+	SharedBufHash = ShmemInitHash("Shared Buffer Lookup Table",
+								  size, size,
+								  &info,
+								  HASH_ELEM | HASH_BLOBS | HASH_PARTITION);
+}
+
+/*
+ * BufTableHashCode
+ *		Compute the hash code associated with a BufferTag
+ *
+ * This must be passed to the lookup/insert/delete routines along with the
+ * tag.  We do it like this because the callers need to know the hash code
+ * in order to determine which buffer partition to lock, and we don't want
+ * to do the hash computation twice (hash_any is a bit slow).
+ */
+uint32
+BufTableHashCode(BufferTag *tagPtr)
+{
+	return get_hash_value(SharedBufHash, (void *) tagPtr);
+}
+
+/*
+ * BufTableLookup
+ *		Lookup the given BufferTag; return buffer ID, or -1 if not found
+ *
+ * Caller must hold at least share lock on BufMappingLock for tag's partition
+ */
+int
+BufTableLookup(BufferTag *tagPtr, uint32 hashcode)
+{
+	BufferLookupEnt *result;
+
+	result = (BufferLookupEnt *)
+		hash_search_with_hash_value(SharedBufHash,
+									(void *) tagPtr,
+									hashcode,
+									HASH_FIND,
+									NULL);
+
+	if (!result)
+		return -1;
+
+	return result->id;
+}
+
+/*
+ * BufTableInsert
+ *		Insert a hashtable entry for given tag and buffer ID,
+ *		unless an entry already exists for that tag
+ *
+ * Returns -1 on successful insertion.  If a conflicting entry exists
+ * already, returns the buffer ID in that entry.
+ *
+ * Caller must hold exclusive lock on BufMappingLock for tag's partition
+ */
+int
+BufTableInsert(BufferTag *tagPtr, uint32 hashcode, int buf_id)
+{
+	BufferLookupEnt *result;
+	bool		found;
+
+	Assert(buf_id >= 0);		/* -1 is reserved for not-in-table */
+	Assert(tagPtr->blockNum != P_NEW);	/* invalid tag */
+
+	result = (BufferLookupEnt *)
+		hash_search_with_hash_value(SharedBufHash,
+									(void *) tagPtr,
+									hashcode,
+									HASH_ENTER,
+									&found);
+
+	if (found)					/* found something already in the table */
+		return result->id;
+
+	result->id = buf_id;
+
+	return -1;
+}
+
+/*
+ * BufTableDelete
+ *		Delete the hashtable entry for given tag (which must exist)
+ *
+ * Caller must hold exclusive lock on BufMappingLock for tag's partition
+ */
+void
+BufTableDelete(BufferTag *tagPtr, uint32 hashcode)
+{
+	BufferLookupEnt *result;
+
+	result = (BufferLookupEnt *)
+		hash_search_with_hash_value(SharedBufHash,
+									(void *) tagPtr,
+									hashcode,
+									HASH_REMOVE,
+									NULL);
+
+	if (!result)				/* shouldn't happen */
+		elog(ERROR, "shared buffer hash table corrupted");
+}
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
new file mode 100644
index 0000000..d2eb69b
--- /dev/null
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -0,0 +1,4892 @@
+/*-------------------------------------------------------------------------
+ *
+ * bufmgr.c
+ *	  buffer manager interface routines
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/storage/buffer/bufmgr.c
+ *
+ *-------------------------------------------------------------------------
+ */
+/*
+ * Principal entry points:
+ *
+ * ReadBuffer() -- find or create a buffer holding the requested page,
+ *		and pin it so that no one can destroy it while this process
+ *		is using it.
+ *
+ * ReleaseBuffer() -- unpin a buffer
+ *
+ * MarkBufferDirty() -- mark a pinned buffer's contents as "dirty".
+ *		The disk write is delayed until buffer replacement or checkpoint.
+ *
+ * See also these files:
+ *		freelist.c -- chooses victim for buffer replacement
+ *		buf_table.c -- manages the buffer lookup table
+ */
+#include "postgres.h"
+
+#include <sys/file.h>
+#include <unistd.h>
+
+#include "access/tableam.h"
+#include "access/xlog.h"
+#include "catalog/catalog.h"
+#include "catalog/storage.h"
+#include "executor/instrument.h"
+#include "lib/binaryheap.h"
+#include "miscadmin.h"
+#include "pg_trace.h"
+#include "pgstat.h"
+#include "postmaster/bgwriter.h"
+#include "storage/buf_internals.h"
+#include "storage/bufmgr.h"
+#include "storage/ipc.h"
+#include "storage/proc.h"
+#include "storage/smgr.h"
+#include "storage/standby.h"
+#include "utils/memdebug.h"
+#include "utils/ps_status.h"
+#include "utils/rel.h"
+#include "utils/resowner_private.h"
+#include "utils/timestamp.h"
+
+
+/* Note: these two macros only work on shared buffers, not local ones! */
+#define BufHdrGetBlock(bufHdr)	((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
+#define BufferGetLSN(bufHdr)	(PageGetLSN(BufHdrGetBlock(bufHdr)))
+
+/* Note: this macro only works on local buffers, not shared ones! */
+#define LocalBufHdrGetBlock(bufHdr) \
+	LocalBufferBlockPointers[-((bufHdr)->buf_id + 2)]
+
+/* Bits in SyncOneBuffer's return value */
+#define BUF_WRITTEN				0x01
+#define BUF_REUSABLE			0x02
+
+#define RELS_BSEARCH_THRESHOLD		20
+
+/*
+ * This is the size (in the number of blocks) above which we scan the
+ * entire buffer pool to remove the buffers for all the pages of relation
+ * being dropped. For the relations with size below this threshold, we find
+ * the buffers by doing lookups in BufMapping table.
+ */
+#define BUF_DROP_FULL_SCAN_THRESHOLD		(uint64) (NBuffers / 32)
+
+typedef struct PrivateRefCountEntry
+{
+	Buffer		buffer;
+	int32		refcount;
+} PrivateRefCountEntry;
+
+/* 64 bytes, about the size of a cache line on common systems */
+#define REFCOUNT_ARRAY_ENTRIES 8
+
+/*
+ * Status of buffers to checkpoint for a particular tablespace, used
+ * internally in BufferSync.
+ */
+typedef struct CkptTsStatus
+{
+	/* oid of the tablespace */
+	Oid			tsId;
+
+	/*
+	 * Checkpoint progress for this tablespace. To make progress comparable
+	 * between tablespaces the progress is, for each tablespace, measured as a
+	 * number between 0 and the total number of to-be-checkpointed pages. Each
+	 * page checkpointed in this tablespace increments this space's progress
+	 * by progress_slice.
+	 */
+	float8		progress;
+	float8		progress_slice;
+
+	/* number of to-be checkpointed pages in this tablespace */
+	int			num_to_scan;
+	/* already processed pages in this tablespace */
+	int			num_scanned;
+
+	/* current offset in CkptBufferIds for this tablespace */
+	int			index;
+} CkptTsStatus;
+
+/*
+ * Type for array used to sort SMgrRelations
+ *
+ * FlushRelationsAllBuffers shares the same comparator function with
+ * DropRelFileNodesAllBuffers. Pointer to this struct and RelFileNode must be
+ * compatible.
+ */
+typedef struct SMgrSortArray
+{
+	RelFileNode rnode;			/* This must be the first member */
+	SMgrRelation srel;
+} SMgrSortArray;
+
+/* GUC variables */
+bool		zero_damaged_pages = false;
+int			bgwriter_lru_maxpages = 100;
+double		bgwriter_lru_multiplier = 2.0;
+bool		track_io_timing = false;
+
+/*
+ * How many buffers PrefetchBuffer callers should try to stay ahead of their
+ * ReadBuffer calls by.  Zero means "never prefetch".  This value is only used
+ * for buffers not belonging to tablespaces that have their
+ * effective_io_concurrency parameter set.
+ */
+int			effective_io_concurrency = 0;
+
+/*
+ * Like effective_io_concurrency, but used by maintenance code paths that might
+ * benefit from a higher setting because they work on behalf of many sessions.
+ * Overridden by the tablespace setting of the same name.
+ */
+int			maintenance_io_concurrency = 0;
+
+/*
+ * GUC variables about triggering kernel writeback for buffers written; OS
+ * dependent defaults are set via the GUC mechanism.
+ */
+int			checkpoint_flush_after = 0;
+int			bgwriter_flush_after = 0;
+int			backend_flush_after = 0;
+
+/* local state for StartBufferIO and related functions */
+static BufferDesc *InProgressBuf = NULL;
+static bool IsForInput;
+
+/* local state for LockBufferForCleanup */
+static BufferDesc *PinCountWaitBuf = NULL;
+
+/*
+ * Backend-Private refcount management:
+ *
+ * Each buffer also has a private refcount that keeps track of the number of
+ * times the buffer is pinned in the current process.  This is so that the
+ * shared refcount needs to be modified only once if a buffer is pinned more
+ * than once by an individual backend.  It's also used to check that no buffers
+ * are still pinned at the end of transactions and when exiting.
+ *
+ *
+ * To avoid - as we used to - requiring an array with NBuffers entries to keep
+ * track of local buffers, we use a small sequentially searched array
+ * (PrivateRefCountArray) and an overflow hash table (PrivateRefCountHash) to
+ * keep track of backend local pins.
+ *
+ * Until no more than REFCOUNT_ARRAY_ENTRIES buffers are pinned at once, all
+ * refcounts are kept track of in the array; after that, new array entries
+ * displace old ones into the hash table. That way a frequently used entry
+ * can't get "stuck" in the hashtable while infrequent ones clog the array.
+ *
+ * Note that in most scenarios the number of pinned buffers will not exceed
+ * REFCOUNT_ARRAY_ENTRIES.
+ *
+ *
+ * To enter a buffer into the refcount tracking mechanism first reserve a free
+ * entry using ReservePrivateRefCountEntry() and then later, if necessary,
+ * fill it with NewPrivateRefCountEntry(). That split lets us avoid doing
+ * memory allocations in NewPrivateRefCountEntry() which can be important
+ * because in some scenarios it's called with a spinlock held...
+ */
+static struct PrivateRefCountEntry PrivateRefCountArray[REFCOUNT_ARRAY_ENTRIES];
+static HTAB *PrivateRefCountHash = NULL;
+static int32 PrivateRefCountOverflowed = 0;
+static uint32 PrivateRefCountClock = 0;
+static PrivateRefCountEntry *ReservedRefCountEntry = NULL;
+
+static void ReservePrivateRefCountEntry(void);
+static PrivateRefCountEntry *NewPrivateRefCountEntry(Buffer buffer);
+static PrivateRefCountEntry *GetPrivateRefCountEntry(Buffer buffer, bool do_move);
+static inline int32 GetPrivateRefCount(Buffer buffer);
+static void ForgetPrivateRefCountEntry(PrivateRefCountEntry *ref);
+
+/*
+ * Ensure that the PrivateRefCountArray has sufficient space to store one more
+ * entry. This has to be called before using NewPrivateRefCountEntry() to fill
+ * a new entry - but it's perfectly fine to not use a reserved entry.
+ */
+static void
+ReservePrivateRefCountEntry(void)
+{
+	/* Already reserved (or freed), nothing to do */
+	if (ReservedRefCountEntry != NULL)
+		return;
+
+	/*
+	 * First search for a free entry the array, that'll be sufficient in the
+	 * majority of cases.
+	 */
+	{
+		int			i;
+
+		for (i = 0; i < REFCOUNT_ARRAY_ENTRIES; i++)
+		{
+			PrivateRefCountEntry *res;
+
+			res = &PrivateRefCountArray[i];
+
+			if (res->buffer == InvalidBuffer)
+			{
+				ReservedRefCountEntry = res;
+				return;
+			}
+		}
+	}
+
+	/*
+	 * No luck. All array entries are full. Move one array entry into the hash
+	 * table.
+	 */
+	{
+		/*
+		 * Move entry from the current clock position in the array into the
+		 * hashtable. Use that slot.
+		 */
+		PrivateRefCountEntry *hashent;
+		bool		found;
+
+		/* select victim slot */
+		ReservedRefCountEntry =
+			&PrivateRefCountArray[PrivateRefCountClock++ % REFCOUNT_ARRAY_ENTRIES];
+
+		/* Better be used, otherwise we shouldn't get here. */
+		Assert(ReservedRefCountEntry->buffer != InvalidBuffer);
+
+		/* enter victim array entry into hashtable */
+		hashent = hash_search(PrivateRefCountHash,
+							  (void *) &(ReservedRefCountEntry->buffer),
+							  HASH_ENTER,
+							  &found);
+		Assert(!found);
+		hashent->refcount = ReservedRefCountEntry->refcount;
+
+		/* clear the now free array slot */
+		ReservedRefCountEntry->buffer = InvalidBuffer;
+		ReservedRefCountEntry->refcount = 0;
+
+		PrivateRefCountOverflowed++;
+	}
+}
+
+/*
+ * Fill a previously reserved refcount entry.
+ */
+static PrivateRefCountEntry *
+NewPrivateRefCountEntry(Buffer buffer)
+{
+	PrivateRefCountEntry *res;
+
+	/* only allowed to be called when a reservation has been made */
+	Assert(ReservedRefCountEntry != NULL);
+
+	/* use up the reserved entry */
+	res = ReservedRefCountEntry;
+	ReservedRefCountEntry = NULL;
+
+	/* and fill it */
+	res->buffer = buffer;
+	res->refcount = 0;
+
+	return res;
+}
+
+/*
+ * Return the PrivateRefCount entry for the passed buffer.
+ *
+ * Returns NULL if a buffer doesn't have a refcount entry. Otherwise, if
+ * do_move is true, and the entry resides in the hashtable the entry is
+ * optimized for frequent access by moving it to the array.
+ */
+static PrivateRefCountEntry *
+GetPrivateRefCountEntry(Buffer buffer, bool do_move)
+{
+	PrivateRefCountEntry *res;
+	int			i;
+
+	Assert(BufferIsValid(buffer));
+	Assert(!BufferIsLocal(buffer));
+
+	/*
+	 * First search for references in the array, that'll be sufficient in the
+	 * majority of cases.
+	 */
+	for (i = 0; i < REFCOUNT_ARRAY_ENTRIES; i++)
+	{
+		res = &PrivateRefCountArray[i];
+
+		if (res->buffer == buffer)
+			return res;
+	}
+
+	/*
+	 * By here we know that the buffer, if already pinned, isn't residing in
+	 * the array.
+	 *
+	 * Only look up the buffer in the hashtable if we've previously overflowed
+	 * into it.
+	 */
+	if (PrivateRefCountOverflowed == 0)
+		return NULL;
+
+	res = hash_search(PrivateRefCountHash,
+					  (void *) &buffer,
+					  HASH_FIND,
+					  NULL);
+
+	if (res == NULL)
+		return NULL;
+	else if (!do_move)
+	{
+		/* caller doesn't want us to move the hash entry into the array */
+		return res;
+	}
+	else
+	{
+		/* move buffer from hashtable into the free array slot */
+		bool		found;
+		PrivateRefCountEntry *free;
+
+		/* Ensure there's a free array slot */
+		ReservePrivateRefCountEntry();
+
+		/* Use up the reserved slot */
+		Assert(ReservedRefCountEntry != NULL);
+		free = ReservedRefCountEntry;
+		ReservedRefCountEntry = NULL;
+		Assert(free->buffer == InvalidBuffer);
+
+		/* and fill it */
+		free->buffer = buffer;
+		free->refcount = res->refcount;
+
+		/* delete from hashtable */
+		hash_search(PrivateRefCountHash,
+					(void *) &buffer,
+					HASH_REMOVE,
+					&found);
+		Assert(found);
+		Assert(PrivateRefCountOverflowed > 0);
+		PrivateRefCountOverflowed--;
+
+		return free;
+	}
+}
+
+/*
+ * Returns how many times the passed buffer is pinned by this backend.
+ *
+ * Only works for shared memory buffers!
+ */
+static inline int32
+GetPrivateRefCount(Buffer buffer)
+{
+	PrivateRefCountEntry *ref;
+
+	Assert(BufferIsValid(buffer));
+	Assert(!BufferIsLocal(buffer));
+
+	/*
+	 * Not moving the entry - that's ok for the current users, but we might
+	 * want to change this one day.
+	 */
+	ref = GetPrivateRefCountEntry(buffer, false);
+
+	if (ref == NULL)
+		return 0;
+	return ref->refcount;
+}
+
+/*
+ * Release resources used to track the reference count of a buffer which we no
+ * longer have pinned and don't want to pin again immediately.
+ */
+static void
+ForgetPrivateRefCountEntry(PrivateRefCountEntry *ref)
+{
+	Assert(ref->refcount == 0);
+
+	if (ref >= &PrivateRefCountArray[0] &&
+		ref < &PrivateRefCountArray[REFCOUNT_ARRAY_ENTRIES])
+	{
+		ref->buffer = InvalidBuffer;
+
+		/*
+		 * Mark the just used entry as reserved - in many scenarios that
+		 * allows us to avoid ever having to search the array/hash for free
+		 * entries.
+		 */
+		ReservedRefCountEntry = ref;
+	}
+	else
+	{
+		bool		found;
+		Buffer		buffer = ref->buffer;
+
+		hash_search(PrivateRefCountHash,
+					(void *) &buffer,
+					HASH_REMOVE,
+					&found);
+		Assert(found);
+		Assert(PrivateRefCountOverflowed > 0);
+		PrivateRefCountOverflowed--;
+	}
+}
+
+/*
+ * BufferIsPinned
+ *		True iff the buffer is pinned (also checks for valid buffer number).
+ *
+ *		NOTE: what we check here is that *this* backend holds a pin on
+ *		the buffer.  We do not care whether some other backend does.
+ */
+#define BufferIsPinned(bufnum) \
+( \
+	!BufferIsValid(bufnum) ? \
+		false \
+	: \
+		BufferIsLocal(bufnum) ? \
+			(LocalRefCount[-(bufnum) - 1] > 0) \
+		: \
+	(GetPrivateRefCount(bufnum) > 0) \
+)
+
+
+static Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
+								ForkNumber forkNum, BlockNumber blockNum,
+								ReadBufferMode mode, BufferAccessStrategy strategy,
+								bool *hit);
+static bool PinBuffer(BufferDesc *buf, BufferAccessStrategy strategy);
+static void PinBuffer_Locked(BufferDesc *buf);
+static void UnpinBuffer(BufferDesc *buf, bool fixOwner);
+static void BufferSync(int flags);
+static uint32 WaitBufHdrUnlocked(BufferDesc *buf);
+static int	SyncOneBuffer(int buf_id, bool skip_recently_used,
+						  WritebackContext *wb_context);
+static void WaitIO(BufferDesc *buf);
+static bool StartBufferIO(BufferDesc *buf, bool forInput);
+static void TerminateBufferIO(BufferDesc *buf, bool clear_dirty,
+							  uint32 set_flag_bits);
+static void shared_buffer_write_error_callback(void *arg);
+static void local_buffer_write_error_callback(void *arg);
+static BufferDesc *BufferAlloc(SMgrRelation smgr,
+							   char relpersistence,
+							   ForkNumber forkNum,
+							   BlockNumber blockNum,
+							   BufferAccessStrategy strategy,
+							   bool *foundPtr);
+static void FlushBuffer(BufferDesc *buf, SMgrRelation reln);
+static void FindAndDropRelFileNodeBuffers(RelFileNode rnode,
+										  ForkNumber forkNum,
+										  BlockNumber nForkBlock,
+										  BlockNumber firstDelBlock);
+static void AtProcExit_Buffers(int code, Datum arg);
+static void CheckForBufferLeaks(void);
+static int	rnode_comparator(const void *p1, const void *p2);
+static inline int buffertag_comparator(const BufferTag *a, const BufferTag *b);
+static inline int ckpt_buforder_comparator(const CkptSortItem *a, const CkptSortItem *b);
+static int	ts_ckpt_progress_comparator(Datum a, Datum b, void *arg);
+
+
+/*
+ * Implementation of PrefetchBuffer() for shared buffers.
+ */
+PrefetchBufferResult
+PrefetchSharedBuffer(SMgrRelation smgr_reln,
+					 ForkNumber forkNum,
+					 BlockNumber blockNum)
+{
+	PrefetchBufferResult result = {InvalidBuffer, false};
+	BufferTag	newTag;			/* identity of requested block */
+	uint32		newHash;		/* hash value for newTag */
+	LWLock	   *newPartitionLock;	/* buffer partition lock for it */
+	int			buf_id;
+
+	Assert(BlockNumberIsValid(blockNum));
+
+	/* create a tag so we can lookup the buffer */
+	INIT_BUFFERTAG(newTag, smgr_reln->smgr_rnode.node,
+				   forkNum, blockNum);
+
+	/* determine its hash code and partition lock ID */
+	newHash = BufTableHashCode(&newTag);
+	newPartitionLock = BufMappingPartitionLock(newHash);
+
+	/* see if the block is in the buffer pool already */
+	LWLockAcquire(newPartitionLock, LW_SHARED);
+	buf_id = BufTableLookup(&newTag, newHash);
+	LWLockRelease(newPartitionLock);
+
+	/* If not in buffers, initiate prefetch */
+	if (buf_id < 0)
+	{
+#ifdef USE_PREFETCH
+		/*
+		 * Try to initiate an asynchronous read.  This returns false in
+		 * recovery if the relation file doesn't exist.
+		 */
+		if (smgrprefetch(smgr_reln, forkNum, blockNum))
+			result.initiated_io = true;
+#endif							/* USE_PREFETCH */
+	}
+	else
+	{
+		/*
+		 * Report the buffer it was in at that time.  The caller may be able
+		 * to avoid a buffer table lookup, but it's not pinned and it must be
+		 * rechecked!
+		 */
+		result.recent_buffer = buf_id + 1;
+	}
+
+	/*
+	 * If the block *is* in buffers, we do nothing.  This is not really ideal:
+	 * the block might be just about to be evicted, which would be stupid
+	 * since we know we are going to need it soon.  But the only easy answer
+	 * is to bump the usage_count, which does not seem like a great solution:
+	 * when the caller does ultimately touch the block, usage_count would get
+	 * bumped again, resulting in too much favoritism for blocks that are
+	 * involved in a prefetch sequence. A real fix would involve some
+	 * additional per-buffer state, and it's not clear that there's enough of
+	 * a problem to justify that.
+	 */
+
+	return result;
+}
+
+/*
+ * PrefetchBuffer -- initiate asynchronous read of a block of a relation
+ *
+ * This is named by analogy to ReadBuffer but doesn't actually allocate a
+ * buffer.  Instead it tries to ensure that a future ReadBuffer for the given
+ * block will not be delayed by the I/O.  Prefetching is optional.
+ *
+ * There are three possible outcomes:
+ *
+ * 1.  If the block is already cached, the result includes a valid buffer that
+ * could be used by the caller to avoid the need for a later buffer lookup, but
+ * it's not pinned, so the caller must recheck it.
+ *
+ * 2.  If the kernel has been asked to initiate I/O, the initiated_io member is
+ * true.  Currently there is no way to know if the data was already cached by
+ * the kernel and therefore didn't really initiate I/O, and no way to know when
+ * the I/O completes other than using synchronous ReadBuffer().
+ *
+ * 3.  Otherwise, the buffer wasn't already cached by PostgreSQL, and either
+ * USE_PREFETCH is not defined (this build doesn't support prefetching due to
+ * lack of a kernel facility), or the underlying relation file wasn't found and
+ * we are in recovery.  (If the relation file wasn't found and we are not in
+ * recovery, an error is raised).
+ */
+PrefetchBufferResult
+PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
+{
+	Assert(RelationIsValid(reln));
+	Assert(BlockNumberIsValid(blockNum));
+
+	/* Open it at the smgr level if not already done */
+	RelationOpenSmgr(reln);
+
+	if (RelationUsesLocalBuffers(reln))
+	{
+		/* see comments in ReadBufferExtended */
+		if (RELATION_IS_OTHER_TEMP(reln))
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("cannot access temporary tables of other sessions")));
+
+		/* pass it off to localbuf.c */
+		return PrefetchLocalBuffer(reln->rd_smgr, forkNum, blockNum);
+	}
+	else
+	{
+		/* pass it to the shared buffer version */
+		return PrefetchSharedBuffer(reln->rd_smgr, forkNum, blockNum);
+	}
+}
+
+/*
+ * ReadRecentBuffer -- try to pin a block in a recently observed buffer
+ *
+ * Compared to ReadBuffer(), this avoids a buffer mapping lookup when it's
+ * successful.  Return true if the buffer is valid and still has the expected
+ * tag.  In that case, the buffer is pinned and the usage count is bumped.
+ */
+bool
+ReadRecentBuffer(RelFileNode rnode, ForkNumber forkNum, BlockNumber blockNum,
+				 Buffer recent_buffer)
+{
+	BufferDesc *bufHdr;
+	BufferTag	tag;
+	uint32		buf_state;
+	bool		have_private_ref;
+
+	Assert(BufferIsValid(recent_buffer));
+
+	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+	ReservePrivateRefCountEntry();
+	INIT_BUFFERTAG(tag, rnode, forkNum, blockNum);
+
+	if (BufferIsLocal(recent_buffer))
+	{
+		int			b = -recent_buffer - 1;
+
+		bufHdr = GetLocalBufferDescriptor(b);
+		buf_state = pg_atomic_read_u32(&bufHdr->state);
+
+		/* Is it still valid and holding the right tag? */
+		if ((buf_state & BM_VALID) && BUFFERTAGS_EQUAL(tag, bufHdr->tag))
+		{
+			/*
+			 * Bump buffer's ref and usage counts. This is equivalent of
+			 * PinBuffer for a shared buffer.
+			 */
+			if (LocalRefCount[b] == 0)
+			{
+				if (BUF_STATE_GET_USAGECOUNT(buf_state) < BM_MAX_USAGE_COUNT)
+				{
+					buf_state += BUF_USAGECOUNT_ONE;
+					pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
+				}
+			}
+			LocalRefCount[b]++;
+			ResourceOwnerRememberBuffer(CurrentResourceOwner, recent_buffer);
+
+			return true;
+		}
+	}
+	else
+	{
+		bufHdr = GetBufferDescriptor(recent_buffer - 1);
+		have_private_ref = GetPrivateRefCount(recent_buffer) > 0;
+
+		/*
+		 * Do we already have this buffer pinned with a private reference?  If
+		 * so, it must be valid and it is safe to check the tag without
+		 * locking.  If not, we have to lock the header first and then check.
+		 */
+		if (have_private_ref)
+			buf_state = pg_atomic_read_u32(&bufHdr->state);
+		else
+			buf_state = LockBufHdr(bufHdr);
+
+		if ((buf_state & BM_VALID) && BUFFERTAGS_EQUAL(tag, bufHdr->tag))
+		{
+			/*
+			 * It's now safe to pin the buffer.  We can't pin first and ask
+			 * questions later, because because it might confuse code paths
+			 * like InvalidateBuffer() if we pinned a random non-matching
+			 * buffer.
+			 */
+			if (have_private_ref)
+				PinBuffer(bufHdr, NULL);	/* bump pin count */
+			else
+				PinBuffer_Locked(bufHdr);	/* pin for first time */
+
+			return true;
+		}
+
+		/* If we locked the header above, now unlock. */
+		if (!have_private_ref)
+			UnlockBufHdr(bufHdr, buf_state);
+	}
+
+	return false;
+}
+
+/*
+ * ReadBuffer -- a shorthand for ReadBufferExtended, for reading from main
+ *		fork with RBM_NORMAL mode and default strategy.
+ */
+Buffer
+ReadBuffer(Relation reln, BlockNumber blockNum)
+{
+	return ReadBufferExtended(reln, MAIN_FORKNUM, blockNum, RBM_NORMAL, NULL);
+}
+
+/*
+ * ReadBufferExtended -- returns a buffer containing the requested
+ *		block of the requested relation.  If the blknum
+ *		requested is P_NEW, extend the relation file and
+ *		allocate a new block.  (Caller is responsible for
+ *		ensuring that only one backend tries to extend a
+ *		relation at the same time!)
+ *
+ * Returns: the buffer number for the buffer containing
+ *		the block read.  The returned buffer has been pinned.
+ *		Does not return on error --- elog's instead.
+ *
+ * Assume when this function is called, that reln has been opened already.
+ *
+ * In RBM_NORMAL mode, the page is read from disk, and the page header is
+ * validated.  An error is thrown if the page header is not valid.  (But
+ * note that an all-zero page is considered "valid"; see
+ * PageIsVerifiedExtended().)
+ *
+ * RBM_ZERO_ON_ERROR is like the normal mode, but if the page header is not
+ * valid, the page is zeroed instead of throwing an error. This is intended
+ * for non-critical data, where the caller is prepared to repair errors.
+ *
+ * In RBM_ZERO_AND_LOCK mode, if the page isn't in buffer cache already, it's
+ * filled with zeros instead of reading it from disk.  Useful when the caller
+ * is going to fill the page from scratch, since this saves I/O and avoids
+ * unnecessary failure if the page-on-disk has corrupt page headers.
+ * The page is returned locked to ensure that the caller has a chance to
+ * initialize the page before it's made visible to others.
+ * Caution: do not use this mode to read a page that is beyond the relation's
+ * current physical EOF; that is likely to cause problems in md.c when
+ * the page is modified and written out. P_NEW is OK, though.
+ *
+ * RBM_ZERO_AND_CLEANUP_LOCK is the same as RBM_ZERO_AND_LOCK, but acquires
+ * a cleanup-strength lock on the page.
+ *
+ * RBM_NORMAL_NO_LOG mode is treated the same as RBM_NORMAL here.
+ *
+ * If strategy is not NULL, a nondefault buffer access strategy is used.
+ * See buffer/README for details.
+ */
+Buffer
+ReadBufferExtended(Relation reln, ForkNumber forkNum, BlockNumber blockNum,
+				   ReadBufferMode mode, BufferAccessStrategy strategy)
+{
+	bool		hit;
+	Buffer		buf;
+
+	/* Open it at the smgr level if not already done */
+	RelationOpenSmgr(reln);
+
+	/*
+	 * Reject attempts to read non-local temporary relations; we would be
+	 * likely to get wrong data since we have no visibility into the owning
+	 * session's local buffers.
+	 */
+	if (RELATION_IS_OTHER_TEMP(reln))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot access temporary tables of other sessions")));
+
+	/*
+	 * Read the buffer, and update pgstat counters to reflect a cache hit or
+	 * miss.
+	 */
+	pgstat_count_buffer_read(reln);
+	buf = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
+							forkNum, blockNum, mode, strategy, &hit);
+	if (hit)
+		pgstat_count_buffer_hit(reln);
+	return buf;
+}
+
+
+/*
+ * ReadBufferWithoutRelcache -- like ReadBufferExtended, but doesn't require
+ *		a relcache entry for the relation.
+ *
+ * NB: At present, this function may only be used on permanent relations, which
+ * is OK, because we only use it during XLOG replay.  If in the future we
+ * want to use it on temporary or unlogged relations, we could pass additional
+ * parameters.
+ */
+Buffer
+ReadBufferWithoutRelcache(RelFileNode rnode, ForkNumber forkNum,
+						  BlockNumber blockNum, ReadBufferMode mode,
+						  BufferAccessStrategy strategy)
+{
+	bool		hit;
+
+	SMgrRelation smgr = smgropen(rnode, InvalidBackendId);
+
+	Assert(InRecovery);
+
+	return ReadBuffer_common(smgr, RELPERSISTENCE_PERMANENT, forkNum, blockNum,
+							 mode, strategy, &hit);
+}
+
+
+/*
+ * ReadBuffer_common -- common logic for all ReadBuffer variants
+ *
+ * *hit is set to true if the request was satisfied from shared buffer cache.
+ */
+static Buffer
+ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
+				  BlockNumber blockNum, ReadBufferMode mode,
+				  BufferAccessStrategy strategy, bool *hit)
+{
+	BufferDesc *bufHdr;
+	Block		bufBlock;
+	bool		found;
+	bool		isExtend;
+	bool		isLocalBuf = SmgrIsTemp(smgr);
+
+	*hit = false;
+
+	/* Make sure we will have room to remember the buffer pin */
+	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+	isExtend = (blockNum == P_NEW);
+
+	TRACE_POSTGRESQL_BUFFER_READ_START(forkNum, blockNum,
+									   smgr->smgr_rnode.node.spcNode,
+									   smgr->smgr_rnode.node.dbNode,
+									   smgr->smgr_rnode.node.relNode,
+									   smgr->smgr_rnode.backend,
+									   isExtend);
+
+	/* Substitute proper block number if caller asked for P_NEW */
+	if (isExtend)
+	{
+		blockNum = smgrnblocks(smgr, forkNum);
+		/* Fail if relation is already at maximum possible length */
+		if (blockNum == P_NEW)
+			ereport(ERROR,
+					(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
+					 errmsg("cannot extend relation %s beyond %u blocks",
+							relpath(smgr->smgr_rnode, forkNum),
+							P_NEW)));
+	}
+
+	if (isLocalBuf)
+	{
+		bufHdr = LocalBufferAlloc(smgr, forkNum, blockNum, &found);
+		if (found)
+			pgBufferUsage.local_blks_hit++;
+		else if (isExtend)
+			pgBufferUsage.local_blks_written++;
+		else if (mode == RBM_NORMAL || mode == RBM_NORMAL_NO_LOG ||
+				 mode == RBM_ZERO_ON_ERROR)
+			pgBufferUsage.local_blks_read++;
+	}
+	else
+	{
+		/*
+		 * lookup the buffer.  IO_IN_PROGRESS is set if the requested block is
+		 * not currently in memory.
+		 */
+		bufHdr = BufferAlloc(smgr, relpersistence, forkNum, blockNum,
+							 strategy, &found);
+		if (found)
+			pgBufferUsage.shared_blks_hit++;
+		else if (isExtend)
+			pgBufferUsage.shared_blks_written++;
+		else if (mode == RBM_NORMAL || mode == RBM_NORMAL_NO_LOG ||
+				 mode == RBM_ZERO_ON_ERROR)
+			pgBufferUsage.shared_blks_read++;
+	}
+
+	/* At this point we do NOT hold any locks. */
+
+	/* if it was already in the buffer pool, we're done */
+	if (found)
+	{
+		if (!isExtend)
+		{
+			/* Just need to update stats before we exit */
+			*hit = true;
+			VacuumPageHit++;
+
+			if (VacuumCostActive)
+				VacuumCostBalance += VacuumCostPageHit;
+
+			TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
+											  smgr->smgr_rnode.node.spcNode,
+											  smgr->smgr_rnode.node.dbNode,
+											  smgr->smgr_rnode.node.relNode,
+											  smgr->smgr_rnode.backend,
+											  isExtend,
+											  found);
+
+			/*
+			 * In RBM_ZERO_AND_LOCK mode the caller expects the page to be
+			 * locked on return.
+			 */
+			if (!isLocalBuf)
+			{
+				if (mode == RBM_ZERO_AND_LOCK)
+					LWLockAcquire(BufferDescriptorGetContentLock(bufHdr),
+								  LW_EXCLUSIVE);
+				else if (mode == RBM_ZERO_AND_CLEANUP_LOCK)
+					LockBufferForCleanup(BufferDescriptorGetBuffer(bufHdr));
+			}
+
+			return BufferDescriptorGetBuffer(bufHdr);
+		}
+
+		/*
+		 * We get here only in the corner case where we are trying to extend
+		 * the relation but we found a pre-existing buffer marked BM_VALID.
+		 * This can happen because mdread doesn't complain about reads beyond
+		 * EOF (when zero_damaged_pages is ON) and so a previous attempt to
+		 * read a block beyond EOF could have left a "valid" zero-filled
+		 * buffer.  Unfortunately, we have also seen this case occurring
+		 * because of buggy Linux kernels that sometimes return an
+		 * lseek(SEEK_END) result that doesn't account for a recent write. In
+		 * that situation, the pre-existing buffer would contain valid data
+		 * that we don't want to overwrite.  Since the legitimate case should
+		 * always have left a zero-filled buffer, complain if not PageIsNew.
+		 */
+		bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
+		if (!PageIsNew((Page) bufBlock))
+			ereport(ERROR,
+					(errmsg("unexpected data beyond EOF in block %u of relation %s",
+							blockNum, relpath(smgr->smgr_rnode, forkNum)),
+					 errhint("This has been seen to occur with buggy kernels; consider updating your system.")));
+
+		/*
+		 * We *must* do smgrextend before succeeding, else the page will not
+		 * be reserved by the kernel, and the next P_NEW call will decide to
+		 * return the same page.  Clear the BM_VALID bit, do the StartBufferIO
+		 * call that BufferAlloc didn't, and proceed.
+		 */
+		if (isLocalBuf)
+		{
+			/* Only need to adjust flags */
+			uint32		buf_state = pg_atomic_read_u32(&bufHdr->state);
+
+			Assert(buf_state & BM_VALID);
+			buf_state &= ~BM_VALID;
+			pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
+		}
+		else
+		{
+			/*
+			 * Loop to handle the very small possibility that someone re-sets
+			 * BM_VALID between our clearing it and StartBufferIO inspecting
+			 * it.
+			 */
+			do
+			{
+				uint32		buf_state = LockBufHdr(bufHdr);
+
+				Assert(buf_state & BM_VALID);
+				buf_state &= ~BM_VALID;
+				UnlockBufHdr(bufHdr, buf_state);
+			} while (!StartBufferIO(bufHdr, true));
+		}
+	}
+
+	/*
+	 * if we have gotten to this point, we have allocated a buffer for the
+	 * page but its contents are not yet valid.  IO_IN_PROGRESS is set for it,
+	 * if it's a shared buffer.
+	 *
+	 * Note: if smgrextend fails, we will end up with a buffer that is
+	 * allocated but not marked BM_VALID.  P_NEW will still select the same
+	 * block number (because the relation didn't get any longer on disk) and
+	 * so future attempts to extend the relation will find the same buffer (if
+	 * it's not been recycled) but come right back here to try smgrextend
+	 * again.
+	 */
+	Assert(!(pg_atomic_read_u32(&bufHdr->state) & BM_VALID));	/* spinlock not needed */
+
+	bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
+
+	if (isExtend)
+	{
+		/* new buffers are zero-filled */
+		MemSet((char *) bufBlock, 0, BLCKSZ);
+		/* don't set checksum for all-zero page */
+		smgrextend(smgr, forkNum, blockNum, (char *) bufBlock, false);
+
+		/*
+		 * NB: we're *not* doing a ScheduleBufferTagForWriteback here;
+		 * although we're essentially performing a write. At least on linux
+		 * doing so defeats the 'delayed allocation' mechanism, leading to
+		 * increased file fragmentation.
+		 */
+	}
+	else
+	{
+		/*
+		 * Read in the page, unless the caller intends to overwrite it and
+		 * just wants us to allocate a buffer.
+		 */
+		if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
+			MemSet((char *) bufBlock, 0, BLCKSZ);
+		else
+		{
+			instr_time	io_start,
+						io_time;
+
+			if (track_io_timing)
+				INSTR_TIME_SET_CURRENT(io_start);
+
+			smgrread(smgr, forkNum, blockNum, (char *) bufBlock);
+
+			if (track_io_timing)
+			{
+				INSTR_TIME_SET_CURRENT(io_time);
+				INSTR_TIME_SUBTRACT(io_time, io_start);
+				pgstat_count_buffer_read_time(INSTR_TIME_GET_MICROSEC(io_time));
+				INSTR_TIME_ADD(pgBufferUsage.blk_read_time, io_time);
+			}
+
+			/* check for garbage data */
+			if (!PageIsVerifiedExtended((Page) bufBlock, blockNum,
+										PIV_LOG_WARNING | PIV_REPORT_STAT))
+			{
+				if (mode == RBM_ZERO_ON_ERROR || zero_damaged_pages)
+				{
+					ereport(WARNING,
+							(errcode(ERRCODE_DATA_CORRUPTED),
+							 errmsg("invalid page in block %u of relation %s; zeroing out page",
+									blockNum,
+									relpath(smgr->smgr_rnode, forkNum))));
+					MemSet((char *) bufBlock, 0, BLCKSZ);
+				}
+				else
+					ereport(ERROR,
+							(errcode(ERRCODE_DATA_CORRUPTED),
+							 errmsg("invalid page in block %u of relation %s",
+									blockNum,
+									relpath(smgr->smgr_rnode, forkNum))));
+			}
+		}
+	}
+
+	/*
+	 * In RBM_ZERO_AND_LOCK mode, grab the buffer content lock before marking
+	 * the page as valid, to make sure that no other backend sees the zeroed
+	 * page before the caller has had a chance to initialize it.
+	 *
+	 * Since no-one else can be looking at the page contents yet, there is no
+	 * difference between an exclusive lock and a cleanup-strength lock. (Note
+	 * that we cannot use LockBuffer() or LockBufferForCleanup() here, because
+	 * they assert that the buffer is already valid.)
+	 */
+	if ((mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK) &&
+		!isLocalBuf)
+	{
+		LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_EXCLUSIVE);
+	}
+
+	if (isLocalBuf)
+	{
+		/* Only need to adjust flags */
+		uint32		buf_state = pg_atomic_read_u32(&bufHdr->state);
+
+		buf_state |= BM_VALID;
+		pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
+	}
+	else
+	{
+		/* Set BM_VALID, terminate IO, and wake up any waiters */
+		TerminateBufferIO(bufHdr, false, BM_VALID);
+	}
+
+	VacuumPageMiss++;
+	if (VacuumCostActive)
+		VacuumCostBalance += VacuumCostPageMiss;
+
+	TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
+									  smgr->smgr_rnode.node.spcNode,
+									  smgr->smgr_rnode.node.dbNode,
+									  smgr->smgr_rnode.node.relNode,
+									  smgr->smgr_rnode.backend,
+									  isExtend,
+									  found);
+
+	return BufferDescriptorGetBuffer(bufHdr);
+}
+
+/*
+ * BufferAlloc -- subroutine for ReadBuffer.  Handles lookup of a shared
+ *		buffer.  If no buffer exists already, selects a replacement
+ *		victim and evicts the old page, but does NOT read in new page.
+ *
+ * "strategy" can be a buffer replacement strategy object, or NULL for
+ * the default strategy.  The selected buffer's usage_count is advanced when
+ * using the default strategy, but otherwise possibly not (see PinBuffer).
+ *
+ * The returned buffer is pinned and is already marked as holding the
+ * desired page.  If it already did have the desired page, *foundPtr is
+ * set true.  Otherwise, *foundPtr is set false and the buffer is marked
+ * as IO_IN_PROGRESS; ReadBuffer will now need to do I/O to fill it.
+ *
+ * *foundPtr is actually redundant with the buffer's BM_VALID flag, but
+ * we keep it for simplicity in ReadBuffer.
+ *
+ * No locks are held either at entry or exit.
+ */
+static BufferDesc *
+BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
+			BlockNumber blockNum,
+			BufferAccessStrategy strategy,
+			bool *foundPtr)
+{
+	BufferTag	newTag;			/* identity of requested block */
+	uint32		newHash;		/* hash value for newTag */
+	LWLock	   *newPartitionLock;	/* buffer partition lock for it */
+	BufferTag	oldTag;			/* previous identity of selected buffer */
+	uint32		oldHash;		/* hash value for oldTag */
+	LWLock	   *oldPartitionLock;	/* buffer partition lock for it */
+	uint32		oldFlags;
+	int			buf_id;
+	BufferDesc *buf;
+	bool		valid;
+	uint32		buf_state;
+
+	/* create a tag so we can lookup the buffer */
+	INIT_BUFFERTAG(newTag, smgr->smgr_rnode.node, forkNum, blockNum);
+
+	/* determine its hash code and partition lock ID */
+	newHash = BufTableHashCode(&newTag);
+	newPartitionLock = BufMappingPartitionLock(newHash);
+
+	/* see if the block is in the buffer pool already */
+	LWLockAcquire(newPartitionLock, LW_SHARED);
+	buf_id = BufTableLookup(&newTag, newHash);
+	if (buf_id >= 0)
+	{
+		/*
+		 * Found it.  Now, pin the buffer so no one can steal it from the
+		 * buffer pool, and check to see if the correct data has been loaded
+		 * into the buffer.
+		 */
+		buf = GetBufferDescriptor(buf_id);
+
+		valid = PinBuffer(buf, strategy);
+
+		/* Can release the mapping lock as soon as we've pinned it */
+		LWLockRelease(newPartitionLock);
+
+		*foundPtr = true;
+
+		if (!valid)
+		{
+			/*
+			 * We can only get here if (a) someone else is still reading in
+			 * the page, or (b) a previous read attempt failed.  We have to
+			 * wait for any active read attempt to finish, and then set up our
+			 * own read attempt if the page is still not BM_VALID.
+			 * StartBufferIO does it all.
+			 */
+			if (StartBufferIO(buf, true))
+			{
+				/*
+				 * If we get here, previous attempts to read the buffer must
+				 * have failed ... but we shall bravely try again.
+				 */
+				*foundPtr = false;
+			}
+		}
+
+		return buf;
+	}
+
+	/*
+	 * Didn't find it in the buffer pool.  We'll have to initialize a new
+	 * buffer.  Remember to unlock the mapping lock while doing the work.
+	 */
+	LWLockRelease(newPartitionLock);
+
+	/* Loop here in case we have to try another victim buffer */
+	for (;;)
+	{
+		/*
+		 * Ensure, while the spinlock's not yet held, that there's a free
+		 * refcount entry.
+		 */
+		ReservePrivateRefCountEntry();
+
+		/*
+		 * Select a victim buffer.  The buffer is returned with its header
+		 * spinlock still held!
+		 */
+		buf = StrategyGetBuffer(strategy, &buf_state);
+
+		Assert(BUF_STATE_GET_REFCOUNT(buf_state) == 0);
+
+		/* Must copy buffer flags while we still hold the spinlock */
+		oldFlags = buf_state & BUF_FLAG_MASK;
+
+		/* Pin the buffer and then release the buffer spinlock */
+		PinBuffer_Locked(buf);
+
+		/*
+		 * If the buffer was dirty, try to write it out.  There is a race
+		 * condition here, in that someone might dirty it after we released it
+		 * above, or even while we are writing it out (since our share-lock
+		 * won't prevent hint-bit updates).  We will recheck the dirty bit
+		 * after re-locking the buffer header.
+		 */
+		if (oldFlags & BM_DIRTY)
+		{
+			/*
+			 * We need a share-lock on the buffer contents to write it out
+			 * (else we might write invalid data, eg because someone else is
+			 * compacting the page contents while we write).  We must use a
+			 * conditional lock acquisition here to avoid deadlock.  Even
+			 * though the buffer was not pinned (and therefore surely not
+			 * locked) when StrategyGetBuffer returned it, someone else could
+			 * have pinned and exclusive-locked it by the time we get here. If
+			 * we try to get the lock unconditionally, we'd block waiting for
+			 * them; if they later block waiting for us, deadlock ensues.
+			 * (This has been observed to happen when two backends are both
+			 * trying to split btree index pages, and the second one just
+			 * happens to be trying to split the page the first one got from
+			 * StrategyGetBuffer.)
+			 */
+			if (LWLockConditionalAcquire(BufferDescriptorGetContentLock(buf),
+										 LW_SHARED))
+			{
+				/*
+				 * If using a nondefault strategy, and writing the buffer
+				 * would require a WAL flush, let the strategy decide whether
+				 * to go ahead and write/reuse the buffer or to choose another
+				 * victim.  We need lock to inspect the page LSN, so this
+				 * can't be done inside StrategyGetBuffer.
+				 */
+				if (strategy != NULL)
+				{
+					XLogRecPtr	lsn;
+
+					/* Read the LSN while holding buffer header lock */
+					buf_state = LockBufHdr(buf);
+					lsn = BufferGetLSN(buf);
+					UnlockBufHdr(buf, buf_state);
+
+					if (XLogNeedsFlush(lsn) &&
+						StrategyRejectBuffer(strategy, buf))
+					{
+						/* Drop lock/pin and loop around for another buffer */
+						LWLockRelease(BufferDescriptorGetContentLock(buf));
+						UnpinBuffer(buf, true);
+						continue;
+					}
+				}
+
+				/* OK, do the I/O */
+				TRACE_POSTGRESQL_BUFFER_WRITE_DIRTY_START(forkNum, blockNum,
+														  smgr->smgr_rnode.node.spcNode,
+														  smgr->smgr_rnode.node.dbNode,
+														  smgr->smgr_rnode.node.relNode);
+
+				FlushBuffer(buf, NULL);
+				LWLockRelease(BufferDescriptorGetContentLock(buf));
+
+				ScheduleBufferTagForWriteback(&BackendWritebackContext,
+											  &buf->tag);
+
+				TRACE_POSTGRESQL_BUFFER_WRITE_DIRTY_DONE(forkNum, blockNum,
+														 smgr->smgr_rnode.node.spcNode,
+														 smgr->smgr_rnode.node.dbNode,
+														 smgr->smgr_rnode.node.relNode);
+			}
+			else
+			{
+				/*
+				 * Someone else has locked the buffer, so give it up and loop
+				 * back to get another one.
+				 */
+				UnpinBuffer(buf, true);
+				continue;
+			}
+		}
+
+		/*
+		 * To change the association of a valid buffer, we'll need to have
+		 * exclusive lock on both the old and new mapping partitions.
+		 */
+		if (oldFlags & BM_TAG_VALID)
+		{
+			/*
+			 * Need to compute the old tag's hashcode and partition lock ID.
+			 * XXX is it worth storing the hashcode in BufferDesc so we need
+			 * not recompute it here?  Probably not.
+			 */
+			oldTag = buf->tag;
+			oldHash = BufTableHashCode(&oldTag);
+			oldPartitionLock = BufMappingPartitionLock(oldHash);
+
+			/*
+			 * Must lock the lower-numbered partition first to avoid
+			 * deadlocks.
+			 */
+			if (oldPartitionLock < newPartitionLock)
+			{
+				LWLockAcquire(oldPartitionLock, LW_EXCLUSIVE);
+				LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
+			}
+			else if (oldPartitionLock > newPartitionLock)
+			{
+				LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
+				LWLockAcquire(oldPartitionLock, LW_EXCLUSIVE);
+			}
+			else
+			{
+				/* only one partition, only one lock */
+				LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
+			}
+		}
+		else
+		{
+			/* if it wasn't valid, we need only the new partition */
+			LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
+			/* remember we have no old-partition lock or tag */
+			oldPartitionLock = NULL;
+			/* keep the compiler quiet about uninitialized variables */
+			oldHash = 0;
+		}
+
+		/*
+		 * Try to make a hashtable entry for the buffer under its new tag.
+		 * This could fail because while we were writing someone else
+		 * allocated another buffer for the same block we want to read in.
+		 * Note that we have not yet removed the hashtable entry for the old
+		 * tag.
+		 */
+		buf_id = BufTableInsert(&newTag, newHash, buf->buf_id);
+
+		if (buf_id >= 0)
+		{
+			/*
+			 * Got a collision. Someone has already done what we were about to
+			 * do. We'll just handle this as if it were found in the buffer
+			 * pool in the first place.  First, give up the buffer we were
+			 * planning to use.
+			 */
+			UnpinBuffer(buf, true);
+
+			/* Can give up that buffer's mapping partition lock now */
+			if (oldPartitionLock != NULL &&
+				oldPartitionLock != newPartitionLock)
+				LWLockRelease(oldPartitionLock);
+
+			/* remaining code should match code at top of routine */
+
+			buf = GetBufferDescriptor(buf_id);
+
+			valid = PinBuffer(buf, strategy);
+
+			/* Can release the mapping lock as soon as we've pinned it */
+			LWLockRelease(newPartitionLock);
+
+			*foundPtr = true;
+
+			if (!valid)
+			{
+				/*
+				 * We can only get here if (a) someone else is still reading
+				 * in the page, or (b) a previous read attempt failed.  We
+				 * have to wait for any active read attempt to finish, and
+				 * then set up our own read attempt if the page is still not
+				 * BM_VALID.  StartBufferIO does it all.
+				 */
+				if (StartBufferIO(buf, true))
+				{
+					/*
+					 * If we get here, previous attempts to read the buffer
+					 * must have failed ... but we shall bravely try again.
+					 */
+					*foundPtr = false;
+				}
+			}
+
+			return buf;
+		}
+
+		/*
+		 * Need to lock the buffer header too in order to change its tag.
+		 */
+		buf_state = LockBufHdr(buf);
+
+		/*
+		 * Somebody could have pinned or re-dirtied the buffer while we were
+		 * doing the I/O and making the new hashtable entry.  If so, we can't
+		 * recycle this buffer; we must undo everything we've done and start
+		 * over with a new victim buffer.
+		 */
+		oldFlags = buf_state & BUF_FLAG_MASK;
+		if (BUF_STATE_GET_REFCOUNT(buf_state) == 1 && !(oldFlags & BM_DIRTY))
+			break;
+
+		UnlockBufHdr(buf, buf_state);
+		BufTableDelete(&newTag, newHash);
+		if (oldPartitionLock != NULL &&
+			oldPartitionLock != newPartitionLock)
+			LWLockRelease(oldPartitionLock);
+		LWLockRelease(newPartitionLock);
+		UnpinBuffer(buf, true);
+	}
+
+	/*
+	 * Okay, it's finally safe to rename the buffer.
+	 *
+	 * Clearing BM_VALID here is necessary, clearing the dirtybits is just
+	 * paranoia.  We also reset the usage_count since any recency of use of
+	 * the old content is no longer relevant.  (The usage_count starts out at
+	 * 1 so that the buffer can survive one clock-sweep pass.)
+	 *
+	 * Make sure BM_PERMANENT is set for buffers that must be written at every
+	 * checkpoint.  Unlogged buffers only need to be written at shutdown
+	 * checkpoints, except for their "init" forks, which need to be treated
+	 * just like permanent relations.
+	 */
+	buf->tag = newTag;
+	buf_state &= ~(BM_VALID | BM_DIRTY | BM_JUST_DIRTIED |
+				   BM_CHECKPOINT_NEEDED | BM_IO_ERROR | BM_PERMANENT |
+				   BUF_USAGECOUNT_MASK);
+	if (relpersistence == RELPERSISTENCE_PERMANENT || forkNum == INIT_FORKNUM)
+		buf_state |= BM_TAG_VALID | BM_PERMANENT | BUF_USAGECOUNT_ONE;
+	else
+		buf_state |= BM_TAG_VALID | BUF_USAGECOUNT_ONE;
+
+	UnlockBufHdr(buf, buf_state);
+
+	if (oldPartitionLock != NULL)
+	{
+		BufTableDelete(&oldTag, oldHash);
+		if (oldPartitionLock != newPartitionLock)
+			LWLockRelease(oldPartitionLock);
+	}
+
+	LWLockRelease(newPartitionLock);
+
+	/*
+	 * Buffer contents are currently invalid.  Try to obtain the right to
+	 * start I/O.  If StartBufferIO returns false, then someone else managed
+	 * to read it before we did, so there's nothing left for BufferAlloc() to
+	 * do.
+	 */
+	if (StartBufferIO(buf, true))
+		*foundPtr = false;
+	else
+		*foundPtr = true;
+
+	return buf;
+}
+
+/*
+ * InvalidateBuffer -- mark a shared buffer invalid and return it to the
+ * freelist.
+ *
+ * The buffer header spinlock must be held at entry.  We drop it before
+ * returning.  (This is sane because the caller must have locked the
+ * buffer in order to be sure it should be dropped.)
+ *
+ * This is used only in contexts such as dropping a relation.  We assume
+ * that no other backend could possibly be interested in using the page,
+ * so the only reason the buffer might be pinned is if someone else is
+ * trying to write it out.  We have to let them finish before we can
+ * reclaim the buffer.
+ *
+ * The buffer could get reclaimed by someone else while we are waiting
+ * to acquire the necessary locks; if so, don't mess it up.
+ */
+static void
+InvalidateBuffer(BufferDesc *buf)
+{
+	BufferTag	oldTag;
+	uint32		oldHash;		/* hash value for oldTag */
+	LWLock	   *oldPartitionLock;	/* buffer partition lock for it */
+	uint32		oldFlags;
+	uint32		buf_state;
+
+	/* Save the original buffer tag before dropping the spinlock */
+	oldTag = buf->tag;
+
+	buf_state = pg_atomic_read_u32(&buf->state);
+	Assert(buf_state & BM_LOCKED);
+	UnlockBufHdr(buf, buf_state);
+
+	/*
+	 * Need to compute the old tag's hashcode and partition lock ID. XXX is it
+	 * worth storing the hashcode in BufferDesc so we need not recompute it
+	 * here?  Probably not.
+	 */
+	oldHash = BufTableHashCode(&oldTag);
+	oldPartitionLock = BufMappingPartitionLock(oldHash);
+
+retry:
+
+	/*
+	 * Acquire exclusive mapping lock in preparation for changing the buffer's
+	 * association.
+	 */
+	LWLockAcquire(oldPartitionLock, LW_EXCLUSIVE);
+
+	/* Re-lock the buffer header */
+	buf_state = LockBufHdr(buf);
+
+	/* If it's changed while we were waiting for lock, do nothing */
+	if (!BUFFERTAGS_EQUAL(buf->tag, oldTag))
+	{
+		UnlockBufHdr(buf, buf_state);
+		LWLockRelease(oldPartitionLock);
+		return;
+	}
+
+	/*
+	 * We assume the only reason for it to be pinned is that someone else is
+	 * flushing the page out.  Wait for them to finish.  (This could be an
+	 * infinite loop if the refcount is messed up... it would be nice to time
+	 * out after awhile, but there seems no way to be sure how many loops may
+	 * be needed.  Note that if the other guy has pinned the buffer but not
+	 * yet done StartBufferIO, WaitIO will fall through and we'll effectively
+	 * be busy-looping here.)
+	 */
+	if (BUF_STATE_GET_REFCOUNT(buf_state) != 0)
+	{
+		UnlockBufHdr(buf, buf_state);
+		LWLockRelease(oldPartitionLock);
+		/* safety check: should definitely not be our *own* pin */
+		if (GetPrivateRefCount(BufferDescriptorGetBuffer(buf)) > 0)
+			elog(ERROR, "buffer is pinned in InvalidateBuffer");
+		WaitIO(buf);
+		goto retry;
+	}
+
+	/*
+	 * Clear out the buffer's tag and flags.  We must do this to ensure that
+	 * linear scans of the buffer array don't think the buffer is valid.
+	 */
+	oldFlags = buf_state & BUF_FLAG_MASK;
+	CLEAR_BUFFERTAG(buf->tag);
+	buf_state &= ~(BUF_FLAG_MASK | BUF_USAGECOUNT_MASK);
+	UnlockBufHdr(buf, buf_state);
+
+	/*
+	 * Remove the buffer from the lookup hashtable, if it was in there.
+	 */
+	if (oldFlags & BM_TAG_VALID)
+		BufTableDelete(&oldTag, oldHash);
+
+	/*
+	 * Done with mapping lock.
+	 */
+	LWLockRelease(oldPartitionLock);
+
+	/*
+	 * Insert the buffer at the head of the list of free buffers.
+	 */
+	StrategyFreeBuffer(buf);
+}
+
+/*
+ * MarkBufferDirty
+ *
+ *		Marks buffer contents as dirty (actual write happens later).
+ *
+ * Buffer must be pinned and exclusive-locked.  (If caller does not hold
+ * exclusive lock, then somebody could be in process of writing the buffer,
+ * leading to risk of bad data written to disk.)
+ */
+void
+MarkBufferDirty(Buffer buffer)
+{
+	BufferDesc *bufHdr;
+	uint32		buf_state;
+	uint32		old_buf_state;
+
+	if (!BufferIsValid(buffer))
+		elog(ERROR, "bad buffer ID: %d", buffer);
+
+	if (BufferIsLocal(buffer))
+	{
+		MarkLocalBufferDirty(buffer);
+		return;
+	}
+
+	bufHdr = GetBufferDescriptor(buffer - 1);
+
+	Assert(BufferIsPinned(buffer));
+	Assert(LWLockHeldByMeInMode(BufferDescriptorGetContentLock(bufHdr),
+								LW_EXCLUSIVE));
+
+	old_buf_state = pg_atomic_read_u32(&bufHdr->state);
+	for (;;)
+	{
+		if (old_buf_state & BM_LOCKED)
+			old_buf_state = WaitBufHdrUnlocked(bufHdr);
+
+		buf_state = old_buf_state;
+
+		Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
+		buf_state |= BM_DIRTY | BM_JUST_DIRTIED;
+
+		if (pg_atomic_compare_exchange_u32(&bufHdr->state, &old_buf_state,
+										   buf_state))
+			break;
+	}
+
+	/*
+	 * If the buffer was not dirty already, do vacuum accounting.
+	 */
+	if (!(old_buf_state & BM_DIRTY))
+	{
+		VacuumPageDirty++;
+		pgBufferUsage.shared_blks_dirtied++;
+		if (VacuumCostActive)
+			VacuumCostBalance += VacuumCostPageDirty;
+	}
+}
+
+/*
+ * ReleaseAndReadBuffer -- combine ReleaseBuffer() and ReadBuffer()
+ *
+ * Formerly, this saved one cycle of acquiring/releasing the BufMgrLock
+ * compared to calling the two routines separately.  Now it's mainly just
+ * a convenience function.  However, if the passed buffer is valid and
+ * already contains the desired block, we just return it as-is; and that
+ * does save considerable work compared to a full release and reacquire.
+ *
+ * Note: it is OK to pass buffer == InvalidBuffer, indicating that no old
+ * buffer actually needs to be released.  This case is the same as ReadBuffer,
+ * but can save some tests in the caller.
+ */
+Buffer
+ReleaseAndReadBuffer(Buffer buffer,
+					 Relation relation,
+					 BlockNumber blockNum)
+{
+	ForkNumber	forkNum = MAIN_FORKNUM;
+	BufferDesc *bufHdr;
+
+	if (BufferIsValid(buffer))
+	{
+		Assert(BufferIsPinned(buffer));
+		if (BufferIsLocal(buffer))
+		{
+			bufHdr = GetLocalBufferDescriptor(-buffer - 1);
+			if (bufHdr->tag.blockNum == blockNum &&
+				RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
+				bufHdr->tag.forkNum == forkNum)
+				return buffer;
+			ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
+			LocalRefCount[-buffer - 1]--;
+		}
+		else
+		{
+			bufHdr = GetBufferDescriptor(buffer - 1);
+			/* we have pin, so it's ok to examine tag without spinlock */
+			if (bufHdr->tag.blockNum == blockNum &&
+				RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
+				bufHdr->tag.forkNum == forkNum)
+				return buffer;
+			UnpinBuffer(bufHdr, true);
+		}
+	}
+
+	return ReadBuffer(relation, blockNum);
+}
+
+/*
+ * PinBuffer -- make buffer unavailable for replacement.
+ *
+ * For the default access strategy, the buffer's usage_count is incremented
+ * when we first pin it; for other strategies we just make sure the usage_count
+ * isn't zero.  (The idea of the latter is that we don't want synchronized
+ * heap scans to inflate the count, but we need it to not be zero to discourage
+ * other backends from stealing buffers from our ring.  As long as we cycle
+ * through the ring faster than the global clock-sweep cycles, buffers in
+ * our ring won't be chosen as victims for replacement by other backends.)
+ *
+ * This should be applied only to shared buffers, never local ones.
+ *
+ * Since buffers are pinned/unpinned very frequently, pin buffers without
+ * taking the buffer header lock; instead update the state variable in loop of
+ * CAS operations. Hopefully it's just a single CAS.
+ *
+ * Note that ResourceOwnerEnlargeBuffers must have been done already.
+ *
+ * Returns true if buffer is BM_VALID, else false.  This provision allows
+ * some callers to avoid an extra spinlock cycle.
+ */
+static bool
+PinBuffer(BufferDesc *buf, BufferAccessStrategy strategy)
+{
+	Buffer		b = BufferDescriptorGetBuffer(buf);
+	bool		result;
+	PrivateRefCountEntry *ref;
+
+	ref = GetPrivateRefCountEntry(b, true);
+
+	if (ref == NULL)
+	{
+		uint32		buf_state;
+		uint32		old_buf_state;
+
+		ReservePrivateRefCountEntry();
+		ref = NewPrivateRefCountEntry(b);
+
+		old_buf_state = pg_atomic_read_u32(&buf->state);
+		for (;;)
+		{
+			if (old_buf_state & BM_LOCKED)
+				old_buf_state = WaitBufHdrUnlocked(buf);
+
+			buf_state = old_buf_state;
+
+			/* increase refcount */
+			buf_state += BUF_REFCOUNT_ONE;
+
+			if (strategy == NULL)
+			{
+				/* Default case: increase usagecount unless already max. */
+				if (BUF_STATE_GET_USAGECOUNT(buf_state) < BM_MAX_USAGE_COUNT)
+					buf_state += BUF_USAGECOUNT_ONE;
+			}
+			else
+			{
+				/*
+				 * Ring buffers shouldn't evict others from pool.  Thus we
+				 * don't make usagecount more than 1.
+				 */
+				if (BUF_STATE_GET_USAGECOUNT(buf_state) == 0)
+					buf_state += BUF_USAGECOUNT_ONE;
+			}
+
+			if (pg_atomic_compare_exchange_u32(&buf->state, &old_buf_state,
+											   buf_state))
+			{
+				result = (buf_state & BM_VALID) != 0;
+
+				/*
+				 * Assume that we acquired a buffer pin for the purposes of
+				 * Valgrind buffer client checks (even in !result case) to
+				 * keep things simple.  Buffers that are unsafe to access are
+				 * not generally guaranteed to be marked undefined or
+				 * non-accessible in any case.
+				 */
+				VALGRIND_MAKE_MEM_DEFINED(BufHdrGetBlock(buf), BLCKSZ);
+				break;
+			}
+		}
+	}
+	else
+	{
+		/*
+		 * If we previously pinned the buffer, it must surely be valid.
+		 *
+		 * Note: We deliberately avoid a Valgrind client request here.
+		 * Individual access methods can optionally superimpose buffer page
+		 * client requests on top of our client requests to enforce that
+		 * buffers are only accessed while locked (and pinned).  It's possible
+		 * that the buffer page is legitimately non-accessible here.  We
+		 * cannot meddle with that.
+		 */
+		result = true;
+	}
+
+	ref->refcount++;
+	Assert(ref->refcount > 0);
+	ResourceOwnerRememberBuffer(CurrentResourceOwner, b);
+	return result;
+}
+
+/*
+ * PinBuffer_Locked -- as above, but caller already locked the buffer header.
+ * The spinlock is released before return.
+ *
+ * As this function is called with the spinlock held, the caller has to
+ * previously call ReservePrivateRefCountEntry().
+ *
+ * Currently, no callers of this function want to modify the buffer's
+ * usage_count at all, so there's no need for a strategy parameter.
+ * Also we don't bother with a BM_VALID test (the caller could check that for
+ * itself).
+ *
+ * Also all callers only ever use this function when it's known that the
+ * buffer can't have a preexisting pin by this backend. That allows us to skip
+ * searching the private refcount array & hash, which is a boon, because the
+ * spinlock is still held.
+ *
+ * Note: use of this routine is frequently mandatory, not just an optimization
+ * to save a spin lock/unlock cycle, because we need to pin a buffer before
+ * its state can change under us.
+ */
+static void
+PinBuffer_Locked(BufferDesc *buf)
+{
+	Buffer		b;
+	PrivateRefCountEntry *ref;
+	uint32		buf_state;
+
+	/*
+	 * As explained, We don't expect any preexisting pins. That allows us to
+	 * manipulate the PrivateRefCount after releasing the spinlock
+	 */
+	Assert(GetPrivateRefCountEntry(BufferDescriptorGetBuffer(buf), false) == NULL);
+
+	/*
+	 * Buffer can't have a preexisting pin, so mark its page as defined to
+	 * Valgrind (this is similar to the PinBuffer() case where the backend
+	 * doesn't already have a buffer pin)
+	 */
+	VALGRIND_MAKE_MEM_DEFINED(BufHdrGetBlock(buf), BLCKSZ);
+
+	/*
+	 * Since we hold the buffer spinlock, we can update the buffer state and
+	 * release the lock in one operation.
+	 */
+	buf_state = pg_atomic_read_u32(&buf->state);
+	Assert(buf_state & BM_LOCKED);
+	buf_state += BUF_REFCOUNT_ONE;
+	UnlockBufHdr(buf, buf_state);
+
+	b = BufferDescriptorGetBuffer(buf);
+
+	ref = NewPrivateRefCountEntry(b);
+	ref->refcount++;
+
+	ResourceOwnerRememberBuffer(CurrentResourceOwner, b);
+}
+
+/*
+ * UnpinBuffer -- make buffer available for replacement.
+ *
+ * This should be applied only to shared buffers, never local ones.
+ *
+ * Most but not all callers want CurrentResourceOwner to be adjusted.
+ * Those that don't should pass fixOwner = false.
+ */
+static void
+UnpinBuffer(BufferDesc *buf, bool fixOwner)
+{
+	PrivateRefCountEntry *ref;
+	Buffer		b = BufferDescriptorGetBuffer(buf);
+
+	/* not moving as we're likely deleting it soon anyway */
+	ref = GetPrivateRefCountEntry(b, false);
+	Assert(ref != NULL);
+
+	if (fixOwner)
+		ResourceOwnerForgetBuffer(CurrentResourceOwner, b);
+
+	Assert(ref->refcount > 0);
+	ref->refcount--;
+	if (ref->refcount == 0)
+	{
+		uint32		buf_state;
+		uint32		old_buf_state;
+
+		/*
+		 * Mark buffer non-accessible to Valgrind.
+		 *
+		 * Note that the buffer may have already been marked non-accessible
+		 * within access method code that enforces that buffers are only
+		 * accessed while a buffer lock is held.
+		 */
+		VALGRIND_MAKE_MEM_NOACCESS(BufHdrGetBlock(buf), BLCKSZ);
+
+		/* I'd better not still hold the buffer content lock */
+		Assert(!LWLockHeldByMe(BufferDescriptorGetContentLock(buf)));
+
+		/*
+		 * Decrement the shared reference count.
+		 *
+		 * Since buffer spinlock holder can update status using just write,
+		 * it's not safe to use atomic decrement here; thus use a CAS loop.
+		 */
+		old_buf_state = pg_atomic_read_u32(&buf->state);
+		for (;;)
+		{
+			if (old_buf_state & BM_LOCKED)
+				old_buf_state = WaitBufHdrUnlocked(buf);
+
+			buf_state = old_buf_state;
+
+			buf_state -= BUF_REFCOUNT_ONE;
+
+			if (pg_atomic_compare_exchange_u32(&buf->state, &old_buf_state,
+											   buf_state))
+				break;
+		}
+
+		/* Support LockBufferForCleanup() */
+		if (buf_state & BM_PIN_COUNT_WAITER)
+		{
+			/*
+			 * Acquire the buffer header lock, re-check that there's a waiter.
+			 * Another backend could have unpinned this buffer, and already
+			 * woken up the waiter.  There's no danger of the buffer being
+			 * replaced after we unpinned it above, as it's pinned by the
+			 * waiter.
+			 */
+			buf_state = LockBufHdr(buf);
+
+			if ((buf_state & BM_PIN_COUNT_WAITER) &&
+				BUF_STATE_GET_REFCOUNT(buf_state) == 1)
+			{
+				/* we just released the last pin other than the waiter's */
+				int			wait_backend_pid = buf->wait_backend_pid;
+
+				buf_state &= ~BM_PIN_COUNT_WAITER;
+				UnlockBufHdr(buf, buf_state);
+				ProcSendSignal(wait_backend_pid);
+			}
+			else
+				UnlockBufHdr(buf, buf_state);
+		}
+		ForgetPrivateRefCountEntry(ref);
+	}
+}
+
+#define ST_SORT sort_checkpoint_bufferids
+#define ST_ELEMENT_TYPE CkptSortItem
+#define ST_COMPARE(a, b) ckpt_buforder_comparator(a, b)
+#define ST_SCOPE static
+#define ST_DEFINE
+#include <lib/sort_template.h>
+
+/*
+ * BufferSync -- Write out all dirty buffers in the pool.
+ *
+ * This is called at checkpoint time to write out all dirty shared buffers.
+ * The checkpoint request flags should be passed in.  If CHECKPOINT_IMMEDIATE
+ * is set, we disable delays between writes; if CHECKPOINT_IS_SHUTDOWN,
+ * CHECKPOINT_END_OF_RECOVERY or CHECKPOINT_FLUSH_ALL is set, we write even
+ * unlogged buffers, which are otherwise skipped.  The remaining flags
+ * currently have no effect here.
+ */
+static void
+BufferSync(int flags)
+{
+	uint32		buf_state;
+	int			buf_id;
+	int			num_to_scan;
+	int			num_spaces;
+	int			num_processed;
+	int			num_written;
+	CkptTsStatus *per_ts_stat = NULL;
+	Oid			last_tsid;
+	binaryheap *ts_heap;
+	int			i;
+	int			mask = BM_DIRTY;
+	WritebackContext wb_context;
+
+	/* Make sure we can handle the pin inside SyncOneBuffer */
+	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+	/*
+	 * Unless this is a shutdown checkpoint or we have been explicitly told,
+	 * we write only permanent, dirty buffers.  But at shutdown or end of
+	 * recovery, we write all dirty buffers.
+	 */
+	if (!((flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY |
+					CHECKPOINT_FLUSH_ALL))))
+		mask |= BM_PERMANENT;
+
+	/*
+	 * Loop over all buffers, and mark the ones that need to be written with
+	 * BM_CHECKPOINT_NEEDED.  Count them as we go (num_to_scan), so that we
+	 * can estimate how much work needs to be done.
+	 *
+	 * This allows us to write only those pages that were dirty when the
+	 * checkpoint began, and not those that get dirtied while it proceeds.
+	 * Whenever a page with BM_CHECKPOINT_NEEDED is written out, either by us
+	 * later in this function, or by normal backends or the bgwriter cleaning
+	 * scan, the flag is cleared.  Any buffer dirtied after this point won't
+	 * have the flag set.
+	 *
+	 * Note that if we fail to write some buffer, we may leave buffers with
+	 * BM_CHECKPOINT_NEEDED still set.  This is OK since any such buffer would
+	 * certainly need to be written for the next checkpoint attempt, too.
+	 */
+	num_to_scan = 0;
+	for (buf_id = 0; buf_id < NBuffers; buf_id++)
+	{
+		BufferDesc *bufHdr = GetBufferDescriptor(buf_id);
+
+		/*
+		 * Header spinlock is enough to examine BM_DIRTY, see comment in
+		 * SyncOneBuffer.
+		 */
+		buf_state = LockBufHdr(bufHdr);
+
+		if ((buf_state & mask) == mask)
+		{
+			CkptSortItem *item;
+
+			buf_state |= BM_CHECKPOINT_NEEDED;
+
+			item = &CkptBufferIds[num_to_scan++];
+			item->buf_id = buf_id;
+			item->tsId = bufHdr->tag.rnode.spcNode;
+			item->relNode = bufHdr->tag.rnode.relNode;
+			item->forkNum = bufHdr->tag.forkNum;
+			item->blockNum = bufHdr->tag.blockNum;
+		}
+
+		UnlockBufHdr(bufHdr, buf_state);
+
+		/* Check for barrier events in case NBuffers is large. */
+		if (ProcSignalBarrierPending)
+			ProcessProcSignalBarrier();
+	}
+
+	if (num_to_scan == 0)
+		return;					/* nothing to do */
+
+	WritebackContextInit(&wb_context, &checkpoint_flush_after);
+
+	TRACE_POSTGRESQL_BUFFER_SYNC_START(NBuffers, num_to_scan);
+
+	/*
+	 * Sort buffers that need to be written to reduce the likelihood of random
+	 * IO. The sorting is also important for the implementation of balancing
+	 * writes between tablespaces. Without balancing writes we'd potentially
+	 * end up writing to the tablespaces one-by-one; possibly overloading the
+	 * underlying system.
+	 */
+	sort_checkpoint_bufferids(CkptBufferIds, num_to_scan);
+
+	num_spaces = 0;
+
+	/*
+	 * Allocate progress status for each tablespace with buffers that need to
+	 * be flushed. This requires the to-be-flushed array to be sorted.
+	 */
+	last_tsid = InvalidOid;
+	for (i = 0; i < num_to_scan; i++)
+	{
+		CkptTsStatus *s;
+		Oid			cur_tsid;
+
+		cur_tsid = CkptBufferIds[i].tsId;
+
+		/*
+		 * Grow array of per-tablespace status structs, every time a new
+		 * tablespace is found.
+		 */
+		if (last_tsid == InvalidOid || last_tsid != cur_tsid)
+		{
+			Size		sz;
+
+			num_spaces++;
+
+			/*
+			 * Not worth adding grow-by-power-of-2 logic here - even with a
+			 * few hundred tablespaces this should be fine.
+			 */
+			sz = sizeof(CkptTsStatus) * num_spaces;
+
+			if (per_ts_stat == NULL)
+				per_ts_stat = (CkptTsStatus *) palloc(sz);
+			else
+				per_ts_stat = (CkptTsStatus *) repalloc(per_ts_stat, sz);
+
+			s = &per_ts_stat[num_spaces - 1];
+			memset(s, 0, sizeof(*s));
+			s->tsId = cur_tsid;
+
+			/*
+			 * The first buffer in this tablespace. As CkptBufferIds is sorted
+			 * by tablespace all (s->num_to_scan) buffers in this tablespace
+			 * will follow afterwards.
+			 */
+			s->index = i;
+
+			/*
+			 * progress_slice will be determined once we know how many buffers
+			 * are in each tablespace, i.e. after this loop.
+			 */
+
+			last_tsid = cur_tsid;
+		}
+		else
+		{
+			s = &per_ts_stat[num_spaces - 1];
+		}
+
+		s->num_to_scan++;
+
+		/* Check for barrier events. */
+		if (ProcSignalBarrierPending)
+			ProcessProcSignalBarrier();
+	}
+
+	Assert(num_spaces > 0);
+
+	/*
+	 * Build a min-heap over the write-progress in the individual tablespaces,
+	 * and compute how large a portion of the total progress a single
+	 * processed buffer is.
+	 */
+	ts_heap = binaryheap_allocate(num_spaces,
+								  ts_ckpt_progress_comparator,
+								  NULL);
+
+	for (i = 0; i < num_spaces; i++)
+	{
+		CkptTsStatus *ts_stat = &per_ts_stat[i];
+
+		ts_stat->progress_slice = (float8) num_to_scan / ts_stat->num_to_scan;
+
+		binaryheap_add_unordered(ts_heap, PointerGetDatum(ts_stat));
+	}
+
+	binaryheap_build(ts_heap);
+
+	/*
+	 * Iterate through to-be-checkpointed buffers and write the ones (still)
+	 * marked with BM_CHECKPOINT_NEEDED. The writes are balanced between
+	 * tablespaces; otherwise the sorting would lead to only one tablespace
+	 * receiving writes at a time, making inefficient use of the hardware.
+	 */
+	num_processed = 0;
+	num_written = 0;
+	while (!binaryheap_empty(ts_heap))
+	{
+		BufferDesc *bufHdr = NULL;
+		CkptTsStatus *ts_stat = (CkptTsStatus *)
+		DatumGetPointer(binaryheap_first(ts_heap));
+
+		buf_id = CkptBufferIds[ts_stat->index].buf_id;
+		Assert(buf_id != -1);
+
+		bufHdr = GetBufferDescriptor(buf_id);
+
+		num_processed++;
+
+		/*
+		 * We don't need to acquire the lock here, because we're only looking
+		 * at a single bit. It's possible that someone else writes the buffer
+		 * and clears the flag right after we check, but that doesn't matter
+		 * since SyncOneBuffer will then do nothing.  However, there is a
+		 * further race condition: it's conceivable that between the time we
+		 * examine the bit here and the time SyncOneBuffer acquires the lock,
+		 * someone else not only wrote the buffer but replaced it with another
+		 * page and dirtied it.  In that improbable case, SyncOneBuffer will
+		 * write the buffer though we didn't need to.  It doesn't seem worth
+		 * guarding against this, though.
+		 */
+		if (pg_atomic_read_u32(&bufHdr->state) & BM_CHECKPOINT_NEEDED)
+		{
+			if (SyncOneBuffer(buf_id, false, &wb_context) & BUF_WRITTEN)
+			{
+				TRACE_POSTGRESQL_BUFFER_SYNC_WRITTEN(buf_id);
+				BgWriterStats.m_buf_written_checkpoints++;
+				num_written++;
+			}
+		}
+
+		/*
+		 * Measure progress independent of actually having to flush the buffer
+		 * - otherwise writing become unbalanced.
+		 */
+		ts_stat->progress += ts_stat->progress_slice;
+		ts_stat->num_scanned++;
+		ts_stat->index++;
+
+		/* Have all the buffers from the tablespace been processed? */
+		if (ts_stat->num_scanned == ts_stat->num_to_scan)
+		{
+			binaryheap_remove_first(ts_heap);
+		}
+		else
+		{
+			/* update heap with the new progress */
+			binaryheap_replace_first(ts_heap, PointerGetDatum(ts_stat));
+		}
+
+		/*
+		 * Sleep to throttle our I/O rate.
+		 *
+		 * (This will check for barrier events even if it doesn't sleep.)
+		 */
+		CheckpointWriteDelay(flags, (double) num_processed / num_to_scan);
+	}
+
+	/* issue all pending flushes */
+	IssuePendingWritebacks(&wb_context);
+
+	pfree(per_ts_stat);
+	per_ts_stat = NULL;
+	binaryheap_free(ts_heap);
+
+	/*
+	 * Update checkpoint statistics. As noted above, this doesn't include
+	 * buffers written by other backends or bgwriter scan.
+	 */
+	CheckpointStats.ckpt_bufs_written += num_written;
+
+	TRACE_POSTGRESQL_BUFFER_SYNC_DONE(NBuffers, num_written, num_to_scan);
+}
+
+/*
+ * BgBufferSync -- Write out some dirty buffers in the pool.
+ *
+ * This is called periodically by the background writer process.
+ *
+ * Returns true if it's appropriate for the bgwriter process to go into
+ * low-power hibernation mode.  (This happens if the strategy clock sweep
+ * has been "lapped" and no buffer allocations have occurred recently,
+ * or if the bgwriter has been effectively disabled by setting
+ * bgwriter_lru_maxpages to 0.)
+ */
+bool
+BgBufferSync(WritebackContext *wb_context)
+{
+	/* info obtained from freelist.c */
+	int			strategy_buf_id;
+	uint32		strategy_passes;
+	uint32		recent_alloc;
+
+	/*
+	 * Information saved between calls so we can determine the strategy
+	 * point's advance rate and avoid scanning already-cleaned buffers.
+	 */
+	static bool saved_info_valid = false;
+	static int	prev_strategy_buf_id;
+	static uint32 prev_strategy_passes;
+	static int	next_to_clean;
+	static uint32 next_passes;
+
+	/* Moving averages of allocation rate and clean-buffer density */
+	static float smoothed_alloc = 0;
+	static float smoothed_density = 10.0;
+
+	/* Potentially these could be tunables, but for now, not */
+	float		smoothing_samples = 16;
+	float		scan_whole_pool_milliseconds = 120000.0;
+
+	/* Used to compute how far we scan ahead */
+	long		strategy_delta;
+	int			bufs_to_lap;
+	int			bufs_ahead;
+	float		scans_per_alloc;
+	int			reusable_buffers_est;
+	int			upcoming_alloc_est;
+	int			min_scan_buffers;
+
+	/* Variables for the scanning loop proper */
+	int			num_to_scan;
+	int			num_written;
+	int			reusable_buffers;
+
+	/* Variables for final smoothed_density update */
+	long		new_strategy_delta;
+	uint32		new_recent_alloc;
+
+	/*
+	 * Find out where the freelist clock sweep currently is, and how many
+	 * buffer allocations have happened since our last call.
+	 */
+	strategy_buf_id = StrategySyncStart(&strategy_passes, &recent_alloc);
+
+	/* Report buffer alloc counts to pgstat */
+	BgWriterStats.m_buf_alloc += recent_alloc;
+
+	/*
+	 * If we're not running the LRU scan, just stop after doing the stats
+	 * stuff.  We mark the saved state invalid so that we can recover sanely
+	 * if LRU scan is turned back on later.
+	 */
+	if (bgwriter_lru_maxpages <= 0)
+	{
+		saved_info_valid = false;
+		return true;
+	}
+
+	/*
+	 * Compute strategy_delta = how many buffers have been scanned by the
+	 * clock sweep since last time.  If first time through, assume none. Then
+	 * see if we are still ahead of the clock sweep, and if so, how many
+	 * buffers we could scan before we'd catch up with it and "lap" it. Note:
+	 * weird-looking coding of xxx_passes comparisons are to avoid bogus
+	 * behavior when the passes counts wrap around.
+	 */
+	if (saved_info_valid)
+	{
+		int32		passes_delta = strategy_passes - prev_strategy_passes;
+
+		strategy_delta = strategy_buf_id - prev_strategy_buf_id;
+		strategy_delta += (long) passes_delta * NBuffers;
+
+		Assert(strategy_delta >= 0);
+
+		if ((int32) (next_passes - strategy_passes) > 0)
+		{
+			/* we're one pass ahead of the strategy point */
+			bufs_to_lap = strategy_buf_id - next_to_clean;
+#ifdef BGW_DEBUG
+			elog(DEBUG2, "bgwriter ahead: bgw %u-%u strategy %u-%u delta=%ld lap=%d",
+				 next_passes, next_to_clean,
+				 strategy_passes, strategy_buf_id,
+				 strategy_delta, bufs_to_lap);
+#endif
+		}
+		else if (next_passes == strategy_passes &&
+				 next_to_clean >= strategy_buf_id)
+		{
+			/* on same pass, but ahead or at least not behind */
+			bufs_to_lap = NBuffers - (next_to_clean - strategy_buf_id);
+#ifdef BGW_DEBUG
+			elog(DEBUG2, "bgwriter ahead: bgw %u-%u strategy %u-%u delta=%ld lap=%d",
+				 next_passes, next_to_clean,
+				 strategy_passes, strategy_buf_id,
+				 strategy_delta, bufs_to_lap);
+#endif
+		}
+		else
+		{
+			/*
+			 * We're behind, so skip forward to the strategy point and start
+			 * cleaning from there.
+			 */
+#ifdef BGW_DEBUG
+			elog(DEBUG2, "bgwriter behind: bgw %u-%u strategy %u-%u delta=%ld",
+				 next_passes, next_to_clean,
+				 strategy_passes, strategy_buf_id,
+				 strategy_delta);
+#endif
+			next_to_clean = strategy_buf_id;
+			next_passes = strategy_passes;
+			bufs_to_lap = NBuffers;
+		}
+	}
+	else
+	{
+		/*
+		 * Initializing at startup or after LRU scanning had been off. Always
+		 * start at the strategy point.
+		 */
+#ifdef BGW_DEBUG
+		elog(DEBUG2, "bgwriter initializing: strategy %u-%u",
+			 strategy_passes, strategy_buf_id);
+#endif
+		strategy_delta = 0;
+		next_to_clean = strategy_buf_id;
+		next_passes = strategy_passes;
+		bufs_to_lap = NBuffers;
+	}
+
+	/* Update saved info for next time */
+	prev_strategy_buf_id = strategy_buf_id;
+	prev_strategy_passes = strategy_passes;
+	saved_info_valid = true;
+
+	/*
+	 * Compute how many buffers had to be scanned for each new allocation, ie,
+	 * 1/density of reusable buffers, and track a moving average of that.
+	 *
+	 * If the strategy point didn't move, we don't update the density estimate
+	 */
+	if (strategy_delta > 0 && recent_alloc > 0)
+	{
+		scans_per_alloc = (float) strategy_delta / (float) recent_alloc;
+		smoothed_density += (scans_per_alloc - smoothed_density) /
+			smoothing_samples;
+	}
+
+	/*
+	 * Estimate how many reusable buffers there are between the current
+	 * strategy point and where we've scanned ahead to, based on the smoothed
+	 * density estimate.
+	 */
+	bufs_ahead = NBuffers - bufs_to_lap;
+	reusable_buffers_est = (float) bufs_ahead / smoothed_density;
+
+	/*
+	 * Track a moving average of recent buffer allocations.  Here, rather than
+	 * a true average we want a fast-attack, slow-decline behavior: we
+	 * immediately follow any increase.
+	 */
+	if (smoothed_alloc <= (float) recent_alloc)
+		smoothed_alloc = recent_alloc;
+	else
+		smoothed_alloc += ((float) recent_alloc - smoothed_alloc) /
+			smoothing_samples;
+
+	/* Scale the estimate by a GUC to allow more aggressive tuning. */
+	upcoming_alloc_est = (int) (smoothed_alloc * bgwriter_lru_multiplier);
+
+	/*
+	 * If recent_alloc remains at zero for many cycles, smoothed_alloc will
+	 * eventually underflow to zero, and the underflows produce annoying
+	 * kernel warnings on some platforms.  Once upcoming_alloc_est has gone to
+	 * zero, there's no point in tracking smaller and smaller values of
+	 * smoothed_alloc, so just reset it to exactly zero to avoid this
+	 * syndrome.  It will pop back up as soon as recent_alloc increases.
+	 */
+	if (upcoming_alloc_est == 0)
+		smoothed_alloc = 0;
+
+	/*
+	 * Even in cases where there's been little or no buffer allocation
+	 * activity, we want to make a small amount of progress through the buffer
+	 * cache so that as many reusable buffers as possible are clean after an
+	 * idle period.
+	 *
+	 * (scan_whole_pool_milliseconds / BgWriterDelay) computes how many times
+	 * the BGW will be called during the scan_whole_pool time; slice the
+	 * buffer pool into that many sections.
+	 */
+	min_scan_buffers = (int) (NBuffers / (scan_whole_pool_milliseconds / BgWriterDelay));
+
+	if (upcoming_alloc_est < (min_scan_buffers + reusable_buffers_est))
+	{
+#ifdef BGW_DEBUG
+		elog(DEBUG2, "bgwriter: alloc_est=%d too small, using min=%d + reusable_est=%d",
+			 upcoming_alloc_est, min_scan_buffers, reusable_buffers_est);
+#endif
+		upcoming_alloc_est = min_scan_buffers + reusable_buffers_est;
+	}
+
+	/*
+	 * Now write out dirty reusable buffers, working forward from the
+	 * next_to_clean point, until we have lapped the strategy scan, or cleaned
+	 * enough buffers to match our estimate of the next cycle's allocation
+	 * requirements, or hit the bgwriter_lru_maxpages limit.
+	 */
+
+	/* Make sure we can handle the pin inside SyncOneBuffer */
+	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+	num_to_scan = bufs_to_lap;
+	num_written = 0;
+	reusable_buffers = reusable_buffers_est;
+
+	/* Execute the LRU scan */
+	while (num_to_scan > 0 && reusable_buffers < upcoming_alloc_est)
+	{
+		int			sync_state = SyncOneBuffer(next_to_clean, true,
+											   wb_context);
+
+		if (++next_to_clean >= NBuffers)
+		{
+			next_to_clean = 0;
+			next_passes++;
+		}
+		num_to_scan--;
+
+		if (sync_state & BUF_WRITTEN)
+		{
+			reusable_buffers++;
+			if (++num_written >= bgwriter_lru_maxpages)
+			{
+				BgWriterStats.m_maxwritten_clean++;
+				break;
+			}
+		}
+		else if (sync_state & BUF_REUSABLE)
+			reusable_buffers++;
+	}
+
+	BgWriterStats.m_buf_written_clean += num_written;
+
+#ifdef BGW_DEBUG
+	elog(DEBUG1, "bgwriter: recent_alloc=%u smoothed=%.2f delta=%ld ahead=%d density=%.2f reusable_est=%d upcoming_est=%d scanned=%d wrote=%d reusable=%d",
+		 recent_alloc, smoothed_alloc, strategy_delta, bufs_ahead,
+		 smoothed_density, reusable_buffers_est, upcoming_alloc_est,
+		 bufs_to_lap - num_to_scan,
+		 num_written,
+		 reusable_buffers - reusable_buffers_est);
+#endif
+
+	/*
+	 * Consider the above scan as being like a new allocation scan.
+	 * Characterize its density and update the smoothed one based on it. This
+	 * effectively halves the moving average period in cases where both the
+	 * strategy and the background writer are doing some useful scanning,
+	 * which is helpful because a long memory isn't as desirable on the
+	 * density estimates.
+	 */
+	new_strategy_delta = bufs_to_lap - num_to_scan;
+	new_recent_alloc = reusable_buffers - reusable_buffers_est;
+	if (new_strategy_delta > 0 && new_recent_alloc > 0)
+	{
+		scans_per_alloc = (float) new_strategy_delta / (float) new_recent_alloc;
+		smoothed_density += (scans_per_alloc - smoothed_density) /
+			smoothing_samples;
+
+#ifdef BGW_DEBUG
+		elog(DEBUG2, "bgwriter: cleaner density alloc=%u scan=%ld density=%.2f new smoothed=%.2f",
+			 new_recent_alloc, new_strategy_delta,
+			 scans_per_alloc, smoothed_density);
+#endif
+	}
+
+	/* Return true if OK to hibernate */
+	return (bufs_to_lap == 0 && recent_alloc == 0);
+}
+
+/*
+ * SyncOneBuffer -- process a single buffer during syncing.
+ *
+ * If skip_recently_used is true, we don't write currently-pinned buffers, nor
+ * buffers marked recently used, as these are not replacement candidates.
+ *
+ * Returns a bitmask containing the following flag bits:
+ *	BUF_WRITTEN: we wrote the buffer.
+ *	BUF_REUSABLE: buffer is available for replacement, ie, it has
+ *		pin count 0 and usage count 0.
+ *
+ * (BUF_WRITTEN could be set in error if FlushBuffer finds the buffer clean
+ * after locking it, but we don't care all that much.)
+ *
+ * Note: caller must have done ResourceOwnerEnlargeBuffers.
+ */
+static int
+SyncOneBuffer(int buf_id, bool skip_recently_used, WritebackContext *wb_context)
+{
+	BufferDesc *bufHdr = GetBufferDescriptor(buf_id);
+	int			result = 0;
+	uint32		buf_state;
+	BufferTag	tag;
+
+	ReservePrivateRefCountEntry();
+
+	/*
+	 * Check whether buffer needs writing.
+	 *
+	 * We can make this check without taking the buffer content lock so long
+	 * as we mark pages dirty in access methods *before* logging changes with
+	 * XLogInsert(): if someone marks the buffer dirty just after our check we
+	 * don't worry because our checkpoint.redo points before log record for
+	 * upcoming changes and so we are not required to write such dirty buffer.
+	 */
+	buf_state = LockBufHdr(bufHdr);
+
+	if (BUF_STATE_GET_REFCOUNT(buf_state) == 0 &&
+		BUF_STATE_GET_USAGECOUNT(buf_state) == 0)
+	{
+		result |= BUF_REUSABLE;
+	}
+	else if (skip_recently_used)
+	{
+		/* Caller told us not to write recently-used buffers */
+		UnlockBufHdr(bufHdr, buf_state);
+		return result;
+	}
+
+	if (!(buf_state & BM_VALID) || !(buf_state & BM_DIRTY))
+	{
+		/* It's clean, so nothing to do */
+		UnlockBufHdr(bufHdr, buf_state);
+		return result;
+	}
+
+	/*
+	 * Pin it, share-lock it, write it.  (FlushBuffer will do nothing if the
+	 * buffer is clean by the time we've locked it.)
+	 */
+	PinBuffer_Locked(bufHdr);
+	LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
+
+	FlushBuffer(bufHdr, NULL);
+
+	LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
+
+	tag = bufHdr->tag;
+
+	UnpinBuffer(bufHdr, true);
+
+	ScheduleBufferTagForWriteback(wb_context, &tag);
+
+	return result | BUF_WRITTEN;
+}
+
+/*
+ *		AtEOXact_Buffers - clean up at end of transaction.
+ *
+ *		As of PostgreSQL 8.0, buffer pins should get released by the
+ *		ResourceOwner mechanism.  This routine is just a debugging
+ *		cross-check that no pins remain.
+ */
+void
+AtEOXact_Buffers(bool isCommit)
+{
+	CheckForBufferLeaks();
+
+	AtEOXact_LocalBuffers(isCommit);
+
+	Assert(PrivateRefCountOverflowed == 0);
+}
+
+/*
+ * Initialize access to shared buffer pool
+ *
+ * This is called during backend startup (whether standalone or under the
+ * postmaster).  It sets up for this backend's access to the already-existing
+ * buffer pool.
+ *
+ * NB: this is called before InitProcess(), so we do not have a PGPROC and
+ * cannot do LWLockAcquire; hence we can't actually access stuff in
+ * shared memory yet.  We are only initializing local data here.
+ * (See also InitBufferPoolBackend)
+ */
+void
+InitBufferPoolAccess(void)
+{
+	HASHCTL		hash_ctl;
+
+	memset(&PrivateRefCountArray, 0, sizeof(PrivateRefCountArray));
+
+	hash_ctl.keysize = sizeof(int32);
+	hash_ctl.entrysize = sizeof(PrivateRefCountEntry);
+
+	PrivateRefCountHash = hash_create("PrivateRefCount", 100, &hash_ctl,
+									  HASH_ELEM | HASH_BLOBS);
+}
+
+/*
+ * InitBufferPoolBackend --- second-stage initialization of a new backend
+ *
+ * This is called after we have acquired a PGPROC and so can safely get
+ * LWLocks.  We don't currently need to do anything at this stage ...
+ * except register a shmem-exit callback.  AtProcExit_Buffers needs LWLock
+ * access, and thereby has to be called at the corresponding phase of
+ * backend shutdown.
+ */
+void
+InitBufferPoolBackend(void)
+{
+	on_shmem_exit(AtProcExit_Buffers, 0);
+}
+
+/*
+ * During backend exit, ensure that we released all shared-buffer locks and
+ * assert that we have no remaining pins.
+ */
+static void
+AtProcExit_Buffers(int code, Datum arg)
+{
+	AbortBufferIO();
+	UnlockBuffers();
+
+	CheckForBufferLeaks();
+
+	/* localbuf.c needs a chance too */
+	AtProcExit_LocalBuffers();
+}
+
+/*
+ *		CheckForBufferLeaks - ensure this backend holds no buffer pins
+ *
+ *		As of PostgreSQL 8.0, buffer pins should get released by the
+ *		ResourceOwner mechanism.  This routine is just a debugging
+ *		cross-check that no pins remain.
+ */
+static void
+CheckForBufferLeaks(void)
+{
+#ifdef USE_ASSERT_CHECKING
+	int			RefCountErrors = 0;
+	PrivateRefCountEntry *res;
+	int			i;
+
+	/* check the array */
+	for (i = 0; i < REFCOUNT_ARRAY_ENTRIES; i++)
+	{
+		res = &PrivateRefCountArray[i];
+
+		if (res->buffer != InvalidBuffer)
+		{
+			PrintBufferLeakWarning(res->buffer);
+			RefCountErrors++;
+		}
+	}
+
+	/* if necessary search the hash */
+	if (PrivateRefCountOverflowed)
+	{
+		HASH_SEQ_STATUS hstat;
+
+		hash_seq_init(&hstat, PrivateRefCountHash);
+		while ((res = (PrivateRefCountEntry *) hash_seq_search(&hstat)) != NULL)
+		{
+			PrintBufferLeakWarning(res->buffer);
+			RefCountErrors++;
+		}
+
+	}
+
+	Assert(RefCountErrors == 0);
+#endif
+}
+
+/*
+ * Helper routine to issue warnings when a buffer is unexpectedly pinned
+ */
+void
+PrintBufferLeakWarning(Buffer buffer)
+{
+	BufferDesc *buf;
+	int32		loccount;
+	char	   *path;
+	BackendId	backend;
+	uint32		buf_state;
+
+	Assert(BufferIsValid(buffer));
+	if (BufferIsLocal(buffer))
+	{
+		buf = GetLocalBufferDescriptor(-buffer - 1);
+		loccount = LocalRefCount[-buffer - 1];
+		backend = MyBackendId;
+	}
+	else
+	{
+		buf = GetBufferDescriptor(buffer - 1);
+		loccount = GetPrivateRefCount(buffer);
+		backend = InvalidBackendId;
+	}
+
+	/* theoretically we should lock the bufhdr here */
+	path = relpathbackend(buf->tag.rnode, backend, buf->tag.forkNum);
+	buf_state = pg_atomic_read_u32(&buf->state);
+	elog(WARNING,
+		 "buffer refcount leak: [%03d] "
+		 "(rel=%s, blockNum=%u, flags=0x%x, refcount=%u %d)",
+		 buffer, path,
+		 buf->tag.blockNum, buf_state & BUF_FLAG_MASK,
+		 BUF_STATE_GET_REFCOUNT(buf_state), loccount);
+	pfree(path);
+}
+
+/*
+ * CheckPointBuffers
+ *
+ * Flush all dirty blocks in buffer pool to disk at checkpoint time.
+ *
+ * Note: temporary relations do not participate in checkpoints, so they don't
+ * need to be flushed.
+ */
+void
+CheckPointBuffers(int flags)
+{
+	BufferSync(flags);
+}
+
+
+/*
+ * Do whatever is needed to prepare for commit at the bufmgr and smgr levels
+ */
+void
+BufmgrCommit(void)
+{
+	/* Nothing to do in bufmgr anymore... */
+}
+
+/*
+ * BufferGetBlockNumber
+ *		Returns the block number associated with a buffer.
+ *
+ * Note:
+ *		Assumes that the buffer is valid and pinned, else the
+ *		value may be obsolete immediately...
+ */
+BlockNumber
+BufferGetBlockNumber(Buffer buffer)
+{
+	BufferDesc *bufHdr;
+
+	Assert(BufferIsPinned(buffer));
+
+	if (BufferIsLocal(buffer))
+		bufHdr = GetLocalBufferDescriptor(-buffer - 1);
+	else
+		bufHdr = GetBufferDescriptor(buffer - 1);
+
+	/* pinned, so OK to read tag without spinlock */
+	return bufHdr->tag.blockNum;
+}
+
+/*
+ * BufferGetTag
+ *		Returns the relfilenode, fork number and block number associated with
+ *		a buffer.
+ */
+void
+BufferGetTag(Buffer buffer, RelFileNode *rnode, ForkNumber *forknum,
+			 BlockNumber *blknum)
+{
+	BufferDesc *bufHdr;
+
+	/* Do the same checks as BufferGetBlockNumber. */
+	Assert(BufferIsPinned(buffer));
+
+	if (BufferIsLocal(buffer))
+		bufHdr = GetLocalBufferDescriptor(-buffer - 1);
+	else
+		bufHdr = GetBufferDescriptor(buffer - 1);
+
+	/* pinned, so OK to read tag without spinlock */
+	*rnode = bufHdr->tag.rnode;
+	*forknum = bufHdr->tag.forkNum;
+	*blknum = bufHdr->tag.blockNum;
+}
+
+/*
+ * FlushBuffer
+ *		Physically write out a shared buffer.
+ *
+ * NOTE: this actually just passes the buffer contents to the kernel; the
+ * real write to disk won't happen until the kernel feels like it.  This
+ * is okay from our point of view since we can redo the changes from WAL.
+ * However, we will need to force the changes to disk via fsync before
+ * we can checkpoint WAL.
+ *
+ * The caller must hold a pin on the buffer and have share-locked the
+ * buffer contents.  (Note: a share-lock does not prevent updates of
+ * hint bits in the buffer, so the page could change while the write
+ * is in progress, but we assume that that will not invalidate the data
+ * written.)
+ *
+ * If the caller has an smgr reference for the buffer's relation, pass it
+ * as the second parameter.  If not, pass NULL.
+ */
+static void
+FlushBuffer(BufferDesc *buf, SMgrRelation reln)
+{
+	XLogRecPtr	recptr;
+	ErrorContextCallback errcallback;
+	instr_time	io_start,
+				io_time;
+	Block		bufBlock;
+	char	   *bufToWrite;
+	uint32		buf_state;
+
+	/*
+	 * Try to start an I/O operation.  If StartBufferIO returns false, then
+	 * someone else flushed the buffer before we could, so we need not do
+	 * anything.
+	 */
+	if (!StartBufferIO(buf, false))
+		return;
+
+	/* Setup error traceback support for ereport() */
+	errcallback.callback = shared_buffer_write_error_callback;
+	errcallback.arg = (void *) buf;
+	errcallback.previous = error_context_stack;
+	error_context_stack = &errcallback;
+
+	/* Find smgr relation for buffer */
+	if (reln == NULL)
+		reln = smgropen(buf->tag.rnode, InvalidBackendId);
+
+	TRACE_POSTGRESQL_BUFFER_FLUSH_START(buf->tag.forkNum,
+										buf->tag.blockNum,
+										reln->smgr_rnode.node.spcNode,
+										reln->smgr_rnode.node.dbNode,
+										reln->smgr_rnode.node.relNode);
+
+	buf_state = LockBufHdr(buf);
+
+	/*
+	 * Run PageGetLSN while holding header lock, since we don't have the
+	 * buffer locked exclusively in all cases.
+	 */
+	recptr = BufferGetLSN(buf);
+
+	/* To check if block content changes while flushing. - vadim 01/17/97 */
+	buf_state &= ~BM_JUST_DIRTIED;
+	UnlockBufHdr(buf, buf_state);
+
+	/*
+	 * Force XLOG flush up to buffer's LSN.  This implements the basic WAL
+	 * rule that log updates must hit disk before any of the data-file changes
+	 * they describe do.
+	 *
+	 * However, this rule does not apply to unlogged relations, which will be
+	 * lost after a crash anyway.  Most unlogged relation pages do not bear
+	 * LSNs since we never emit WAL records for them, and therefore flushing
+	 * up through the buffer LSN would be useless, but harmless.  However,
+	 * GiST indexes use LSNs internally to track page-splits, and therefore
+	 * unlogged GiST pages bear "fake" LSNs generated by
+	 * GetFakeLSNForUnloggedRel.  It is unlikely but possible that the fake
+	 * LSN counter could advance past the WAL insertion point; and if it did
+	 * happen, attempting to flush WAL through that location would fail, with
+	 * disastrous system-wide consequences.  To make sure that can't happen,
+	 * skip the flush if the buffer isn't permanent.
+	 */
+	if (buf_state & BM_PERMANENT)
+		XLogFlush(recptr);
+
+	/*
+	 * Now it's safe to write buffer to disk. Note that no one else should
+	 * have been able to write it while we were busy with log flushing because
+	 * only one process at a time can set the BM_IO_IN_PROGRESS bit.
+	 */
+	bufBlock = BufHdrGetBlock(buf);
+
+	/*
+	 * Update page checksum if desired.  Since we have only shared lock on the
+	 * buffer, other processes might be updating hint bits in it, so we must
+	 * copy the page to private storage if we do checksumming.
+	 */
+	bufToWrite = PageSetChecksumCopy((Page) bufBlock, buf->tag.blockNum);
+
+	if (track_io_timing)
+		INSTR_TIME_SET_CURRENT(io_start);
+
+	/*
+	 * bufToWrite is either the shared buffer or a copy, as appropriate.
+	 */
+	smgrwrite(reln,
+			  buf->tag.forkNum,
+			  buf->tag.blockNum,
+			  bufToWrite,
+			  false);
+
+	if (track_io_timing)
+	{
+		INSTR_TIME_SET_CURRENT(io_time);
+		INSTR_TIME_SUBTRACT(io_time, io_start);
+		pgstat_count_buffer_write_time(INSTR_TIME_GET_MICROSEC(io_time));
+		INSTR_TIME_ADD(pgBufferUsage.blk_write_time, io_time);
+	}
+
+	pgBufferUsage.shared_blks_written++;
+
+	/*
+	 * Mark the buffer as clean (unless BM_JUST_DIRTIED has become set) and
+	 * end the BM_IO_IN_PROGRESS state.
+	 */
+	TerminateBufferIO(buf, true, 0);
+
+	TRACE_POSTGRESQL_BUFFER_FLUSH_DONE(buf->tag.forkNum,
+									   buf->tag.blockNum,
+									   reln->smgr_rnode.node.spcNode,
+									   reln->smgr_rnode.node.dbNode,
+									   reln->smgr_rnode.node.relNode);
+
+	/* Pop the error context stack */
+	error_context_stack = errcallback.previous;
+}
+
+/*
+ * RelationGetNumberOfBlocksInFork
+ *		Determines the current number of pages in the specified relation fork.
+ *
+ * Note that the accuracy of the result will depend on the details of the
+ * relation's storage. For builtin AMs it'll be accurate, but for external AMs
+ * it might not be.
+ */
+BlockNumber
+RelationGetNumberOfBlocksInFork(Relation relation, ForkNumber forkNum)
+{
+	switch (relation->rd_rel->relkind)
+	{
+		case RELKIND_SEQUENCE:
+		case RELKIND_INDEX:
+		case RELKIND_PARTITIONED_INDEX:
+			/* Open it at the smgr level if not already done */
+			RelationOpenSmgr(relation);
+
+			return smgrnblocks(relation->rd_smgr, forkNum);
+
+		case RELKIND_RELATION:
+		case RELKIND_TOASTVALUE:
+		case RELKIND_MATVIEW:
+			{
+				/*
+				 * Not every table AM uses BLCKSZ wide fixed size blocks.
+				 * Therefore tableam returns the size in bytes - but for the
+				 * purpose of this routine, we want the number of blocks.
+				 * Therefore divide, rounding up.
+				 */
+				uint64		szbytes;
+
+				szbytes = table_relation_size(relation, forkNum);
+
+				return (szbytes + (BLCKSZ - 1)) / BLCKSZ;
+			}
+		case RELKIND_VIEW:
+		case RELKIND_COMPOSITE_TYPE:
+		case RELKIND_FOREIGN_TABLE:
+		case RELKIND_PARTITIONED_TABLE:
+		default:
+			Assert(false);
+			break;
+	}
+
+	return 0;					/* keep compiler quiet */
+}
+
+/*
+ * BufferIsPermanent
+ *		Determines whether a buffer will potentially still be around after
+ *		a crash.  Caller must hold a buffer pin.
+ */
+bool
+BufferIsPermanent(Buffer buffer)
+{
+	BufferDesc *bufHdr;
+
+	/* Local buffers are used only for temp relations. */
+	if (BufferIsLocal(buffer))
+		return false;
+
+	/* Make sure we've got a real buffer, and that we hold a pin on it. */
+	Assert(BufferIsValid(buffer));
+	Assert(BufferIsPinned(buffer));
+
+	/*
+	 * BM_PERMANENT can't be changed while we hold a pin on the buffer, so we
+	 * need not bother with the buffer header spinlock.  Even if someone else
+	 * changes the buffer header state while we're doing this, the state is
+	 * changed atomically, so we'll read the old value or the new value, but
+	 * not random garbage.
+	 */
+	bufHdr = GetBufferDescriptor(buffer - 1);
+	return (pg_atomic_read_u32(&bufHdr->state) & BM_PERMANENT) != 0;
+}
+
+/*
+ * BufferGetLSNAtomic
+ *		Retrieves the LSN of the buffer atomically using a buffer header lock.
+ *		This is necessary for some callers who may not have an exclusive lock
+ *		on the buffer.
+ */
+XLogRecPtr
+BufferGetLSNAtomic(Buffer buffer)
+{
+	BufferDesc *bufHdr = GetBufferDescriptor(buffer - 1);
+	char	   *page = BufferGetPage(buffer);
+	XLogRecPtr	lsn;
+	uint32		buf_state;
+
+	/*
+	 * If we don't need locking for correctness, fastpath out.
+	 */
+	if (!XLogHintBitIsNeeded() || BufferIsLocal(buffer))
+		return PageGetLSN(page);
+
+	/* Make sure we've got a real buffer, and that we hold a pin on it. */
+	Assert(BufferIsValid(buffer));
+	Assert(BufferIsPinned(buffer));
+
+	buf_state = LockBufHdr(bufHdr);
+	lsn = PageGetLSN(page);
+	UnlockBufHdr(bufHdr, buf_state);
+
+	return lsn;
+}
+
+/* ---------------------------------------------------------------------
+ *		DropRelFileNodeBuffers
+ *
+ *		This function removes from the buffer pool all the pages of the
+ *		specified relation forks that have block numbers >= firstDelBlock.
+ *		(In particular, with firstDelBlock = 0, all pages are removed.)
+ *		Dirty pages are simply dropped, without bothering to write them
+ *		out first.  Therefore, this is NOT rollback-able, and so should be
+ *		used only with extreme caution!
+ *
+ *		Currently, this is called only from smgr.c when the underlying file
+ *		is about to be deleted or truncated (firstDelBlock is needed for
+ *		the truncation case).  The data in the affected pages would therefore
+ *		be deleted momentarily anyway, and there is no point in writing it.
+ *		It is the responsibility of higher-level code to ensure that the
+ *		deletion or truncation does not lose any data that could be needed
+ *		later.  It is also the responsibility of higher-level code to ensure
+ *		that no other process could be trying to load more pages of the
+ *		relation into buffers.
+ * --------------------------------------------------------------------
+ */
+void
+DropRelFileNodeBuffers(SMgrRelation smgr_reln, ForkNumber *forkNum,
+					   int nforks, BlockNumber *firstDelBlock)
+{
+	int			i;
+	int			j;
+	RelFileNodeBackend rnode;
+	BlockNumber nForkBlock[MAX_FORKNUM];
+	uint64		nBlocksToInvalidate = 0;
+
+	rnode = smgr_reln->smgr_rnode;
+
+	/* If it's a local relation, it's localbuf.c's problem. */
+	if (RelFileNodeBackendIsTemp(rnode))
+	{
+		if (rnode.backend == MyBackendId)
+		{
+			for (j = 0; j < nforks; j++)
+				DropRelFileNodeLocalBuffers(rnode.node, forkNum[j],
+											firstDelBlock[j]);
+		}
+		return;
+	}
+
+	/*
+	 * To remove all the pages of the specified relation forks from the buffer
+	 * pool, we need to scan the entire buffer pool but we can optimize it by
+	 * finding the buffers from BufMapping table provided we know the exact
+	 * size of each fork of the relation. The exact size is required to ensure
+	 * that we don't leave any buffer for the relation being dropped as
+	 * otherwise the background writer or checkpointer can lead to a PANIC
+	 * error while flushing buffers corresponding to files that don't exist.
+	 *
+	 * To know the exact size, we rely on the size cached for each fork by us
+	 * during recovery which limits the optimization to recovery and on
+	 * standbys but we can easily extend it once we have shared cache for
+	 * relation size.
+	 *
+	 * In recovery, we cache the value returned by the first lseek(SEEK_END)
+	 * and the future writes keeps the cached value up-to-date. See
+	 * smgrextend. It is possible that the value of the first lseek is smaller
+	 * than the actual number of existing blocks in the file due to buggy
+	 * Linux kernels that might not have accounted for the recent write. But
+	 * that should be fine because there must not be any buffers after that
+	 * file size.
+	 */
+	for (i = 0; i < nforks; i++)
+	{
+		/* Get the number of blocks for a relation's fork */
+		nForkBlock[i] = smgrnblocks_cached(smgr_reln, forkNum[i]);
+
+		if (nForkBlock[i] == InvalidBlockNumber)
+		{
+			nBlocksToInvalidate = InvalidBlockNumber;
+			break;
+		}
+
+		/* calculate the number of blocks to be invalidated */
+		nBlocksToInvalidate += (nForkBlock[i] - firstDelBlock[i]);
+	}
+
+	/*
+	 * We apply the optimization iff the total number of blocks to invalidate
+	 * is below the BUF_DROP_FULL_SCAN_THRESHOLD.
+	 */
+	if (BlockNumberIsValid(nBlocksToInvalidate) &&
+		nBlocksToInvalidate < BUF_DROP_FULL_SCAN_THRESHOLD)
+	{
+		for (j = 0; j < nforks; j++)
+			FindAndDropRelFileNodeBuffers(rnode.node, forkNum[j],
+										  nForkBlock[j], firstDelBlock[j]);
+		return;
+	}
+
+	for (i = 0; i < NBuffers; i++)
+	{
+		BufferDesc *bufHdr = GetBufferDescriptor(i);
+		uint32		buf_state;
+
+		/*
+		 * We can make this a tad faster by prechecking the buffer tag before
+		 * we attempt to lock the buffer; this saves a lot of lock
+		 * acquisitions in typical cases.  It should be safe because the
+		 * caller must have AccessExclusiveLock on the relation, or some other
+		 * reason to be certain that no one is loading new pages of the rel
+		 * into the buffer pool.  (Otherwise we might well miss such pages
+		 * entirely.)  Therefore, while the tag might be changing while we
+		 * look at it, it can't be changing *to* a value we care about, only
+		 * *away* from such a value.  So false negatives are impossible, and
+		 * false positives are safe because we'll recheck after getting the
+		 * buffer lock.
+		 *
+		 * We could check forkNum and blockNum as well as the rnode, but the
+		 * incremental win from doing so seems small.
+		 */
+		if (!RelFileNodeEquals(bufHdr->tag.rnode, rnode.node))
+			continue;
+
+		buf_state = LockBufHdr(bufHdr);
+
+		for (j = 0; j < nforks; j++)
+		{
+			if (RelFileNodeEquals(bufHdr->tag.rnode, rnode.node) &&
+				bufHdr->tag.forkNum == forkNum[j] &&
+				bufHdr->tag.blockNum >= firstDelBlock[j])
+			{
+				InvalidateBuffer(bufHdr);	/* releases spinlock */
+				break;
+			}
+		}
+		if (j >= nforks)
+			UnlockBufHdr(bufHdr, buf_state);
+	}
+}
+
+/* ---------------------------------------------------------------------
+ *		DropRelFileNodesAllBuffers
+ *
+ *		This function removes from the buffer pool all the pages of all
+ *		forks of the specified relations.  It's equivalent to calling
+ *		DropRelFileNodeBuffers once per fork per relation with
+ *		firstDelBlock = 0.
+ * --------------------------------------------------------------------
+ */
+void
+DropRelFileNodesAllBuffers(SMgrRelation *smgr_reln, int nnodes)
+{
+	int			i;
+	int			j;
+	int			n = 0;
+	SMgrRelation *rels;
+	BlockNumber (*block)[MAX_FORKNUM + 1];
+	uint64		nBlocksToInvalidate = 0;
+	RelFileNode *nodes;
+	bool		cached = true;
+	bool		use_bsearch;
+
+	if (nnodes == 0)
+		return;
+
+	rels = palloc(sizeof(SMgrRelation) * nnodes);	/* non-local relations */
+
+	/* If it's a local relation, it's localbuf.c's problem. */
+	for (i = 0; i < nnodes; i++)
+	{
+		if (RelFileNodeBackendIsTemp(smgr_reln[i]->smgr_rnode))
+		{
+			if (smgr_reln[i]->smgr_rnode.backend == MyBackendId)
+				DropRelFileNodeAllLocalBuffers(smgr_reln[i]->smgr_rnode.node);
+		}
+		else
+			rels[n++] = smgr_reln[i];
+	}
+
+	/*
+	 * If there are no non-local relations, then we're done. Release the
+	 * memory and return.
+	 */
+	if (n == 0)
+	{
+		pfree(rels);
+		return;
+	}
+
+	/*
+	 * This is used to remember the number of blocks for all the relations
+	 * forks.
+	 */
+	block = (BlockNumber (*)[MAX_FORKNUM + 1])
+		palloc(sizeof(BlockNumber) * n * (MAX_FORKNUM + 1));
+
+	/*
+	 * We can avoid scanning the entire buffer pool if we know the exact size
+	 * of each of the given relation forks. See DropRelFileNodeBuffers.
+	 */
+	for (i = 0; i < n && cached; i++)
+	{
+		for (j = 0; j <= MAX_FORKNUM; j++)
+		{
+			/* Get the number of blocks for a relation's fork. */
+			block[i][j] = smgrnblocks_cached(rels[i], j);
+
+			/* We need to only consider the relation forks that exists. */
+			if (block[i][j] == InvalidBlockNumber)
+			{
+				if (!smgrexists(rels[i], j))
+					continue;
+				cached = false;
+				break;
+			}
+
+			/* calculate the total number of blocks to be invalidated */
+			nBlocksToInvalidate += block[i][j];
+		}
+	}
+
+	/*
+	 * We apply the optimization iff the total number of blocks to invalidate
+	 * is below the BUF_DROP_FULL_SCAN_THRESHOLD.
+	 */
+	if (cached && nBlocksToInvalidate < BUF_DROP_FULL_SCAN_THRESHOLD)
+	{
+		for (i = 0; i < n; i++)
+		{
+			for (j = 0; j <= MAX_FORKNUM; j++)
+			{
+				/* ignore relation forks that doesn't exist */
+				if (!BlockNumberIsValid(block[i][j]))
+					continue;
+
+				/* drop all the buffers for a particular relation fork */
+				FindAndDropRelFileNodeBuffers(rels[i]->smgr_rnode.node,
+											  j, block[i][j], 0);
+			}
+		}
+
+		pfree(block);
+		pfree(rels);
+		return;
+	}
+
+	pfree(block);
+	nodes = palloc(sizeof(RelFileNode) * n);	/* non-local relations */
+	for (i = 0; i < n; i++)
+		nodes[i] = rels[i]->smgr_rnode.node;
+
+	/*
+	 * For low number of relations to drop just use a simple walk through, to
+	 * save the bsearch overhead. The threshold to use is rather a guess than
+	 * an exactly determined value, as it depends on many factors (CPU and RAM
+	 * speeds, amount of shared buffers etc.).
+	 */
+	use_bsearch = n > RELS_BSEARCH_THRESHOLD;
+
+	/* sort the list of rnodes if necessary */
+	if (use_bsearch)
+		pg_qsort(nodes, n, sizeof(RelFileNode), rnode_comparator);
+
+	for (i = 0; i < NBuffers; i++)
+	{
+		RelFileNode *rnode = NULL;
+		BufferDesc *bufHdr = GetBufferDescriptor(i);
+		uint32		buf_state;
+
+		/*
+		 * As in DropRelFileNodeBuffers, an unlocked precheck should be safe
+		 * and saves some cycles.
+		 */
+
+		if (!use_bsearch)
+		{
+			int			j;
+
+			for (j = 0; j < n; j++)
+			{
+				if (RelFileNodeEquals(bufHdr->tag.rnode, nodes[j]))
+				{
+					rnode = &nodes[j];
+					break;
+				}
+			}
+		}
+		else
+		{
+			rnode = bsearch((const void *) &(bufHdr->tag.rnode),
+							nodes, n, sizeof(RelFileNode),
+							rnode_comparator);
+		}
+
+		/* buffer doesn't belong to any of the given relfilenodes; skip it */
+		if (rnode == NULL)
+			continue;
+
+		buf_state = LockBufHdr(bufHdr);
+		if (RelFileNodeEquals(bufHdr->tag.rnode, (*rnode)))
+			InvalidateBuffer(bufHdr);	/* releases spinlock */
+		else
+			UnlockBufHdr(bufHdr, buf_state);
+	}
+
+	pfree(nodes);
+	pfree(rels);
+}
+
+/* ---------------------------------------------------------------------
+ *		FindAndDropRelFileNodeBuffers
+ *
+ *		This function performs look up in BufMapping table and removes from the
+ *		buffer pool all the pages of the specified relation fork that has block
+ *		number >= firstDelBlock. (In particular, with firstDelBlock = 0, all
+ *		pages are removed.)
+ * --------------------------------------------------------------------
+ */
+static void
+FindAndDropRelFileNodeBuffers(RelFileNode rnode, ForkNumber forkNum,
+							  BlockNumber nForkBlock,
+							  BlockNumber firstDelBlock)
+{
+	BlockNumber curBlock;
+
+	for (curBlock = firstDelBlock; curBlock < nForkBlock; curBlock++)
+	{
+		uint32		bufHash;	/* hash value for tag */
+		BufferTag	bufTag;		/* identity of requested block */
+		LWLock	   *bufPartitionLock;	/* buffer partition lock for it */
+		int			buf_id;
+		BufferDesc *bufHdr;
+		uint32		buf_state;
+
+		/* create a tag so we can lookup the buffer */
+		INIT_BUFFERTAG(bufTag, rnode, forkNum, curBlock);
+
+		/* determine its hash code and partition lock ID */
+		bufHash = BufTableHashCode(&bufTag);
+		bufPartitionLock = BufMappingPartitionLock(bufHash);
+
+		/* Check that it is in the buffer pool. If not, do nothing. */
+		LWLockAcquire(bufPartitionLock, LW_SHARED);
+		buf_id = BufTableLookup(&bufTag, bufHash);
+		LWLockRelease(bufPartitionLock);
+
+		if (buf_id < 0)
+			continue;
+
+		bufHdr = GetBufferDescriptor(buf_id);
+
+		/*
+		 * We need to lock the buffer header and recheck if the buffer is
+		 * still associated with the same block because the buffer could be
+		 * evicted by some other backend loading blocks for a different
+		 * relation after we release lock on the BufMapping table.
+		 */
+		buf_state = LockBufHdr(bufHdr);
+
+		if (RelFileNodeEquals(bufHdr->tag.rnode, rnode) &&
+			bufHdr->tag.forkNum == forkNum &&
+			bufHdr->tag.blockNum >= firstDelBlock)
+			InvalidateBuffer(bufHdr);	/* releases spinlock */
+		else
+			UnlockBufHdr(bufHdr, buf_state);
+	}
+}
+
+/* ---------------------------------------------------------------------
+ *		DropDatabaseBuffers
+ *
+ *		This function removes all the buffers in the buffer cache for a
+ *		particular database.  Dirty pages are simply dropped, without
+ *		bothering to write them out first.  This is used when we destroy a
+ *		database, to avoid trying to flush data to disk when the directory
+ *		tree no longer exists.  Implementation is pretty similar to
+ *		DropRelFileNodeBuffers() which is for destroying just one relation.
+ * --------------------------------------------------------------------
+ */
+void
+DropDatabaseBuffers(Oid dbid)
+{
+	int			i;
+
+	/*
+	 * We needn't consider local buffers, since by assumption the target
+	 * database isn't our own.
+	 */
+
+	for (i = 0; i < NBuffers; i++)
+	{
+		BufferDesc *bufHdr = GetBufferDescriptor(i);
+		uint32		buf_state;
+
+		/*
+		 * As in DropRelFileNodeBuffers, an unlocked precheck should be safe
+		 * and saves some cycles.
+		 */
+		if (bufHdr->tag.rnode.dbNode != dbid)
+			continue;
+
+		buf_state = LockBufHdr(bufHdr);
+		if (bufHdr->tag.rnode.dbNode == dbid)
+			InvalidateBuffer(bufHdr);	/* releases spinlock */
+		else
+			UnlockBufHdr(bufHdr, buf_state);
+	}
+}
+
+/* -----------------------------------------------------------------
+ *		PrintBufferDescs
+ *
+ *		this function prints all the buffer descriptors, for debugging
+ *		use only.
+ * -----------------------------------------------------------------
+ */
+#ifdef NOT_USED
+void
+PrintBufferDescs(void)
+{
+	int			i;
+
+	for (i = 0; i < NBuffers; ++i)
+	{
+		BufferDesc *buf = GetBufferDescriptor(i);
+		Buffer		b = BufferDescriptorGetBuffer(buf);
+
+		/* theoretically we should lock the bufhdr here */
+		elog(LOG,
+			 "[%02d] (freeNext=%d, rel=%s, "
+			 "blockNum=%u, flags=0x%x, refcount=%u %d)",
+			 i, buf->freeNext,
+			 relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum),
+			 buf->tag.blockNum, buf->flags,
+			 buf->refcount, GetPrivateRefCount(b));
+	}
+}
+#endif
+
+#ifdef NOT_USED
+void
+PrintPinnedBufs(void)
+{
+	int			i;
+
+	for (i = 0; i < NBuffers; ++i)
+	{
+		BufferDesc *buf = GetBufferDescriptor(i);
+		Buffer		b = BufferDescriptorGetBuffer(buf);
+
+		if (GetPrivateRefCount(b) > 0)
+		{
+			/* theoretically we should lock the bufhdr here */
+			elog(LOG,
+				 "[%02d] (freeNext=%d, rel=%s, "
+				 "blockNum=%u, flags=0x%x, refcount=%u %d)",
+				 i, buf->freeNext,
+				 relpathperm(buf->tag.rnode, buf->tag.forkNum),
+				 buf->tag.blockNum, buf->flags,
+				 buf->refcount, GetPrivateRefCount(b));
+		}
+	}
+}
+#endif
+
+/* ---------------------------------------------------------------------
+ *		FlushRelationBuffers
+ *
+ *		This function writes all dirty pages of a relation out to disk
+ *		(or more accurately, out to kernel disk buffers), ensuring that the
+ *		kernel has an up-to-date view of the relation.
+ *
+ *		Generally, the caller should be holding AccessExclusiveLock on the
+ *		target relation to ensure that no other backend is busy dirtying
+ *		more blocks of the relation; the effects can't be expected to last
+ *		after the lock is released.
+ *
+ *		XXX currently it sequentially searches the buffer pool, should be
+ *		changed to more clever ways of searching.  This routine is not
+ *		used in any performance-critical code paths, so it's not worth
+ *		adding additional overhead to normal paths to make it go faster.
+ * --------------------------------------------------------------------
+ */
+void
+FlushRelationBuffers(Relation rel)
+{
+	int			i;
+	BufferDesc *bufHdr;
+
+	/* Open rel at the smgr level if not already done */
+	RelationOpenSmgr(rel);
+
+	if (RelationUsesLocalBuffers(rel))
+	{
+		for (i = 0; i < NLocBuffer; i++)
+		{
+			uint32		buf_state;
+
+			bufHdr = GetLocalBufferDescriptor(i);
+			if (RelFileNodeEquals(bufHdr->tag.rnode, rel->rd_node) &&
+				((buf_state = pg_atomic_read_u32(&bufHdr->state)) &
+				 (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
+			{
+				ErrorContextCallback errcallback;
+				Page		localpage;
+
+				localpage = (char *) LocalBufHdrGetBlock(bufHdr);
+
+				/* Setup error traceback support for ereport() */
+				errcallback.callback = local_buffer_write_error_callback;
+				errcallback.arg = (void *) bufHdr;
+				errcallback.previous = error_context_stack;
+				error_context_stack = &errcallback;
+
+				PageSetChecksumInplace(localpage, bufHdr->tag.blockNum);
+
+				smgrwrite(rel->rd_smgr,
+						  bufHdr->tag.forkNum,
+						  bufHdr->tag.blockNum,
+						  localpage,
+						  false);
+
+				buf_state &= ~(BM_DIRTY | BM_JUST_DIRTIED);
+				pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
+
+				/* Pop the error context stack */
+				error_context_stack = errcallback.previous;
+			}
+		}
+
+		return;
+	}
+
+	/* Make sure we can handle the pin inside the loop */
+	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+	for (i = 0; i < NBuffers; i++)
+	{
+		uint32		buf_state;
+
+		bufHdr = GetBufferDescriptor(i);
+
+		/*
+		 * As in DropRelFileNodeBuffers, an unlocked precheck should be safe
+		 * and saves some cycles.
+		 */
+		if (!RelFileNodeEquals(bufHdr->tag.rnode, rel->rd_node))
+			continue;
+
+		ReservePrivateRefCountEntry();
+
+		buf_state = LockBufHdr(bufHdr);
+		if (RelFileNodeEquals(bufHdr->tag.rnode, rel->rd_node) &&
+			(buf_state & (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
+		{
+			PinBuffer_Locked(bufHdr);
+			LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
+			FlushBuffer(bufHdr, rel->rd_smgr);
+			LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
+			UnpinBuffer(bufHdr, true);
+		}
+		else
+			UnlockBufHdr(bufHdr, buf_state);
+	}
+}
+
+/* ---------------------------------------------------------------------
+ *		FlushRelationsAllBuffers
+ *
+ *		This function flushes out of the buffer pool all the pages of all
+ *		forks of the specified smgr relations.  It's equivalent to calling
+ *		FlushRelationBuffers once per fork per relation.  The relations are
+ *		assumed not to use local buffers.
+ * --------------------------------------------------------------------
+ */
+void
+FlushRelationsAllBuffers(SMgrRelation *smgrs, int nrels)
+{
+	int			i;
+	SMgrSortArray *srels;
+	bool		use_bsearch;
+
+	if (nrels == 0)
+		return;
+
+	/* fill-in array for qsort */
+	srels = palloc(sizeof(SMgrSortArray) * nrels);
+
+	for (i = 0; i < nrels; i++)
+	{
+		Assert(!RelFileNodeBackendIsTemp(smgrs[i]->smgr_rnode));
+
+		srels[i].rnode = smgrs[i]->smgr_rnode.node;
+		srels[i].srel = smgrs[i];
+	}
+
+	/*
+	 * Save the bsearch overhead for low number of relations to sync. See
+	 * DropRelFileNodesAllBuffers for details.
+	 */
+	use_bsearch = nrels > RELS_BSEARCH_THRESHOLD;
+
+	/* sort the list of SMgrRelations if necessary */
+	if (use_bsearch)
+		pg_qsort(srels, nrels, sizeof(SMgrSortArray), rnode_comparator);
+
+	/* Make sure we can handle the pin inside the loop */
+	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+	for (i = 0; i < NBuffers; i++)
+	{
+		SMgrSortArray *srelent = NULL;
+		BufferDesc *bufHdr = GetBufferDescriptor(i);
+		uint32		buf_state;
+
+		/*
+		 * As in DropRelFileNodeBuffers, an unlocked precheck should be safe
+		 * and saves some cycles.
+		 */
+
+		if (!use_bsearch)
+		{
+			int			j;
+
+			for (j = 0; j < nrels; j++)
+			{
+				if (RelFileNodeEquals(bufHdr->tag.rnode, srels[j].rnode))
+				{
+					srelent = &srels[j];
+					break;
+				}
+			}
+
+		}
+		else
+		{
+			srelent = bsearch((const void *) &(bufHdr->tag.rnode),
+							  srels, nrels, sizeof(SMgrSortArray),
+							  rnode_comparator);
+		}
+
+		/* buffer doesn't belong to any of the given relfilenodes; skip it */
+		if (srelent == NULL)
+			continue;
+
+		ReservePrivateRefCountEntry();
+
+		buf_state = LockBufHdr(bufHdr);
+		if (RelFileNodeEquals(bufHdr->tag.rnode, srelent->rnode) &&
+			(buf_state & (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
+		{
+			PinBuffer_Locked(bufHdr);
+			LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
+			FlushBuffer(bufHdr, srelent->srel);
+			LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
+			UnpinBuffer(bufHdr, true);
+		}
+		else
+			UnlockBufHdr(bufHdr, buf_state);
+	}
+
+	pfree(srels);
+}
+
+/* ---------------------------------------------------------------------
+ *		FlushDatabaseBuffers
+ *
+ *		This function writes all dirty pages of a database out to disk
+ *		(or more accurately, out to kernel disk buffers), ensuring that the
+ *		kernel has an up-to-date view of the database.
+ *
+ *		Generally, the caller should be holding an appropriate lock to ensure
+ *		no other backend is active in the target database; otherwise more
+ *		pages could get dirtied.
+ *
+ *		Note we don't worry about flushing any pages of temporary relations.
+ *		It's assumed these wouldn't be interesting.
+ * --------------------------------------------------------------------
+ */
+void
+FlushDatabaseBuffers(Oid dbid)
+{
+	int			i;
+	BufferDesc *bufHdr;
+
+	/* Make sure we can handle the pin inside the loop */
+	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+	for (i = 0; i < NBuffers; i++)
+	{
+		uint32		buf_state;
+
+		bufHdr = GetBufferDescriptor(i);
+
+		/*
+		 * As in DropRelFileNodeBuffers, an unlocked precheck should be safe
+		 * and saves some cycles.
+		 */
+		if (bufHdr->tag.rnode.dbNode != dbid)
+			continue;
+
+		ReservePrivateRefCountEntry();
+
+		buf_state = LockBufHdr(bufHdr);
+		if (bufHdr->tag.rnode.dbNode == dbid &&
+			(buf_state & (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
+		{
+			PinBuffer_Locked(bufHdr);
+			LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
+			FlushBuffer(bufHdr, NULL);
+			LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
+			UnpinBuffer(bufHdr, true);
+		}
+		else
+			UnlockBufHdr(bufHdr, buf_state);
+	}
+}
+
+/*
+ * Flush a previously, shared or exclusively, locked and pinned buffer to the
+ * OS.
+ */
+void
+FlushOneBuffer(Buffer buffer)
+{
+	BufferDesc *bufHdr;
+
+	/* currently not needed, but no fundamental reason not to support */
+	Assert(!BufferIsLocal(buffer));
+
+	Assert(BufferIsPinned(buffer));
+
+	bufHdr = GetBufferDescriptor(buffer - 1);
+
+	Assert(LWLockHeldByMe(BufferDescriptorGetContentLock(bufHdr)));
+
+	FlushBuffer(bufHdr, NULL);
+}
+
+/*
+ * ReleaseBuffer -- release the pin on a buffer
+ */
+void
+ReleaseBuffer(Buffer buffer)
+{
+	if (!BufferIsValid(buffer))
+		elog(ERROR, "bad buffer ID: %d", buffer);
+
+	if (BufferIsLocal(buffer))
+	{
+		ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
+
+		Assert(LocalRefCount[-buffer - 1] > 0);
+		LocalRefCount[-buffer - 1]--;
+		return;
+	}
+
+	UnpinBuffer(GetBufferDescriptor(buffer - 1), true);
+}
+
+/*
+ * UnlockReleaseBuffer -- release the content lock and pin on a buffer
+ *
+ * This is just a shorthand for a common combination.
+ */
+void
+UnlockReleaseBuffer(Buffer buffer)
+{
+	LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
+	ReleaseBuffer(buffer);
+}
+
+/*
+ * IncrBufferRefCount
+ *		Increment the pin count on a buffer that we have *already* pinned
+ *		at least once.
+ *
+ *		This function cannot be used on a buffer we do not have pinned,
+ *		because it doesn't change the shared buffer state.
+ */
+void
+IncrBufferRefCount(Buffer buffer)
+{
+	Assert(BufferIsPinned(buffer));
+	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+	if (BufferIsLocal(buffer))
+		LocalRefCount[-buffer - 1]++;
+	else
+	{
+		PrivateRefCountEntry *ref;
+
+		ref = GetPrivateRefCountEntry(buffer, true);
+		Assert(ref != NULL);
+		ref->refcount++;
+	}
+	ResourceOwnerRememberBuffer(CurrentResourceOwner, buffer);
+}
+
+/*
+ * MarkBufferDirtyHint
+ *
+ *	Mark a buffer dirty for non-critical changes.
+ *
+ * This is essentially the same as MarkBufferDirty, except:
+ *
+ * 1. The caller does not write WAL; so if checksums are enabled, we may need
+ *	  to write an XLOG_FPI_FOR_HINT WAL record to protect against torn pages.
+ * 2. The caller might have only share-lock instead of exclusive-lock on the
+ *	  buffer's content lock.
+ * 3. This function does not guarantee that the buffer is always marked dirty
+ *	  (due to a race condition), so it cannot be used for important changes.
+ */
+void
+MarkBufferDirtyHint(Buffer buffer, bool buffer_std)
+{
+	BufferDesc *bufHdr;
+	Page		page = BufferGetPage(buffer);
+
+	if (!BufferIsValid(buffer))
+		elog(ERROR, "bad buffer ID: %d", buffer);
+
+	if (BufferIsLocal(buffer))
+	{
+		MarkLocalBufferDirty(buffer);
+		return;
+	}
+
+	bufHdr = GetBufferDescriptor(buffer - 1);
+
+	Assert(GetPrivateRefCount(buffer) > 0);
+	/* here, either share or exclusive lock is OK */
+	Assert(LWLockHeldByMe(BufferDescriptorGetContentLock(bufHdr)));
+
+	/*
+	 * This routine might get called many times on the same page, if we are
+	 * making the first scan after commit of an xact that added/deleted many
+	 * tuples. So, be as quick as we can if the buffer is already dirty.  We
+	 * do this by not acquiring spinlock if it looks like the status bits are
+	 * already set.  Since we make this test unlocked, there's a chance we
+	 * might fail to notice that the flags have just been cleared, and failed
+	 * to reset them, due to memory-ordering issues.  But since this function
+	 * is only intended to be used in cases where failing to write out the
+	 * data would be harmless anyway, it doesn't really matter.
+	 */
+	if ((pg_atomic_read_u32(&bufHdr->state) & (BM_DIRTY | BM_JUST_DIRTIED)) !=
+		(BM_DIRTY | BM_JUST_DIRTIED))
+	{
+		XLogRecPtr	lsn = InvalidXLogRecPtr;
+		bool		dirtied = false;
+		bool		delayChkpt = false;
+		uint32		buf_state;
+
+		/*
+		 * If we need to protect hint bit updates from torn writes, WAL-log a
+		 * full page image of the page. This full page image is only necessary
+		 * if the hint bit update is the first change to the page since the
+		 * last checkpoint.
+		 *
+		 * We don't check full_page_writes here because that logic is included
+		 * when we call XLogInsert() since the value changes dynamically.
+		 */
+		if (XLogHintBitIsNeeded() &&
+			(pg_atomic_read_u32(&bufHdr->state) & BM_PERMANENT))
+		{
+			/*
+			 * If we must not write WAL, due to a relfilenode-specific
+			 * condition or being in recovery, don't dirty the page.  We can
+			 * set the hint, just not dirty the page as a result so the hint
+			 * is lost when we evict the page or shutdown.
+			 *
+			 * See src/backend/storage/page/README for longer discussion.
+			 */
+			if (RecoveryInProgress() ||
+				RelFileNodeSkippingWAL(bufHdr->tag.rnode))
+				return;
+
+			/*
+			 * If the block is already dirty because we either made a change
+			 * or set a hint already, then we don't need to write a full page
+			 * image.  Note that aggressive cleaning of blocks dirtied by hint
+			 * bit setting would increase the call rate. Bulk setting of hint
+			 * bits would reduce the call rate...
+			 *
+			 * We must issue the WAL record before we mark the buffer dirty.
+			 * Otherwise we might write the page before we write the WAL. That
+			 * causes a race condition, since a checkpoint might occur between
+			 * writing the WAL record and marking the buffer dirty. We solve
+			 * that with a kluge, but one that is already in use during
+			 * transaction commit to prevent race conditions. Basically, we
+			 * simply prevent the checkpoint WAL record from being written
+			 * until we have marked the buffer dirty. We don't start the
+			 * checkpoint flush until we have marked dirty, so our checkpoint
+			 * must flush the change to disk successfully or the checkpoint
+			 * never gets written, so crash recovery will fix.
+			 *
+			 * It's possible we may enter here without an xid, so it is
+			 * essential that CreateCheckpoint waits for virtual transactions
+			 * rather than full transactionids.
+			 */
+			Assert(!MyProc->delayChkpt);
+			MyProc->delayChkpt = true;
+			delayChkpt = true;
+			lsn = XLogSaveBufferForHint(buffer, buffer_std);
+		}
+
+		buf_state = LockBufHdr(bufHdr);
+
+		Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
+
+		if (!(buf_state & BM_DIRTY))
+		{
+			dirtied = true;		/* Means "will be dirtied by this action" */
+
+			/*
+			 * Set the page LSN if we wrote a backup block. We aren't supposed
+			 * to set this when only holding a share lock but as long as we
+			 * serialise it somehow we're OK. We choose to set LSN while
+			 * holding the buffer header lock, which causes any reader of an
+			 * LSN who holds only a share lock to also obtain a buffer header
+			 * lock before using PageGetLSN(), which is enforced in
+			 * BufferGetLSNAtomic().
+			 *
+			 * If checksums are enabled, you might think we should reset the
+			 * checksum here. That will happen when the page is written
+			 * sometime later in this checkpoint cycle.
+			 */
+			if (!XLogRecPtrIsInvalid(lsn))
+				PageSetLSN(page, lsn);
+		}
+
+		buf_state |= BM_DIRTY | BM_JUST_DIRTIED;
+		UnlockBufHdr(bufHdr, buf_state);
+
+		if (delayChkpt)
+			MyProc->delayChkpt = false;
+
+		if (dirtied)
+		{
+			VacuumPageDirty++;
+			pgBufferUsage.shared_blks_dirtied++;
+			if (VacuumCostActive)
+				VacuumCostBalance += VacuumCostPageDirty;
+		}
+	}
+}
+
+/*
+ * Release buffer content locks for shared buffers.
+ *
+ * Used to clean up after errors.
+ *
+ * Currently, we can expect that lwlock.c's LWLockReleaseAll() took care
+ * of releasing buffer content locks per se; the only thing we need to deal
+ * with here is clearing any PIN_COUNT request that was in progress.
+ */
+void
+UnlockBuffers(void)
+{
+	BufferDesc *buf = PinCountWaitBuf;
+
+	if (buf)
+	{
+		uint32		buf_state;
+
+		buf_state = LockBufHdr(buf);
+
+		/*
+		 * Don't complain if flag bit not set; it could have been reset but we
+		 * got a cancel/die interrupt before getting the signal.
+		 */
+		if ((buf_state & BM_PIN_COUNT_WAITER) != 0 &&
+			buf->wait_backend_pid == MyProcPid)
+			buf_state &= ~BM_PIN_COUNT_WAITER;
+
+		UnlockBufHdr(buf, buf_state);
+
+		PinCountWaitBuf = NULL;
+	}
+}
+
+/*
+ * Acquire or release the content_lock for the buffer.
+ */
+void
+LockBuffer(Buffer buffer, int mode)
+{
+	BufferDesc *buf;
+
+	Assert(BufferIsPinned(buffer));
+	if (BufferIsLocal(buffer))
+		return;					/* local buffers need no lock */
+
+	buf = GetBufferDescriptor(buffer - 1);
+
+	if (mode == BUFFER_LOCK_UNLOCK)
+		LWLockRelease(BufferDescriptorGetContentLock(buf));
+	else if (mode == BUFFER_LOCK_SHARE)
+		LWLockAcquire(BufferDescriptorGetContentLock(buf), LW_SHARED);
+	else if (mode == BUFFER_LOCK_EXCLUSIVE)
+		LWLockAcquire(BufferDescriptorGetContentLock(buf), LW_EXCLUSIVE);
+	else
+		elog(ERROR, "unrecognized buffer lock mode: %d", mode);
+}
+
+/*
+ * Acquire the content_lock for the buffer, but only if we don't have to wait.
+ *
+ * This assumes the caller wants BUFFER_LOCK_EXCLUSIVE mode.
+ */
+bool
+ConditionalLockBuffer(Buffer buffer)
+{
+	BufferDesc *buf;
+
+	Assert(BufferIsPinned(buffer));
+	if (BufferIsLocal(buffer))
+		return true;			/* act as though we got it */
+
+	buf = GetBufferDescriptor(buffer - 1);
+
+	return LWLockConditionalAcquire(BufferDescriptorGetContentLock(buf),
+									LW_EXCLUSIVE);
+}
+
+/*
+ * LockBufferForCleanup - lock a buffer in preparation for deleting items
+ *
+ * Items may be deleted from a disk page only when the caller (a) holds an
+ * exclusive lock on the buffer and (b) has observed that no other backend
+ * holds a pin on the buffer.  If there is a pin, then the other backend
+ * might have a pointer into the buffer (for example, a heapscan reference
+ * to an item --- see README for more details).  It's OK if a pin is added
+ * after the cleanup starts, however; the newly-arrived backend will be
+ * unable to look at the page until we release the exclusive lock.
+ *
+ * To implement this protocol, a would-be deleter must pin the buffer and
+ * then call LockBufferForCleanup().  LockBufferForCleanup() is similar to
+ * LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE), except that it loops until
+ * it has successfully observed pin count = 1.
+ */
+void
+LockBufferForCleanup(Buffer buffer)
+{
+	BufferDesc *bufHdr;
+	char	   *new_status = NULL;
+	TimestampTz waitStart = 0;
+	bool		logged_recovery_conflict = false;
+
+	Assert(BufferIsPinned(buffer));
+	Assert(PinCountWaitBuf == NULL);
+
+	if (BufferIsLocal(buffer))
+	{
+		/* There should be exactly one pin */
+		if (LocalRefCount[-buffer - 1] != 1)
+			elog(ERROR, "incorrect local pin count: %d",
+				 LocalRefCount[-buffer - 1]);
+		/* Nobody else to wait for */
+		return;
+	}
+
+	/* There should be exactly one local pin */
+	if (GetPrivateRefCount(buffer) != 1)
+		elog(ERROR, "incorrect local pin count: %d",
+			 GetPrivateRefCount(buffer));
+
+	bufHdr = GetBufferDescriptor(buffer - 1);
+
+	for (;;)
+	{
+		uint32		buf_state;
+
+		/* Try to acquire lock */
+		LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
+		buf_state = LockBufHdr(bufHdr);
+
+		Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
+		if (BUF_STATE_GET_REFCOUNT(buf_state) == 1)
+		{
+			/* Successfully acquired exclusive lock with pincount 1 */
+			UnlockBufHdr(bufHdr, buf_state);
+
+			/*
+			 * Emit the log message if recovery conflict on buffer pin was
+			 * resolved but the startup process waited longer than
+			 * deadlock_timeout for it.
+			 */
+			if (logged_recovery_conflict)
+				LogRecoveryConflict(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN,
+									waitStart, GetCurrentTimestamp(),
+									NULL, false);
+
+			/* Report change to non-waiting status */
+			if (new_status)
+			{
+				set_ps_display(new_status);
+				pfree(new_status);
+			}
+			return;
+		}
+		/* Failed, so mark myself as waiting for pincount 1 */
+		if (buf_state & BM_PIN_COUNT_WAITER)
+		{
+			UnlockBufHdr(bufHdr, buf_state);
+			LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
+			elog(ERROR, "multiple backends attempting to wait for pincount 1");
+		}
+		bufHdr->wait_backend_pid = MyProcPid;
+		PinCountWaitBuf = bufHdr;
+		buf_state |= BM_PIN_COUNT_WAITER;
+		UnlockBufHdr(bufHdr, buf_state);
+		LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
+
+		/* Wait to be signaled by UnpinBuffer() */
+		if (InHotStandby)
+		{
+			/* Report change to waiting status */
+			if (update_process_title && new_status == NULL)
+			{
+				const char *old_status;
+				int			len;
+
+				old_status = get_ps_display(&len);
+				new_status = (char *) palloc(len + 8 + 1);
+				memcpy(new_status, old_status, len);
+				strcpy(new_status + len, " waiting");
+				set_ps_display(new_status);
+				new_status[len] = '\0'; /* truncate off " waiting" */
+			}
+
+			/*
+			 * Emit the log message if the startup process is waiting longer
+			 * than deadlock_timeout for recovery conflict on buffer pin.
+			 *
+			 * Skip this if first time through because the startup process has
+			 * not started waiting yet in this case. So, the wait start
+			 * timestamp is set after this logic.
+			 */
+			if (waitStart != 0 && !logged_recovery_conflict)
+			{
+				TimestampTz now = GetCurrentTimestamp();
+
+				if (TimestampDifferenceExceeds(waitStart, now,
+											   DeadlockTimeout))
+				{
+					LogRecoveryConflict(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN,
+										waitStart, now, NULL, true);
+					logged_recovery_conflict = true;
+				}
+			}
+
+			/*
+			 * Set the wait start timestamp if logging is enabled and first
+			 * time through.
+			 */
+			if (log_recovery_conflict_waits && waitStart == 0)
+				waitStart = GetCurrentTimestamp();
+
+			/* Publish the bufid that Startup process waits on */
+			SetStartupBufferPinWaitBufId(buffer - 1);
+			/* Set alarm and then wait to be signaled by UnpinBuffer() */
+			ResolveRecoveryConflictWithBufferPin();
+			/* Reset the published bufid */
+			SetStartupBufferPinWaitBufId(-1);
+		}
+		else
+			ProcWaitForSignal(PG_WAIT_BUFFER_PIN);
+
+		/*
+		 * Remove flag marking us as waiter. Normally this will not be set
+		 * anymore, but ProcWaitForSignal() can return for other signals as
+		 * well.  We take care to only reset the flag if we're the waiter, as
+		 * theoretically another backend could have started waiting. That's
+		 * impossible with the current usages due to table level locking, but
+		 * better be safe.
+		 */
+		buf_state = LockBufHdr(bufHdr);
+		if ((buf_state & BM_PIN_COUNT_WAITER) != 0 &&
+			bufHdr->wait_backend_pid == MyProcPid)
+			buf_state &= ~BM_PIN_COUNT_WAITER;
+		UnlockBufHdr(bufHdr, buf_state);
+
+		PinCountWaitBuf = NULL;
+		/* Loop back and try again */
+	}
+}
+
+/*
+ * Check called from RecoveryConflictInterrupt handler when Startup
+ * process requests cancellation of all pin holders that are blocking it.
+ */
+bool
+HoldingBufferPinThatDelaysRecovery(void)
+{
+	int			bufid = GetStartupBufferPinWaitBufId();
+
+	/*
+	 * If we get woken slowly then it's possible that the Startup process was
+	 * already woken by other backends before we got here. Also possible that
+	 * we get here by multiple interrupts or interrupts at inappropriate
+	 * times, so make sure we do nothing if the bufid is not set.
+	 */
+	if (bufid < 0)
+		return false;
+
+	if (GetPrivateRefCount(bufid + 1) > 0)
+		return true;
+
+	return false;
+}
+
+/*
+ * ConditionalLockBufferForCleanup - as above, but don't wait to get the lock
+ *
+ * We won't loop, but just check once to see if the pin count is OK.  If
+ * not, return false with no lock held.
+ */
+bool
+ConditionalLockBufferForCleanup(Buffer buffer)
+{
+	BufferDesc *bufHdr;
+	uint32		buf_state,
+				refcount;
+
+	Assert(BufferIsValid(buffer));
+
+	if (BufferIsLocal(buffer))
+	{
+		refcount = LocalRefCount[-buffer - 1];
+		/* There should be exactly one pin */
+		Assert(refcount > 0);
+		if (refcount != 1)
+			return false;
+		/* Nobody else to wait for */
+		return true;
+	}
+
+	/* There should be exactly one local pin */
+	refcount = GetPrivateRefCount(buffer);
+	Assert(refcount);
+	if (refcount != 1)
+		return false;
+
+	/* Try to acquire lock */
+	if (!ConditionalLockBuffer(buffer))
+		return false;
+
+	bufHdr = GetBufferDescriptor(buffer - 1);
+	buf_state = LockBufHdr(bufHdr);
+	refcount = BUF_STATE_GET_REFCOUNT(buf_state);
+
+	Assert(refcount > 0);
+	if (refcount == 1)
+	{
+		/* Successfully acquired exclusive lock with pincount 1 */
+		UnlockBufHdr(bufHdr, buf_state);
+		return true;
+	}
+
+	/* Failed, so release the lock */
+	UnlockBufHdr(bufHdr, buf_state);
+	LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
+	return false;
+}
+
+/*
+ * IsBufferCleanupOK - as above, but we already have the lock
+ *
+ * Check whether it's OK to perform cleanup on a buffer we've already
+ * locked.  If we observe that the pin count is 1, our exclusive lock
+ * happens to be a cleanup lock, and we can proceed with anything that
+ * would have been allowable had we sought a cleanup lock originally.
+ */
+bool
+IsBufferCleanupOK(Buffer buffer)
+{
+	BufferDesc *bufHdr;
+	uint32		buf_state;
+
+	Assert(BufferIsValid(buffer));
+
+	if (BufferIsLocal(buffer))
+	{
+		/* There should be exactly one pin */
+		if (LocalRefCount[-buffer - 1] != 1)
+			return false;
+		/* Nobody else to wait for */
+		return true;
+	}
+
+	/* There should be exactly one local pin */
+	if (GetPrivateRefCount(buffer) != 1)
+		return false;
+
+	bufHdr = GetBufferDescriptor(buffer - 1);
+
+	/* caller must hold exclusive lock on buffer */
+	Assert(LWLockHeldByMeInMode(BufferDescriptorGetContentLock(bufHdr),
+								LW_EXCLUSIVE));
+
+	buf_state = LockBufHdr(bufHdr);
+
+	Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
+	if (BUF_STATE_GET_REFCOUNT(buf_state) == 1)
+	{
+		/* pincount is OK. */
+		UnlockBufHdr(bufHdr, buf_state);
+		return true;
+	}
+
+	UnlockBufHdr(bufHdr, buf_state);
+	return false;
+}
+
+
+/*
+ *	Functions for buffer I/O handling
+ *
+ *	Note: We assume that nested buffer I/O never occurs.
+ *	i.e at most one BM_IO_IN_PROGRESS bit is set per proc.
+ *
+ *	Also note that these are used only for shared buffers, not local ones.
+ */
+
+/*
+ * WaitIO -- Block until the IO_IN_PROGRESS flag on 'buf' is cleared.
+ */
+static void
+WaitIO(BufferDesc *buf)
+{
+	ConditionVariable *cv = BufferDescriptorGetIOCV(buf);
+
+	ConditionVariablePrepareToSleep(cv);
+	for (;;)
+	{
+		uint32		buf_state;
+
+		/*
+		 * It may not be necessary to acquire the spinlock to check the flag
+		 * here, but since this test is essential for correctness, we'd better
+		 * play it safe.
+		 */
+		buf_state = LockBufHdr(buf);
+		UnlockBufHdr(buf, buf_state);
+
+		if (!(buf_state & BM_IO_IN_PROGRESS))
+			break;
+		ConditionVariableSleep(cv, WAIT_EVENT_BUFFER_IO);
+	}
+	ConditionVariableCancelSleep();
+}
+
+/*
+ * StartBufferIO: begin I/O on this buffer
+ *	(Assumptions)
+ *	My process is executing no IO
+ *	The buffer is Pinned
+ *
+ * In some scenarios there are race conditions in which multiple backends
+ * could attempt the same I/O operation concurrently.  If someone else
+ * has already started I/O on this buffer then we will block on the
+ * I/O condition variable until he's done.
+ *
+ * Input operations are only attempted on buffers that are not BM_VALID,
+ * and output operations only on buffers that are BM_VALID and BM_DIRTY,
+ * so we can always tell if the work is already done.
+ *
+ * Returns true if we successfully marked the buffer as I/O busy,
+ * false if someone else already did the work.
+ */
+static bool
+StartBufferIO(BufferDesc *buf, bool forInput)
+{
+	uint32		buf_state;
+
+	Assert(!InProgressBuf);
+
+	for (;;)
+	{
+		buf_state = LockBufHdr(buf);
+
+		if (!(buf_state & BM_IO_IN_PROGRESS))
+			break;
+		UnlockBufHdr(buf, buf_state);
+		WaitIO(buf);
+	}
+
+	/* Once we get here, there is definitely no I/O active on this buffer */
+
+	if (forInput ? (buf_state & BM_VALID) : !(buf_state & BM_DIRTY))
+	{
+		/* someone else already did the I/O */
+		UnlockBufHdr(buf, buf_state);
+		return false;
+	}
+
+	buf_state |= BM_IO_IN_PROGRESS;
+	UnlockBufHdr(buf, buf_state);
+
+	InProgressBuf = buf;
+	IsForInput = forInput;
+
+	return true;
+}
+
+/*
+ * TerminateBufferIO: release a buffer we were doing I/O on
+ *	(Assumptions)
+ *	My process is executing IO for the buffer
+ *	BM_IO_IN_PROGRESS bit is set for the buffer
+ *	The buffer is Pinned
+ *
+ * If clear_dirty is true and BM_JUST_DIRTIED is not set, we clear the
+ * buffer's BM_DIRTY flag.  This is appropriate when terminating a
+ * successful write.  The check on BM_JUST_DIRTIED is necessary to avoid
+ * marking the buffer clean if it was re-dirtied while we were writing.
+ *
+ * set_flag_bits gets ORed into the buffer's flags.  It must include
+ * BM_IO_ERROR in a failure case.  For successful completion it could
+ * be 0, or BM_VALID if we just finished reading in the page.
+ */
+static void
+TerminateBufferIO(BufferDesc *buf, bool clear_dirty, uint32 set_flag_bits)
+{
+	uint32		buf_state;
+
+	Assert(buf == InProgressBuf);
+
+	buf_state = LockBufHdr(buf);
+
+	Assert(buf_state & BM_IO_IN_PROGRESS);
+
+	buf_state &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR);
+	if (clear_dirty && !(buf_state & BM_JUST_DIRTIED))
+		buf_state &= ~(BM_DIRTY | BM_CHECKPOINT_NEEDED);
+
+	buf_state |= set_flag_bits;
+	UnlockBufHdr(buf, buf_state);
+
+	InProgressBuf = NULL;
+
+	ConditionVariableBroadcast(BufferDescriptorGetIOCV(buf));
+}
+
+/*
+ * AbortBufferIO: Clean up any active buffer I/O after an error.
+ *
+ *	All LWLocks we might have held have been released,
+ *	but we haven't yet released buffer pins, so the buffer is still pinned.
+ *
+ *	If I/O was in progress, we always set BM_IO_ERROR, even though it's
+ *	possible the error condition wasn't related to the I/O.
+ */
+void
+AbortBufferIO(void)
+{
+	BufferDesc *buf = InProgressBuf;
+
+	if (buf)
+	{
+		uint32		buf_state;
+
+		buf_state = LockBufHdr(buf);
+		Assert(buf_state & BM_IO_IN_PROGRESS);
+		if (IsForInput)
+		{
+			Assert(!(buf_state & BM_DIRTY));
+
+			/* We'd better not think buffer is valid yet */
+			Assert(!(buf_state & BM_VALID));
+			UnlockBufHdr(buf, buf_state);
+		}
+		else
+		{
+			Assert(buf_state & BM_DIRTY);
+			UnlockBufHdr(buf, buf_state);
+			/* Issue notice if this is not the first failure... */
+			if (buf_state & BM_IO_ERROR)
+			{
+				/* Buffer is pinned, so we can read tag without spinlock */
+				char	   *path;
+
+				path = relpathperm(buf->tag.rnode, buf->tag.forkNum);
+				ereport(WARNING,
+						(errcode(ERRCODE_IO_ERROR),
+						 errmsg("could not write block %u of %s",
+								buf->tag.blockNum, path),
+						 errdetail("Multiple failures --- write error might be permanent.")));
+				pfree(path);
+			}
+		}
+		TerminateBufferIO(buf, false, BM_IO_ERROR);
+	}
+}
+
+/*
+ * Error context callback for errors occurring during shared buffer writes.
+ */
+static void
+shared_buffer_write_error_callback(void *arg)
+{
+	BufferDesc *bufHdr = (BufferDesc *) arg;
+
+	/* Buffer is pinned, so we can read the tag without locking the spinlock */
+	if (bufHdr != NULL)
+	{
+		char	   *path = relpathperm(bufHdr->tag.rnode, bufHdr->tag.forkNum);
+
+		errcontext("writing block %u of relation %s",
+				   bufHdr->tag.blockNum, path);
+		pfree(path);
+	}
+}
+
+/*
+ * Error context callback for errors occurring during local buffer writes.
+ */
+static void
+local_buffer_write_error_callback(void *arg)
+{
+	BufferDesc *bufHdr = (BufferDesc *) arg;
+
+	if (bufHdr != NULL)
+	{
+		char	   *path = relpathbackend(bufHdr->tag.rnode, MyBackendId,
+										  bufHdr->tag.forkNum);
+
+		errcontext("writing block %u of relation %s",
+				   bufHdr->tag.blockNum, path);
+		pfree(path);
+	}
+}
+
+/*
+ * RelFileNode qsort/bsearch comparator; see RelFileNodeEquals.
+ */
+static int
+rnode_comparator(const void *p1, const void *p2)
+{
+	RelFileNode n1 = *(const RelFileNode *) p1;
+	RelFileNode n2 = *(const RelFileNode *) p2;
+
+	if (n1.relNode < n2.relNode)
+		return -1;
+	else if (n1.relNode > n2.relNode)
+		return 1;
+
+	if (n1.dbNode < n2.dbNode)
+		return -1;
+	else if (n1.dbNode > n2.dbNode)
+		return 1;
+
+	if (n1.spcNode < n2.spcNode)
+		return -1;
+	else if (n1.spcNode > n2.spcNode)
+		return 1;
+	else
+		return 0;
+}
+
+/*
+ * Lock buffer header - set BM_LOCKED in buffer state.
+ */
+uint32
+LockBufHdr(BufferDesc *desc)
+{
+	SpinDelayStatus delayStatus;
+	uint32		old_buf_state;
+
+	init_local_spin_delay(&delayStatus);
+
+	while (true)
+	{
+		/* set BM_LOCKED flag */
+		old_buf_state = pg_atomic_fetch_or_u32(&desc->state, BM_LOCKED);
+		/* if it wasn't set before we're OK */
+		if (!(old_buf_state & BM_LOCKED))
+			break;
+		perform_spin_delay(&delayStatus);
+	}
+	finish_spin_delay(&delayStatus);
+	return old_buf_state | BM_LOCKED;
+}
+
+/*
+ * Wait until the BM_LOCKED flag isn't set anymore and return the buffer's
+ * state at that point.
+ *
+ * Obviously the buffer could be locked by the time the value is returned, so
+ * this is primarily useful in CAS style loops.
+ */
+static uint32
+WaitBufHdrUnlocked(BufferDesc *buf)
+{
+	SpinDelayStatus delayStatus;
+	uint32		buf_state;
+
+	init_local_spin_delay(&delayStatus);
+
+	buf_state = pg_atomic_read_u32(&buf->state);
+
+	while (buf_state & BM_LOCKED)
+	{
+		perform_spin_delay(&delayStatus);
+		buf_state = pg_atomic_read_u32(&buf->state);
+	}
+
+	finish_spin_delay(&delayStatus);
+
+	return buf_state;
+}
+
+/*
+ * BufferTag comparator.
+ */
+static inline int
+buffertag_comparator(const BufferTag *ba, const BufferTag *bb)
+{
+	int			ret;
+
+	ret = rnode_comparator(&ba->rnode, &bb->rnode);
+
+	if (ret != 0)
+		return ret;
+
+	if (ba->forkNum < bb->forkNum)
+		return -1;
+	if (ba->forkNum > bb->forkNum)
+		return 1;
+
+	if (ba->blockNum < bb->blockNum)
+		return -1;
+	if (ba->blockNum > bb->blockNum)
+		return 1;
+
+	return 0;
+}
+
+/*
+ * Comparator determining the writeout order in a checkpoint.
+ *
+ * It is important that tablespaces are compared first, the logic balancing
+ * writes between tablespaces relies on it.
+ */
+static inline int
+ckpt_buforder_comparator(const CkptSortItem *a, const CkptSortItem *b)
+{
+	/* compare tablespace */
+	if (a->tsId < b->tsId)
+		return -1;
+	else if (a->tsId > b->tsId)
+		return 1;
+	/* compare relation */
+	if (a->relNode < b->relNode)
+		return -1;
+	else if (a->relNode > b->relNode)
+		return 1;
+	/* compare fork */
+	else if (a->forkNum < b->forkNum)
+		return -1;
+	else if (a->forkNum > b->forkNum)
+		return 1;
+	/* compare block number */
+	else if (a->blockNum < b->blockNum)
+		return -1;
+	else if (a->blockNum > b->blockNum)
+		return 1;
+	/* equal page IDs are unlikely, but not impossible */
+	return 0;
+}
+
+/*
+ * Comparator for a Min-Heap over the per-tablespace checkpoint completion
+ * progress.
+ */
+static int
+ts_ckpt_progress_comparator(Datum a, Datum b, void *arg)
+{
+	CkptTsStatus *sa = (CkptTsStatus *) a;
+	CkptTsStatus *sb = (CkptTsStatus *) b;
+
+	/* we want a min-heap, so return 1 for the a < b */
+	if (sa->progress < sb->progress)
+		return 1;
+	else if (sa->progress == sb->progress)
+		return 0;
+	else
+		return -1;
+}
+
+/*
+ * Initialize a writeback context, discarding potential previous state.
+ *
+ * *max_pending is a pointer instead of an immediate value, so the coalesce
+ * limits can easily changed by the GUC mechanism, and so calling code does
+ * not have to check the current configuration. A value of 0 means that no
+ * writeback control will be performed.
+ */
+void
+WritebackContextInit(WritebackContext *context, int *max_pending)
+{
+	Assert(*max_pending <= WRITEBACK_MAX_PENDING_FLUSHES);
+
+	context->max_pending = max_pending;
+	context->nr_pending = 0;
+}
+
+/*
+ * Add buffer to list of pending writeback requests.
+ */
+void
+ScheduleBufferTagForWriteback(WritebackContext *context, BufferTag *tag)
+{
+	PendingWriteback *pending;
+
+	/*
+	 * Add buffer to the pending writeback array, unless writeback control is
+	 * disabled.
+	 */
+	if (*context->max_pending > 0)
+	{
+		Assert(*context->max_pending <= WRITEBACK_MAX_PENDING_FLUSHES);
+
+		pending = &context->pending_writebacks[context->nr_pending++];
+
+		pending->tag = *tag;
+	}
+
+	/*
+	 * Perform pending flushes if the writeback limit is exceeded. This
+	 * includes the case where previously an item has been added, but control
+	 * is now disabled.
+	 */
+	if (context->nr_pending >= *context->max_pending)
+		IssuePendingWritebacks(context);
+}
+
+#define ST_SORT sort_pending_writebacks
+#define ST_ELEMENT_TYPE PendingWriteback
+#define ST_COMPARE(a, b) buffertag_comparator(&a->tag, &b->tag)
+#define ST_SCOPE static
+#define ST_DEFINE
+#include <lib/sort_template.h>
+
+/*
+ * Issue all pending writeback requests, previously scheduled with
+ * ScheduleBufferTagForWriteback, to the OS.
+ *
+ * Because this is only used to improve the OSs IO scheduling we try to never
+ * error out - it's just a hint.
+ */
+void
+IssuePendingWritebacks(WritebackContext *context)
+{
+	int			i;
+
+	if (context->nr_pending == 0)
+		return;
+
+	/*
+	 * Executing the writes in-order can make them a lot faster, and allows to
+	 * merge writeback requests to consecutive blocks into larger writebacks.
+	 */
+	sort_pending_writebacks(context->pending_writebacks, context->nr_pending);
+
+	/*
+	 * Coalesce neighbouring writes, but nothing else. For that we iterate
+	 * through the, now sorted, array of pending flushes, and look forward to
+	 * find all neighbouring (or identical) writes.
+	 */
+	for (i = 0; i < context->nr_pending; i++)
+	{
+		PendingWriteback *cur;
+		PendingWriteback *next;
+		SMgrRelation reln;
+		int			ahead;
+		BufferTag	tag;
+		Size		nblocks = 1;
+
+		cur = &context->pending_writebacks[i];
+		tag = cur->tag;
+
+		/*
+		 * Peek ahead, into following writeback requests, to see if they can
+		 * be combined with the current one.
+		 */
+		for (ahead = 0; i + ahead + 1 < context->nr_pending; ahead++)
+		{
+			next = &context->pending_writebacks[i + ahead + 1];
+
+			/* different file, stop */
+			if (!RelFileNodeEquals(cur->tag.rnode, next->tag.rnode) ||
+				cur->tag.forkNum != next->tag.forkNum)
+				break;
+
+			/* ok, block queued twice, skip */
+			if (cur->tag.blockNum == next->tag.blockNum)
+				continue;
+
+			/* only merge consecutive writes */
+			if (cur->tag.blockNum + 1 != next->tag.blockNum)
+				break;
+
+			nblocks++;
+			cur = next;
+		}
+
+		i += ahead;
+
+		/* and finally tell the kernel to write the data to storage */
+		reln = smgropen(tag.rnode, InvalidBackendId);
+		smgrwriteback(reln, tag.forkNum, tag.blockNum, nblocks);
+	}
+
+	context->nr_pending = 0;
+}
+
+
+/*
+ * Implement slower/larger portions of TestForOldSnapshot
+ *
+ * Smaller/faster portions are put inline, but the entire set of logic is too
+ * big for that.
+ */
+void
+TestForOldSnapshot_impl(Snapshot snapshot, Relation relation)
+{
+	if (RelationAllowsEarlyPruning(relation)
+		&& (snapshot)->whenTaken < GetOldSnapshotThresholdTimestamp())
+		ereport(ERROR,
+				(errcode(ERRCODE_SNAPSHOT_TOO_OLD),
+				 errmsg("snapshot too old")));
+}
diff --git a/src/backend/storage/buffer/freelist.c b/src/backend/storage/buffer/freelist.c
new file mode 100644
index 0000000..6be8047
--- /dev/null
+++ b/src/backend/storage/buffer/freelist.c
@@ -0,0 +1,704 @@
+/*-------------------------------------------------------------------------
+ *
+ * freelist.c
+ *	  routines for managing the buffer pool's replacement strategy.
+ *
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/storage/buffer/freelist.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "port/atomics.h"
+#include "storage/buf_internals.h"
+#include "storage/bufmgr.h"
+#include "storage/proc.h"
+
+#define INT_ACCESS_ONCE(var)	((int)(*((volatile int *)&(var))))
+
+
+/*
+ * The shared freelist control information.
+ */
+typedef struct
+{
+	/* Spinlock: protects the values below */
+	slock_t		buffer_strategy_lock;
+
+	/*
+	 * Clock sweep hand: index of next buffer to consider grabbing. Note that
+	 * this isn't a concrete buffer - we only ever increase the value. So, to
+	 * get an actual buffer, it needs to be used modulo NBuffers.
+	 */
+	pg_atomic_uint32 nextVictimBuffer;
+
+	int			firstFreeBuffer;	/* Head of list of unused buffers */
+	int			lastFreeBuffer; /* Tail of list of unused buffers */
+
+	/*
+	 * NOTE: lastFreeBuffer is undefined when firstFreeBuffer is -1 (that is,
+	 * when the list is empty)
+	 */
+
+	/*
+	 * Statistics.  These counters should be wide enough that they can't
+	 * overflow during a single bgwriter cycle.
+	 */
+	uint32		completePasses; /* Complete cycles of the clock sweep */
+	pg_atomic_uint32 numBufferAllocs;	/* Buffers allocated since last reset */
+
+	/*
+	 * Bgworker process to be notified upon activity or -1 if none. See
+	 * StrategyNotifyBgWriter.
+	 */
+	int			bgwprocno;
+} BufferStrategyControl;
+
+/* Pointers to shared state */
+static BufferStrategyControl *StrategyControl = NULL;
+
+/*
+ * Private (non-shared) state for managing a ring of shared buffers to re-use.
+ * This is currently the only kind of BufferAccessStrategy object, but someday
+ * we might have more kinds.
+ */
+typedef struct BufferAccessStrategyData
+{
+	/* Overall strategy type */
+	BufferAccessStrategyType btype;
+	/* Number of elements in buffers[] array */
+	int			ring_size;
+
+	/*
+	 * Index of the "current" slot in the ring, ie, the one most recently
+	 * returned by GetBufferFromRing.
+	 */
+	int			current;
+
+	/*
+	 * True if the buffer just returned by StrategyGetBuffer had been in the
+	 * ring already.
+	 */
+	bool		current_was_in_ring;
+
+	/*
+	 * Array of buffer numbers.  InvalidBuffer (that is, zero) indicates we
+	 * have not yet selected a buffer for this ring slot.  For allocation
+	 * simplicity this is palloc'd together with the fixed fields of the
+	 * struct.
+	 */
+	Buffer		buffers[FLEXIBLE_ARRAY_MEMBER];
+}			BufferAccessStrategyData;
+
+
+/* Prototypes for internal functions */
+static BufferDesc *GetBufferFromRing(BufferAccessStrategy strategy,
+									 uint32 *buf_state);
+static void AddBufferToRing(BufferAccessStrategy strategy,
+							BufferDesc *buf);
+
+/*
+ * ClockSweepTick - Helper routine for StrategyGetBuffer()
+ *
+ * Move the clock hand one buffer ahead of its current position and return the
+ * id of the buffer now under the hand.
+ */
+static inline uint32
+ClockSweepTick(void)
+{
+	uint32		victim;
+
+	/*
+	 * Atomically move hand ahead one buffer - if there's several processes
+	 * doing this, this can lead to buffers being returned slightly out of
+	 * apparent order.
+	 */
+	victim =
+		pg_atomic_fetch_add_u32(&StrategyControl->nextVictimBuffer, 1);
+
+	if (victim >= NBuffers)
+	{
+		uint32		originalVictim = victim;
+
+		/* always wrap what we look up in BufferDescriptors */
+		victim = victim % NBuffers;
+
+		/*
+		 * If we're the one that just caused a wraparound, force
+		 * completePasses to be incremented while holding the spinlock. We
+		 * need the spinlock so StrategySyncStart() can return a consistent
+		 * value consisting of nextVictimBuffer and completePasses.
+		 */
+		if (victim == 0)
+		{
+			uint32		expected;
+			uint32		wrapped;
+			bool		success = false;
+
+			expected = originalVictim + 1;
+
+			while (!success)
+			{
+				/*
+				 * Acquire the spinlock while increasing completePasses. That
+				 * allows other readers to read nextVictimBuffer and
+				 * completePasses in a consistent manner which is required for
+				 * StrategySyncStart().  In theory delaying the increment
+				 * could lead to an overflow of nextVictimBuffers, but that's
+				 * highly unlikely and wouldn't be particularly harmful.
+				 */
+				SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
+
+				wrapped = expected % NBuffers;
+
+				success = pg_atomic_compare_exchange_u32(&StrategyControl->nextVictimBuffer,
+														 &expected, wrapped);
+				if (success)
+					StrategyControl->completePasses++;
+				SpinLockRelease(&StrategyControl->buffer_strategy_lock);
+			}
+		}
+	}
+	return victim;
+}
+
+/*
+ * have_free_buffer -- a lockless check to see if there is a free buffer in
+ *					   buffer pool.
+ *
+ * If the result is true that will become stale once free buffers are moved out
+ * by other operations, so the caller who strictly want to use a free buffer
+ * should not call this.
+ */
+bool
+have_free_buffer(void)
+{
+	if (StrategyControl->firstFreeBuffer >= 0)
+		return true;
+	else
+		return false;
+}
+
+/*
+ * StrategyGetBuffer
+ *
+ *	Called by the bufmgr to get the next candidate buffer to use in
+ *	BufferAlloc(). The only hard requirement BufferAlloc() has is that
+ *	the selected buffer must not currently be pinned by anyone.
+ *
+ *	strategy is a BufferAccessStrategy object, or NULL for default strategy.
+ *
+ *	To ensure that no one else can pin the buffer before we do, we must
+ *	return the buffer with the buffer header spinlock still held.
+ */
+BufferDesc *
+StrategyGetBuffer(BufferAccessStrategy strategy, uint32 *buf_state)
+{
+	BufferDesc *buf;
+	int			bgwprocno;
+	int			trycounter;
+	uint32		local_buf_state;	/* to avoid repeated (de-)referencing */
+
+	/*
+	 * If given a strategy object, see whether it can select a buffer. We
+	 * assume strategy objects don't need buffer_strategy_lock.
+	 */
+	if (strategy != NULL)
+	{
+		buf = GetBufferFromRing(strategy, buf_state);
+		if (buf != NULL)
+			return buf;
+	}
+
+	/*
+	 * If asked, we need to waken the bgwriter. Since we don't want to rely on
+	 * a spinlock for this we force a read from shared memory once, and then
+	 * set the latch based on that value. We need to go through that length
+	 * because otherwise bgwprocno might be reset while/after we check because
+	 * the compiler might just reread from memory.
+	 *
+	 * This can possibly set the latch of the wrong process if the bgwriter
+	 * dies in the wrong moment. But since PGPROC->procLatch is never
+	 * deallocated the worst consequence of that is that we set the latch of
+	 * some arbitrary process.
+	 */
+	bgwprocno = INT_ACCESS_ONCE(StrategyControl->bgwprocno);
+	if (bgwprocno != -1)
+	{
+		/* reset bgwprocno first, before setting the latch */
+		StrategyControl->bgwprocno = -1;
+
+		/*
+		 * Not acquiring ProcArrayLock here which is slightly icky. It's
+		 * actually fine because procLatch isn't ever freed, so we just can
+		 * potentially set the wrong process' (or no process') latch.
+		 */
+		SetLatch(&ProcGlobal->allProcs[bgwprocno].procLatch);
+	}
+
+	/*
+	 * We count buffer allocation requests so that the bgwriter can estimate
+	 * the rate of buffer consumption.  Note that buffers recycled by a
+	 * strategy object are intentionally not counted here.
+	 */
+	pg_atomic_fetch_add_u32(&StrategyControl->numBufferAllocs, 1);
+
+	/*
+	 * First check, without acquiring the lock, whether there's buffers in the
+	 * freelist. Since we otherwise don't require the spinlock in every
+	 * StrategyGetBuffer() invocation, it'd be sad to acquire it here -
+	 * uselessly in most cases. That obviously leaves a race where a buffer is
+	 * put on the freelist but we don't see the store yet - but that's pretty
+	 * harmless, it'll just get used during the next buffer acquisition.
+	 *
+	 * If there's buffers on the freelist, acquire the spinlock to pop one
+	 * buffer of the freelist. Then check whether that buffer is usable and
+	 * repeat if not.
+	 *
+	 * Note that the freeNext fields are considered to be protected by the
+	 * buffer_strategy_lock not the individual buffer spinlocks, so it's OK to
+	 * manipulate them without holding the spinlock.
+	 */
+	if (StrategyControl->firstFreeBuffer >= 0)
+	{
+		while (true)
+		{
+			/* Acquire the spinlock to remove element from the freelist */
+			SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
+
+			if (StrategyControl->firstFreeBuffer < 0)
+			{
+				SpinLockRelease(&StrategyControl->buffer_strategy_lock);
+				break;
+			}
+
+			buf = GetBufferDescriptor(StrategyControl->firstFreeBuffer);
+			Assert(buf->freeNext != FREENEXT_NOT_IN_LIST);
+
+			/* Unconditionally remove buffer from freelist */
+			StrategyControl->firstFreeBuffer = buf->freeNext;
+			buf->freeNext = FREENEXT_NOT_IN_LIST;
+
+			/*
+			 * Release the lock so someone else can access the freelist while
+			 * we check out this buffer.
+			 */
+			SpinLockRelease(&StrategyControl->buffer_strategy_lock);
+
+			/*
+			 * If the buffer is pinned or has a nonzero usage_count, we cannot
+			 * use it; discard it and retry.  (This can only happen if VACUUM
+			 * put a valid buffer in the freelist and then someone else used
+			 * it before we got to it.  It's probably impossible altogether as
+			 * of 8.3, but we'd better check anyway.)
+			 */
+			local_buf_state = LockBufHdr(buf);
+			if (BUF_STATE_GET_REFCOUNT(local_buf_state) == 0
+				&& BUF_STATE_GET_USAGECOUNT(local_buf_state) == 0)
+			{
+				if (strategy != NULL)
+					AddBufferToRing(strategy, buf);
+				*buf_state = local_buf_state;
+				return buf;
+			}
+			UnlockBufHdr(buf, local_buf_state);
+
+		}
+	}
+
+	/* Nothing on the freelist, so run the "clock sweep" algorithm */
+	trycounter = NBuffers;
+	for (;;)
+	{
+		buf = GetBufferDescriptor(ClockSweepTick());
+
+		/*
+		 * If the buffer is pinned or has a nonzero usage_count, we cannot use
+		 * it; decrement the usage_count (unless pinned) and keep scanning.
+		 */
+		local_buf_state = LockBufHdr(buf);
+
+		if (BUF_STATE_GET_REFCOUNT(local_buf_state) == 0)
+		{
+			if (BUF_STATE_GET_USAGECOUNT(local_buf_state) != 0)
+			{
+				local_buf_state -= BUF_USAGECOUNT_ONE;
+
+				trycounter = NBuffers;
+			}
+			else
+			{
+				/* Found a usable buffer */
+				if (strategy != NULL)
+					AddBufferToRing(strategy, buf);
+				*buf_state = local_buf_state;
+				return buf;
+			}
+		}
+		else if (--trycounter == 0)
+		{
+			/*
+			 * We've scanned all the buffers without making any state changes,
+			 * so all the buffers are pinned (or were when we looked at them).
+			 * We could hope that someone will free one eventually, but it's
+			 * probably better to fail than to risk getting stuck in an
+			 * infinite loop.
+			 */
+			UnlockBufHdr(buf, local_buf_state);
+			elog(ERROR, "no unpinned buffers available");
+		}
+		UnlockBufHdr(buf, local_buf_state);
+	}
+}
+
+/*
+ * StrategyFreeBuffer: put a buffer on the freelist
+ */
+void
+StrategyFreeBuffer(BufferDesc *buf)
+{
+	SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
+
+	/*
+	 * It is possible that we are told to put something in the freelist that
+	 * is already in it; don't screw up the list if so.
+	 */
+	if (buf->freeNext == FREENEXT_NOT_IN_LIST)
+	{
+		buf->freeNext = StrategyControl->firstFreeBuffer;
+		if (buf->freeNext < 0)
+			StrategyControl->lastFreeBuffer = buf->buf_id;
+		StrategyControl->firstFreeBuffer = buf->buf_id;
+	}
+
+	SpinLockRelease(&StrategyControl->buffer_strategy_lock);
+}
+
+/*
+ * StrategySyncStart -- tell BufferSync where to start syncing
+ *
+ * The result is the buffer index of the best buffer to sync first.
+ * BufferSync() will proceed circularly around the buffer array from there.
+ *
+ * In addition, we return the completed-pass count (which is effectively
+ * the higher-order bits of nextVictimBuffer) and the count of recent buffer
+ * allocs if non-NULL pointers are passed.  The alloc count is reset after
+ * being read.
+ */
+int
+StrategySyncStart(uint32 *complete_passes, uint32 *num_buf_alloc)
+{
+	uint32		nextVictimBuffer;
+	int			result;
+
+	SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
+	nextVictimBuffer = pg_atomic_read_u32(&StrategyControl->nextVictimBuffer);
+	result = nextVictimBuffer % NBuffers;
+
+	if (complete_passes)
+	{
+		*complete_passes = StrategyControl->completePasses;
+
+		/*
+		 * Additionally add the number of wraparounds that happened before
+		 * completePasses could be incremented. C.f. ClockSweepTick().
+		 */
+		*complete_passes += nextVictimBuffer / NBuffers;
+	}
+
+	if (num_buf_alloc)
+	{
+		*num_buf_alloc = pg_atomic_exchange_u32(&StrategyControl->numBufferAllocs, 0);
+	}
+	SpinLockRelease(&StrategyControl->buffer_strategy_lock);
+	return result;
+}
+
+/*
+ * StrategyNotifyBgWriter -- set or clear allocation notification latch
+ *
+ * If bgwprocno isn't -1, the next invocation of StrategyGetBuffer will
+ * set that latch.  Pass -1 to clear the pending notification before it
+ * happens.  This feature is used by the bgwriter process to wake itself up
+ * from hibernation, and is not meant for anybody else to use.
+ */
+void
+StrategyNotifyBgWriter(int bgwprocno)
+{
+	/*
+	 * We acquire buffer_strategy_lock just to ensure that the store appears
+	 * atomic to StrategyGetBuffer.  The bgwriter should call this rather
+	 * infrequently, so there's no performance penalty from being safe.
+	 */
+	SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
+	StrategyControl->bgwprocno = bgwprocno;
+	SpinLockRelease(&StrategyControl->buffer_strategy_lock);
+}
+
+
+/*
+ * StrategyShmemSize
+ *
+ * estimate the size of shared memory used by the freelist-related structures.
+ *
+ * Note: for somewhat historical reasons, the buffer lookup hashtable size
+ * is also determined here.
+ */
+Size
+StrategyShmemSize(void)
+{
+	Size		size = 0;
+
+	/* size of lookup hash table ... see comment in StrategyInitialize */
+	size = add_size(size, BufTableShmemSize(NBuffers + NUM_BUFFER_PARTITIONS));
+
+	/* size of the shared replacement strategy control block */
+	size = add_size(size, MAXALIGN(sizeof(BufferStrategyControl)));
+
+	return size;
+}
+
+/*
+ * StrategyInitialize -- initialize the buffer cache replacement
+ *		strategy.
+ *
+ * Assumes: All of the buffers are already built into a linked list.
+ *		Only called by postmaster and only during initialization.
+ */
+void
+StrategyInitialize(bool init)
+{
+	bool		found;
+
+	/*
+	 * Initialize the shared buffer lookup hashtable.
+	 *
+	 * Since we can't tolerate running out of lookup table entries, we must be
+	 * sure to specify an adequate table size here.  The maximum steady-state
+	 * usage is of course NBuffers entries, but BufferAlloc() tries to insert
+	 * a new entry before deleting the old.  In principle this could be
+	 * happening in each partition concurrently, so we could need as many as
+	 * NBuffers + NUM_BUFFER_PARTITIONS entries.
+	 */
+	InitBufTable(NBuffers + NUM_BUFFER_PARTITIONS);
+
+	/*
+	 * Get or create the shared strategy control block
+	 */
+	StrategyControl = (BufferStrategyControl *)
+		ShmemInitStruct("Buffer Strategy Status",
+						sizeof(BufferStrategyControl),
+						&found);
+
+	if (!found)
+	{
+		/*
+		 * Only done once, usually in postmaster
+		 */
+		Assert(init);
+
+		SpinLockInit(&StrategyControl->buffer_strategy_lock);
+
+		/*
+		 * Grab the whole linked list of free buffers for our strategy. We
+		 * assume it was previously set up by InitBufferPool().
+		 */
+		StrategyControl->firstFreeBuffer = 0;
+		StrategyControl->lastFreeBuffer = NBuffers - 1;
+
+		/* Initialize the clock sweep pointer */
+		pg_atomic_init_u32(&StrategyControl->nextVictimBuffer, 0);
+
+		/* Clear statistics */
+		StrategyControl->completePasses = 0;
+		pg_atomic_init_u32(&StrategyControl->numBufferAllocs, 0);
+
+		/* No pending notification */
+		StrategyControl->bgwprocno = -1;
+	}
+	else
+		Assert(!init);
+}
+
+
+/* ----------------------------------------------------------------
+ *				Backend-private buffer ring management
+ * ----------------------------------------------------------------
+ */
+
+
+/*
+ * GetAccessStrategy -- create a BufferAccessStrategy object
+ *
+ * The object is allocated in the current memory context.
+ */
+BufferAccessStrategy
+GetAccessStrategy(BufferAccessStrategyType btype)
+{
+	BufferAccessStrategy strategy;
+	int			ring_size;
+
+	/*
+	 * Select ring size to use.  See buffer/README for rationales.
+	 *
+	 * Note: if you change the ring size for BAS_BULKREAD, see also
+	 * SYNC_SCAN_REPORT_INTERVAL in access/heap/syncscan.c.
+	 */
+	switch (btype)
+	{
+		case BAS_NORMAL:
+			/* if someone asks for NORMAL, just give 'em a "default" object */
+			return NULL;
+
+		case BAS_BULKREAD:
+			ring_size = 256 * 1024 / BLCKSZ;
+			break;
+		case BAS_BULKWRITE:
+			ring_size = 16 * 1024 * 1024 / BLCKSZ;
+			break;
+		case BAS_VACUUM:
+			ring_size = 256 * 1024 / BLCKSZ;
+			break;
+
+		default:
+			elog(ERROR, "unrecognized buffer access strategy: %d",
+				 (int) btype);
+			return NULL;		/* keep compiler quiet */
+	}
+
+	/* Make sure ring isn't an undue fraction of shared buffers */
+	ring_size = Min(NBuffers / 8, ring_size);
+
+	/* Allocate the object and initialize all elements to zeroes */
+	strategy = (BufferAccessStrategy)
+		palloc0(offsetof(BufferAccessStrategyData, buffers) +
+				ring_size * sizeof(Buffer));
+
+	/* Set fields that don't start out zero */
+	strategy->btype = btype;
+	strategy->ring_size = ring_size;
+
+	return strategy;
+}
+
+/*
+ * FreeAccessStrategy -- release a BufferAccessStrategy object
+ *
+ * A simple pfree would do at the moment, but we would prefer that callers
+ * don't assume that much about the representation of BufferAccessStrategy.
+ */
+void
+FreeAccessStrategy(BufferAccessStrategy strategy)
+{
+	/* don't crash if called on a "default" strategy */
+	if (strategy != NULL)
+		pfree(strategy);
+}
+
+/*
+ * GetBufferFromRing -- returns a buffer from the ring, or NULL if the
+ *		ring is empty.
+ *
+ * The bufhdr spin lock is held on the returned buffer.
+ */
+static BufferDesc *
+GetBufferFromRing(BufferAccessStrategy strategy, uint32 *buf_state)
+{
+	BufferDesc *buf;
+	Buffer		bufnum;
+	uint32		local_buf_state;	/* to avoid repeated (de-)referencing */
+
+
+	/* Advance to next ring slot */
+	if (++strategy->current >= strategy->ring_size)
+		strategy->current = 0;
+
+	/*
+	 * If the slot hasn't been filled yet, tell the caller to allocate a new
+	 * buffer with the normal allocation strategy.  He will then fill this
+	 * slot by calling AddBufferToRing with the new buffer.
+	 */
+	bufnum = strategy->buffers[strategy->current];
+	if (bufnum == InvalidBuffer)
+	{
+		strategy->current_was_in_ring = false;
+		return NULL;
+	}
+
+	/*
+	 * If the buffer is pinned we cannot use it under any circumstances.
+	 *
+	 * If usage_count is 0 or 1 then the buffer is fair game (we expect 1,
+	 * since our own previous usage of the ring element would have left it
+	 * there, but it might've been decremented by clock sweep since then). A
+	 * higher usage_count indicates someone else has touched the buffer, so we
+	 * shouldn't re-use it.
+	 */
+	buf = GetBufferDescriptor(bufnum - 1);
+	local_buf_state = LockBufHdr(buf);
+	if (BUF_STATE_GET_REFCOUNT(local_buf_state) == 0
+		&& BUF_STATE_GET_USAGECOUNT(local_buf_state) <= 1)
+	{
+		strategy->current_was_in_ring = true;
+		*buf_state = local_buf_state;
+		return buf;
+	}
+	UnlockBufHdr(buf, local_buf_state);
+
+	/*
+	 * Tell caller to allocate a new buffer with the normal allocation
+	 * strategy.  He'll then replace this ring element via AddBufferToRing.
+	 */
+	strategy->current_was_in_ring = false;
+	return NULL;
+}
+
+/*
+ * AddBufferToRing -- add a buffer to the buffer ring
+ *
+ * Caller must hold the buffer header spinlock on the buffer.  Since this
+ * is called with the spinlock held, it had better be quite cheap.
+ */
+static void
+AddBufferToRing(BufferAccessStrategy strategy, BufferDesc *buf)
+{
+	strategy->buffers[strategy->current] = BufferDescriptorGetBuffer(buf);
+}
+
+/*
+ * StrategyRejectBuffer -- consider rejecting a dirty buffer
+ *
+ * When a nondefault strategy is used, the buffer manager calls this function
+ * when it turns out that the buffer selected by StrategyGetBuffer needs to
+ * be written out and doing so would require flushing WAL too.  This gives us
+ * a chance to choose a different victim.
+ *
+ * Returns true if buffer manager should ask for a new victim, and false
+ * if this buffer should be written and re-used.
+ */
+bool
+StrategyRejectBuffer(BufferAccessStrategy strategy, BufferDesc *buf)
+{
+	/* We only do this in bulkread mode */
+	if (strategy->btype != BAS_BULKREAD)
+		return false;
+
+	/* Don't muck with behavior of normal buffer-replacement strategy */
+	if (!strategy->current_was_in_ring ||
+		strategy->buffers[strategy->current] != BufferDescriptorGetBuffer(buf))
+		return false;
+
+	/*
+	 * Remove the dirty buffer from the ring; necessary to prevent infinite
+	 * loop if all ring members are dirty.
+	 */
+	strategy->buffers[strategy->current] = InvalidBuffer;
+
+	return true;
+}
diff --git a/src/backend/storage/buffer/localbuf.c b/src/backend/storage/buffer/localbuf.c
new file mode 100644
index 0000000..04b3558
--- /dev/null
+++ b/src/backend/storage/buffer/localbuf.c
@@ -0,0 +1,596 @@
+/*-------------------------------------------------------------------------
+ *
+ * localbuf.c
+ *	  local buffer manager. Fast buffer manager for temporary tables,
+ *	  which never need to be WAL-logged or checkpointed, etc.
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994-5, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/storage/buffer/localbuf.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "access/parallel.h"
+#include "catalog/catalog.h"
+#include "executor/instrument.h"
+#include "storage/buf_internals.h"
+#include "storage/bufmgr.h"
+#include "utils/guc.h"
+#include "utils/memutils.h"
+#include "utils/resowner_private.h"
+
+
+/*#define LBDEBUG*/
+
+/* entry for buffer lookup hashtable */
+typedef struct
+{
+	BufferTag	key;			/* Tag of a disk page */
+	int			id;				/* Associated local buffer's index */
+} LocalBufferLookupEnt;
+
+/* Note: this macro only works on local buffers, not shared ones! */
+#define LocalBufHdrGetBlock(bufHdr) \
+	LocalBufferBlockPointers[-((bufHdr)->buf_id + 2)]
+
+int			NLocBuffer = 0;		/* until buffers are initialized */
+
+BufferDesc *LocalBufferDescriptors = NULL;
+Block	   *LocalBufferBlockPointers = NULL;
+int32	   *LocalRefCount = NULL;
+
+static int	nextFreeLocalBuf = 0;
+
+static HTAB *LocalBufHash = NULL;
+
+
+static void InitLocalBuffers(void);
+static Block GetLocalBufferStorage(void);
+
+
+/*
+ * PrefetchLocalBuffer -
+ *	  initiate asynchronous read of a block of a relation
+ *
+ * Do PrefetchBuffer's work for temporary relations.
+ * No-op if prefetching isn't compiled in.
+ */
+PrefetchBufferResult
+PrefetchLocalBuffer(SMgrRelation smgr, ForkNumber forkNum,
+					BlockNumber blockNum)
+{
+	PrefetchBufferResult result = {InvalidBuffer, false};
+	BufferTag	newTag;			/* identity of requested block */
+	LocalBufferLookupEnt *hresult;
+
+	INIT_BUFFERTAG(newTag, smgr->smgr_rnode.node, forkNum, blockNum);
+
+	/* Initialize local buffers if first request in this session */
+	if (LocalBufHash == NULL)
+		InitLocalBuffers();
+
+	/* See if the desired buffer already exists */
+	hresult = (LocalBufferLookupEnt *)
+		hash_search(LocalBufHash, (void *) &newTag, HASH_FIND, NULL);
+
+	if (hresult)
+	{
+		/* Yes, so nothing to do */
+		result.recent_buffer = -hresult->id - 1;
+	}
+	else
+	{
+#ifdef USE_PREFETCH
+		/* Not in buffers, so initiate prefetch */
+		smgrprefetch(smgr, forkNum, blockNum);
+		result.initiated_io = true;
+#endif							/* USE_PREFETCH */
+	}
+
+	return result;
+}
+
+
+/*
+ * LocalBufferAlloc -
+ *	  Find or create a local buffer for the given page of the given relation.
+ *
+ * API is similar to bufmgr.c's BufferAlloc, except that we do not need
+ * to do any locking since this is all local.   Also, IO_IN_PROGRESS
+ * does not get set.  Lastly, we support only default access strategy
+ * (hence, usage_count is always advanced).
+ */
+BufferDesc *
+LocalBufferAlloc(SMgrRelation smgr, ForkNumber forkNum, BlockNumber blockNum,
+				 bool *foundPtr)
+{
+	BufferTag	newTag;			/* identity of requested block */
+	LocalBufferLookupEnt *hresult;
+	BufferDesc *bufHdr;
+	int			b;
+	int			trycounter;
+	bool		found;
+	uint32		buf_state;
+
+	INIT_BUFFERTAG(newTag, smgr->smgr_rnode.node, forkNum, blockNum);
+
+	/* Initialize local buffers if first request in this session */
+	if (LocalBufHash == NULL)
+		InitLocalBuffers();
+
+	/* See if the desired buffer already exists */
+	hresult = (LocalBufferLookupEnt *)
+		hash_search(LocalBufHash, (void *) &newTag, HASH_FIND, NULL);
+
+	if (hresult)
+	{
+		b = hresult->id;
+		bufHdr = GetLocalBufferDescriptor(b);
+		Assert(BUFFERTAGS_EQUAL(bufHdr->tag, newTag));
+#ifdef LBDEBUG
+		fprintf(stderr, "LB ALLOC (%u,%d,%d) %d\n",
+				smgr->smgr_rnode.node.relNode, forkNum, blockNum, -b - 1);
+#endif
+		buf_state = pg_atomic_read_u32(&bufHdr->state);
+
+		/* this part is equivalent to PinBuffer for a shared buffer */
+		if (LocalRefCount[b] == 0)
+		{
+			if (BUF_STATE_GET_USAGECOUNT(buf_state) < BM_MAX_USAGE_COUNT)
+			{
+				buf_state += BUF_USAGECOUNT_ONE;
+				pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
+			}
+		}
+		LocalRefCount[b]++;
+		ResourceOwnerRememberBuffer(CurrentResourceOwner,
+									BufferDescriptorGetBuffer(bufHdr));
+		if (buf_state & BM_VALID)
+			*foundPtr = true;
+		else
+		{
+			/* Previous read attempt must have failed; try again */
+			*foundPtr = false;
+		}
+		return bufHdr;
+	}
+
+#ifdef LBDEBUG
+	fprintf(stderr, "LB ALLOC (%u,%d,%d) %d\n",
+			smgr->smgr_rnode.node.relNode, forkNum, blockNum,
+			-nextFreeLocalBuf - 1);
+#endif
+
+	/*
+	 * Need to get a new buffer.  We use a clock sweep algorithm (essentially
+	 * the same as what freelist.c does now...)
+	 */
+	trycounter = NLocBuffer;
+	for (;;)
+	{
+		b = nextFreeLocalBuf;
+
+		if (++nextFreeLocalBuf >= NLocBuffer)
+			nextFreeLocalBuf = 0;
+
+		bufHdr = GetLocalBufferDescriptor(b);
+
+		if (LocalRefCount[b] == 0)
+		{
+			buf_state = pg_atomic_read_u32(&bufHdr->state);
+
+			if (BUF_STATE_GET_USAGECOUNT(buf_state) > 0)
+			{
+				buf_state -= BUF_USAGECOUNT_ONE;
+				pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
+				trycounter = NLocBuffer;
+			}
+			else
+			{
+				/* Found a usable buffer */
+				LocalRefCount[b]++;
+				ResourceOwnerRememberBuffer(CurrentResourceOwner,
+											BufferDescriptorGetBuffer(bufHdr));
+				break;
+			}
+		}
+		else if (--trycounter == 0)
+			ereport(ERROR,
+					(errcode(ERRCODE_INSUFFICIENT_RESOURCES),
+					 errmsg("no empty local buffer available")));
+	}
+
+	/*
+	 * this buffer is not referenced but it might still be dirty. if that's
+	 * the case, write it out before reusing it!
+	 */
+	if (buf_state & BM_DIRTY)
+	{
+		SMgrRelation oreln;
+		Page		localpage = (char *) LocalBufHdrGetBlock(bufHdr);
+
+		/* Find smgr relation for buffer */
+		oreln = smgropen(bufHdr->tag.rnode, MyBackendId);
+
+		PageSetChecksumInplace(localpage, bufHdr->tag.blockNum);
+
+		/* And write... */
+		smgrwrite(oreln,
+				  bufHdr->tag.forkNum,
+				  bufHdr->tag.blockNum,
+				  localpage,
+				  false);
+
+		/* Mark not-dirty now in case we error out below */
+		buf_state &= ~BM_DIRTY;
+		pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
+
+		pgBufferUsage.local_blks_written++;
+	}
+
+	/*
+	 * lazy memory allocation: allocate space on first use of a buffer.
+	 */
+	if (LocalBufHdrGetBlock(bufHdr) == NULL)
+	{
+		/* Set pointer for use by BufferGetBlock() macro */
+		LocalBufHdrGetBlock(bufHdr) = GetLocalBufferStorage();
+	}
+
+	/*
+	 * Update the hash table: remove old entry, if any, and make new one.
+	 */
+	if (buf_state & BM_TAG_VALID)
+	{
+		hresult = (LocalBufferLookupEnt *)
+			hash_search(LocalBufHash, (void *) &bufHdr->tag,
+						HASH_REMOVE, NULL);
+		if (!hresult)			/* shouldn't happen */
+			elog(ERROR, "local buffer hash table corrupted");
+		/* mark buffer invalid just in case hash insert fails */
+		CLEAR_BUFFERTAG(bufHdr->tag);
+		buf_state &= ~(BM_VALID | BM_TAG_VALID);
+		pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
+	}
+
+	hresult = (LocalBufferLookupEnt *)
+		hash_search(LocalBufHash, (void *) &newTag, HASH_ENTER, &found);
+	if (found)					/* shouldn't happen */
+		elog(ERROR, "local buffer hash table corrupted");
+	hresult->id = b;
+
+	/*
+	 * it's all ours now.
+	 */
+	bufHdr->tag = newTag;
+	buf_state &= ~(BM_VALID | BM_DIRTY | BM_JUST_DIRTIED | BM_IO_ERROR);
+	buf_state |= BM_TAG_VALID;
+	buf_state &= ~BUF_USAGECOUNT_MASK;
+	buf_state += BUF_USAGECOUNT_ONE;
+	pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
+
+	*foundPtr = false;
+	return bufHdr;
+}
+
+/*
+ * MarkLocalBufferDirty -
+ *	  mark a local buffer dirty
+ */
+void
+MarkLocalBufferDirty(Buffer buffer)
+{
+	int			bufid;
+	BufferDesc *bufHdr;
+	uint32		buf_state;
+
+	Assert(BufferIsLocal(buffer));
+
+#ifdef LBDEBUG
+	fprintf(stderr, "LB DIRTY %d\n", buffer);
+#endif
+
+	bufid = -(buffer + 1);
+
+	Assert(LocalRefCount[bufid] > 0);
+
+	bufHdr = GetLocalBufferDescriptor(bufid);
+
+	buf_state = pg_atomic_read_u32(&bufHdr->state);
+
+	if (!(buf_state & BM_DIRTY))
+		pgBufferUsage.local_blks_dirtied++;
+
+	buf_state |= BM_DIRTY;
+
+	pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
+}
+
+/*
+ * DropRelFileNodeLocalBuffers
+ *		This function removes from the buffer pool all the pages of the
+ *		specified relation that have block numbers >= firstDelBlock.
+ *		(In particular, with firstDelBlock = 0, all pages are removed.)
+ *		Dirty pages are simply dropped, without bothering to write them
+ *		out first.  Therefore, this is NOT rollback-able, and so should be
+ *		used only with extreme caution!
+ *
+ *		See DropRelFileNodeBuffers in bufmgr.c for more notes.
+ */
+void
+DropRelFileNodeLocalBuffers(RelFileNode rnode, ForkNumber forkNum,
+							BlockNumber firstDelBlock)
+{
+	int			i;
+
+	for (i = 0; i < NLocBuffer; i++)
+	{
+		BufferDesc *bufHdr = GetLocalBufferDescriptor(i);
+		LocalBufferLookupEnt *hresult;
+		uint32		buf_state;
+
+		buf_state = pg_atomic_read_u32(&bufHdr->state);
+
+		if ((buf_state & BM_TAG_VALID) &&
+			RelFileNodeEquals(bufHdr->tag.rnode, rnode) &&
+			bufHdr->tag.forkNum == forkNum &&
+			bufHdr->tag.blockNum >= firstDelBlock)
+		{
+			if (LocalRefCount[i] != 0)
+				elog(ERROR, "block %u of %s is still referenced (local %u)",
+					 bufHdr->tag.blockNum,
+					 relpathbackend(bufHdr->tag.rnode, MyBackendId,
+									bufHdr->tag.forkNum),
+					 LocalRefCount[i]);
+			/* Remove entry from hashtable */
+			hresult = (LocalBufferLookupEnt *)
+				hash_search(LocalBufHash, (void *) &bufHdr->tag,
+							HASH_REMOVE, NULL);
+			if (!hresult)		/* shouldn't happen */
+				elog(ERROR, "local buffer hash table corrupted");
+			/* Mark buffer invalid */
+			CLEAR_BUFFERTAG(bufHdr->tag);
+			buf_state &= ~BUF_FLAG_MASK;
+			buf_state &= ~BUF_USAGECOUNT_MASK;
+			pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
+		}
+	}
+}
+
+/*
+ * DropRelFileNodeAllLocalBuffers
+ *		This function removes from the buffer pool all pages of all forks
+ *		of the specified relation.
+ *
+ *		See DropRelFileNodesAllBuffers in bufmgr.c for more notes.
+ */
+void
+DropRelFileNodeAllLocalBuffers(RelFileNode rnode)
+{
+	int			i;
+
+	for (i = 0; i < NLocBuffer; i++)
+	{
+		BufferDesc *bufHdr = GetLocalBufferDescriptor(i);
+		LocalBufferLookupEnt *hresult;
+		uint32		buf_state;
+
+		buf_state = pg_atomic_read_u32(&bufHdr->state);
+
+		if ((buf_state & BM_TAG_VALID) &&
+			RelFileNodeEquals(bufHdr->tag.rnode, rnode))
+		{
+			if (LocalRefCount[i] != 0)
+				elog(ERROR, "block %u of %s is still referenced (local %u)",
+					 bufHdr->tag.blockNum,
+					 relpathbackend(bufHdr->tag.rnode, MyBackendId,
+									bufHdr->tag.forkNum),
+					 LocalRefCount[i]);
+			/* Remove entry from hashtable */
+			hresult = (LocalBufferLookupEnt *)
+				hash_search(LocalBufHash, (void *) &bufHdr->tag,
+							HASH_REMOVE, NULL);
+			if (!hresult)		/* shouldn't happen */
+				elog(ERROR, "local buffer hash table corrupted");
+			/* Mark buffer invalid */
+			CLEAR_BUFFERTAG(bufHdr->tag);
+			buf_state &= ~BUF_FLAG_MASK;
+			buf_state &= ~BUF_USAGECOUNT_MASK;
+			pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
+		}
+	}
+}
+
+/*
+ * InitLocalBuffers -
+ *	  init the local buffer cache. Since most queries (esp. multi-user ones)
+ *	  don't involve local buffers, we delay allocating actual memory for the
+ *	  buffers until we need them; just make the buffer headers here.
+ */
+static void
+InitLocalBuffers(void)
+{
+	int			nbufs = num_temp_buffers;
+	HASHCTL		info;
+	int			i;
+
+	/*
+	 * Parallel workers can't access data in temporary tables, because they
+	 * have no visibility into the local buffers of their leader.  This is a
+	 * convenient, low-cost place to provide a backstop check for that.  Note
+	 * that we don't wish to prevent a parallel worker from accessing catalog
+	 * metadata about a temp table, so checks at higher levels would be
+	 * inappropriate.
+	 */
+	if (IsParallelWorker())
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TRANSACTION_STATE),
+				 errmsg("cannot access temporary tables during a parallel operation")));
+
+	/* Allocate and zero buffer headers and auxiliary arrays */
+	LocalBufferDescriptors = (BufferDesc *) calloc(nbufs, sizeof(BufferDesc));
+	LocalBufferBlockPointers = (Block *) calloc(nbufs, sizeof(Block));
+	LocalRefCount = (int32 *) calloc(nbufs, sizeof(int32));
+	if (!LocalBufferDescriptors || !LocalBufferBlockPointers || !LocalRefCount)
+		ereport(FATAL,
+				(errcode(ERRCODE_OUT_OF_MEMORY),
+				 errmsg("out of memory")));
+
+	nextFreeLocalBuf = 0;
+
+	/* initialize fields that need to start off nonzero */
+	for (i = 0; i < nbufs; i++)
+	{
+		BufferDesc *buf = GetLocalBufferDescriptor(i);
+
+		/*
+		 * negative to indicate local buffer. This is tricky: shared buffers
+		 * start with 0. We have to start with -2. (Note that the routine
+		 * BufferDescriptorGetBuffer adds 1 to buf_id so our first buffer id
+		 * is -1.)
+		 */
+		buf->buf_id = -i - 2;
+
+		/*
+		 * Intentionally do not initialize the buffer's atomic variable
+		 * (besides zeroing the underlying memory above). That way we get
+		 * errors on platforms without atomics, if somebody (re-)introduces
+		 * atomic operations for local buffers.
+		 */
+	}
+
+	/* Create the lookup hash table */
+	info.keysize = sizeof(BufferTag);
+	info.entrysize = sizeof(LocalBufferLookupEnt);
+
+	LocalBufHash = hash_create("Local Buffer Lookup Table",
+							   nbufs,
+							   &info,
+							   HASH_ELEM | HASH_BLOBS);
+
+	if (!LocalBufHash)
+		elog(ERROR, "could not initialize local buffer hash table");
+
+	/* Initialization done, mark buffers allocated */
+	NLocBuffer = nbufs;
+}
+
+/*
+ * GetLocalBufferStorage - allocate memory for a local buffer
+ *
+ * The idea of this function is to aggregate our requests for storage
+ * so that the memory manager doesn't see a whole lot of relatively small
+ * requests.  Since we'll never give back a local buffer once it's created
+ * within a particular process, no point in burdening memmgr with separately
+ * managed chunks.
+ */
+static Block
+GetLocalBufferStorage(void)
+{
+	static char *cur_block = NULL;
+	static int	next_buf_in_block = 0;
+	static int	num_bufs_in_block = 0;
+	static int	total_bufs_allocated = 0;
+	static MemoryContext LocalBufferContext = NULL;
+
+	char	   *this_buf;
+
+	Assert(total_bufs_allocated < NLocBuffer);
+
+	if (next_buf_in_block >= num_bufs_in_block)
+	{
+		/* Need to make a new request to memmgr */
+		int			num_bufs;
+
+		/*
+		 * We allocate local buffers in a context of their own, so that the
+		 * space eaten for them is easily recognizable in MemoryContextStats
+		 * output.  Create the context on first use.
+		 */
+		if (LocalBufferContext == NULL)
+			LocalBufferContext =
+				AllocSetContextCreate(TopMemoryContext,
+									  "LocalBufferContext",
+									  ALLOCSET_DEFAULT_SIZES);
+
+		/* Start with a 16-buffer request; subsequent ones double each time */
+		num_bufs = Max(num_bufs_in_block * 2, 16);
+		/* But not more than what we need for all remaining local bufs */
+		num_bufs = Min(num_bufs, NLocBuffer - total_bufs_allocated);
+		/* And don't overflow MaxAllocSize, either */
+		num_bufs = Min(num_bufs, MaxAllocSize / BLCKSZ);
+
+		cur_block = (char *) MemoryContextAlloc(LocalBufferContext,
+												num_bufs * BLCKSZ);
+		next_buf_in_block = 0;
+		num_bufs_in_block = num_bufs;
+	}
+
+	/* Allocate next buffer in current memory block */
+	this_buf = cur_block + next_buf_in_block * BLCKSZ;
+	next_buf_in_block++;
+	total_bufs_allocated++;
+
+	return (Block) this_buf;
+}
+
+/*
+ * CheckForLocalBufferLeaks - ensure this backend holds no local buffer pins
+ *
+ * This is just like CheckForBufferLeaks(), but for local buffers.
+ */
+static void
+CheckForLocalBufferLeaks(void)
+{
+#ifdef USE_ASSERT_CHECKING
+	if (LocalRefCount)
+	{
+		int			RefCountErrors = 0;
+		int			i;
+
+		for (i = 0; i < NLocBuffer; i++)
+		{
+			if (LocalRefCount[i] != 0)
+			{
+				Buffer		b = -i - 1;
+
+				PrintBufferLeakWarning(b);
+				RefCountErrors++;
+			}
+		}
+		Assert(RefCountErrors == 0);
+	}
+#endif
+}
+
+/*
+ * AtEOXact_LocalBuffers - clean up at end of transaction.
+ *
+ * This is just like AtEOXact_Buffers, but for local buffers.
+ */
+void
+AtEOXact_LocalBuffers(bool isCommit)
+{
+	CheckForLocalBufferLeaks();
+}
+
+/*
+ * AtProcExit_LocalBuffers - ensure we have dropped pins during backend exit.
+ *
+ * This is just like AtProcExit_Buffers, but for local buffers.
+ */
+void
+AtProcExit_LocalBuffers(void)
+{
+	/*
+	 * We shouldn't be holding any remaining pins; if we are, and assertions
+	 * aren't enabled, we'll fail later in DropRelFileNodeBuffers while trying
+	 * to drop the temp rels.
+	 */
+	CheckForLocalBufferLeaks();
+}