Adding upstream version 16.2.upstream/16.2

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

1 files changed, 1615 insertions, 0 deletions

diff --git a/src/backend/access/transam/slru.c b/src/backend/access/transam/slru.c
new file mode 100644
index 0000000..71ac70f
--- /dev/null
+++ b/src/backend/access/transam/slru.c
@@ -0,0 +1,1615 @@
+/*-------------------------------------------------------------------------
+ *
+ * slru.c
+ *		Simple LRU buffering for transaction status logfiles
+ *
+ * We use a simple least-recently-used scheme to manage a pool of page
+ * buffers.  Under ordinary circumstances we expect that write
+ * traffic will occur mostly to the latest page (and to the just-prior
+ * page, soon after a page transition).  Read traffic will probably touch
+ * a larger span of pages, but in any case a fairly small number of page
+ * buffers should be sufficient.  So, we just search the buffers using plain
+ * linear search; there's no need for a hashtable or anything fancy.
+ * The management algorithm is straight LRU except that we will never swap
+ * out the latest page (since we know it's going to be hit again eventually).
+ *
+ * We use a control LWLock to protect the shared data structures, plus
+ * per-buffer LWLocks that synchronize I/O for each buffer.  The control lock
+ * must be held to examine or modify any shared state.  A process that is
+ * reading in or writing out a page buffer does not hold the control lock,
+ * only the per-buffer lock for the buffer it is working on.
+ *
+ * "Holding the control lock" means exclusive lock in all cases except for
+ * SimpleLruReadPage_ReadOnly(); see comments for SlruRecentlyUsed() for
+ * the implications of that.
+ *
+ * When initiating I/O on a buffer, we acquire the per-buffer lock exclusively
+ * before releasing the control lock.  The per-buffer lock is released after
+ * completing the I/O, re-acquiring the control lock, and updating the shared
+ * state.  (Deadlock is not possible here, because we never try to initiate
+ * I/O when someone else is already doing I/O on the same buffer.)
+ * To wait for I/O to complete, release the control lock, acquire the
+ * per-buffer lock in shared mode, immediately release the per-buffer lock,
+ * reacquire the control lock, and then recheck state (since arbitrary things
+ * could have happened while we didn't have the lock).
+ *
+ * As with the regular buffer manager, it is possible for another process
+ * to re-dirty a page that is currently being written out.  This is handled
+ * by re-setting the page's page_dirty flag.
+ *
+ *
+ * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/backend/access/transam/slru.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include <fcntl.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+#include "access/slru.h"
+#include "access/transam.h"
+#include "access/xlog.h"
+#include "access/xlogutils.h"
+#include "miscadmin.h"
+#include "pgstat.h"
+#include "storage/fd.h"
+#include "storage/shmem.h"
+
+#define SlruFileName(ctl, path, seg) \
+	snprintf(path, MAXPGPATH, "%s/%04X", (ctl)->Dir, seg)
+
+/*
+ * During SimpleLruWriteAll(), we will usually not need to write more than one
+ * or two physical files, but we may need to write several pages per file.  We
+ * can consolidate the I/O requests by leaving files open until control returns
+ * to SimpleLruWriteAll().  This data structure remembers which files are open.
+ */
+#define MAX_WRITEALL_BUFFERS	16
+
+typedef struct SlruWriteAllData
+{
+	int			num_files;		/* # files actually open */
+	int			fd[MAX_WRITEALL_BUFFERS];	/* their FD's */
+	int			segno[MAX_WRITEALL_BUFFERS];	/* their log seg#s */
+} SlruWriteAllData;
+
+typedef struct SlruWriteAllData *SlruWriteAll;
+
+/*
+ * Populate a file tag describing a segment file.  We only use the segment
+ * number, since we can derive everything else we need by having separate
+ * sync handler functions for clog, multixact etc.
+ */
+#define INIT_SLRUFILETAG(a,xx_handler,xx_segno) \
+( \
+	memset(&(a), 0, sizeof(FileTag)), \
+	(a).handler = (xx_handler), \
+	(a).segno = (xx_segno) \
+)
+
+/*
+ * Macro to mark a buffer slot "most recently used".  Note multiple evaluation
+ * of arguments!
+ *
+ * The reason for the if-test is that there are often many consecutive
+ * accesses to the same page (particularly the latest page).  By suppressing
+ * useless increments of cur_lru_count, we reduce the probability that old
+ * pages' counts will "wrap around" and make them appear recently used.
+ *
+ * We allow this code to be executed concurrently by multiple processes within
+ * SimpleLruReadPage_ReadOnly().  As long as int reads and writes are atomic,
+ * this should not cause any completely-bogus values to enter the computation.
+ * However, it is possible for either cur_lru_count or individual
+ * page_lru_count entries to be "reset" to lower values than they should have,
+ * in case a process is delayed while it executes this macro.  With care in
+ * SlruSelectLRUPage(), this does little harm, and in any case the absolute
+ * worst possible consequence is a nonoptimal choice of page to evict.  The
+ * gain from allowing concurrent reads of SLRU pages seems worth it.
+ */
+#define SlruRecentlyUsed(shared, slotno)	\
+	do { \
+		int		new_lru_count = (shared)->cur_lru_count; \
+		if (new_lru_count != (shared)->page_lru_count[slotno]) { \
+			(shared)->cur_lru_count = ++new_lru_count; \
+			(shared)->page_lru_count[slotno] = new_lru_count; \
+		} \
+	} while (0)
+
+/* Saved info for SlruReportIOError */
+typedef enum
+{
+	SLRU_OPEN_FAILED,
+	SLRU_SEEK_FAILED,
+	SLRU_READ_FAILED,
+	SLRU_WRITE_FAILED,
+	SLRU_FSYNC_FAILED,
+	SLRU_CLOSE_FAILED
+} SlruErrorCause;
+
+static SlruErrorCause slru_errcause;
+static int	slru_errno;
+
+
+static void SimpleLruZeroLSNs(SlruCtl ctl, int slotno);
+static void SimpleLruWaitIO(SlruCtl ctl, int slotno);
+static void SlruInternalWritePage(SlruCtl ctl, int slotno, SlruWriteAll fdata);
+static bool SlruPhysicalReadPage(SlruCtl ctl, int pageno, int slotno);
+static bool SlruPhysicalWritePage(SlruCtl ctl, int pageno, int slotno,
+								  SlruWriteAll fdata);
+static void SlruReportIOError(SlruCtl ctl, int pageno, TransactionId xid);
+static int	SlruSelectLRUPage(SlruCtl ctl, int pageno);
+
+static bool SlruScanDirCbDeleteCutoff(SlruCtl ctl, char *filename,
+									  int segpage, void *data);
+static void SlruInternalDeleteSegment(SlruCtl ctl, int segno);
+
+/*
+ * Initialization of shared memory
+ */
+
+Size
+SimpleLruShmemSize(int nslots, int nlsns)
+{
+	Size		sz;
+
+	/* we assume nslots isn't so large as to risk overflow */
+	sz = MAXALIGN(sizeof(SlruSharedData));
+	sz += MAXALIGN(nslots * sizeof(char *));	/* page_buffer[] */
+	sz += MAXALIGN(nslots * sizeof(SlruPageStatus));	/* page_status[] */
+	sz += MAXALIGN(nslots * sizeof(bool));	/* page_dirty[] */
+	sz += MAXALIGN(nslots * sizeof(int));	/* page_number[] */
+	sz += MAXALIGN(nslots * sizeof(int));	/* page_lru_count[] */
+	sz += MAXALIGN(nslots * sizeof(LWLockPadded));	/* buffer_locks[] */
+
+	if (nlsns > 0)
+		sz += MAXALIGN(nslots * nlsns * sizeof(XLogRecPtr));	/* group_lsn[] */
+
+	return BUFFERALIGN(sz) + BLCKSZ * nslots;
+}
+
+/*
+ * Initialize, or attach to, a simple LRU cache in shared memory.
+ *
+ * ctl: address of local (unshared) control structure.
+ * name: name of SLRU.  (This is user-visible, pick with care!)
+ * nslots: number of page slots to use.
+ * nlsns: number of LSN groups per page (set to zero if not relevant).
+ * ctllock: LWLock to use to control access to the shared control structure.
+ * subdir: PGDATA-relative subdirectory that will contain the files.
+ * tranche_id: LWLock tranche ID to use for the SLRU's per-buffer LWLocks.
+ * sync_handler: which set of functions to use to handle sync requests
+ */
+void
+SimpleLruInit(SlruCtl ctl, const char *name, int nslots, int nlsns,
+			  LWLock *ctllock, const char *subdir, int tranche_id,
+			  SyncRequestHandler sync_handler)
+{
+	SlruShared	shared;
+	bool		found;
+
+	shared = (SlruShared) ShmemInitStruct(name,
+										  SimpleLruShmemSize(nslots, nlsns),
+										  &found);
+
+	if (!IsUnderPostmaster)
+	{
+		/* Initialize locks and shared memory area */
+		char	   *ptr;
+		Size		offset;
+		int			slotno;
+
+		Assert(!found);
+
+		memset(shared, 0, sizeof(SlruSharedData));
+
+		shared->ControlLock = ctllock;
+
+		shared->num_slots = nslots;
+		shared->lsn_groups_per_page = nlsns;
+
+		shared->cur_lru_count = 0;
+
+		/* shared->latest_page_number will be set later */
+
+		shared->slru_stats_idx = pgstat_get_slru_index(name);
+
+		ptr = (char *) shared;
+		offset = MAXALIGN(sizeof(SlruSharedData));
+		shared->page_buffer = (char **) (ptr + offset);
+		offset += MAXALIGN(nslots * sizeof(char *));
+		shared->page_status = (SlruPageStatus *) (ptr + offset);
+		offset += MAXALIGN(nslots * sizeof(SlruPageStatus));
+		shared->page_dirty = (bool *) (ptr + offset);
+		offset += MAXALIGN(nslots * sizeof(bool));
+		shared->page_number = (int *) (ptr + offset);
+		offset += MAXALIGN(nslots * sizeof(int));
+		shared->page_lru_count = (int *) (ptr + offset);
+		offset += MAXALIGN(nslots * sizeof(int));
+
+		/* Initialize LWLocks */
+		shared->buffer_locks = (LWLockPadded *) (ptr + offset);
+		offset += MAXALIGN(nslots * sizeof(LWLockPadded));
+
+		if (nlsns > 0)
+		{
+			shared->group_lsn = (XLogRecPtr *) (ptr + offset);
+			offset += MAXALIGN(nslots * nlsns * sizeof(XLogRecPtr));
+		}
+
+		ptr += BUFFERALIGN(offset);
+		for (slotno = 0; slotno < nslots; slotno++)
+		{
+			LWLockInitialize(&shared->buffer_locks[slotno].lock,
+							 tranche_id);
+
+			shared->page_buffer[slotno] = ptr;
+			shared->page_status[slotno] = SLRU_PAGE_EMPTY;
+			shared->page_dirty[slotno] = false;
+			shared->page_lru_count[slotno] = 0;
+			ptr += BLCKSZ;
+		}
+
+		/* Should fit to estimated shmem size */
+		Assert(ptr - (char *) shared <= SimpleLruShmemSize(nslots, nlsns));
+	}
+	else
+		Assert(found);
+
+	/*
+	 * Initialize the unshared control struct, including directory path. We
+	 * assume caller set PagePrecedes.
+	 */
+	ctl->shared = shared;
+	ctl->sync_handler = sync_handler;
+	strlcpy(ctl->Dir, subdir, sizeof(ctl->Dir));
+}
+
+/*
+ * Initialize (or reinitialize) a page to zeroes.
+ *
+ * The page is not actually written, just set up in shared memory.
+ * The slot number of the new page is returned.
+ *
+ * Control lock must be held at entry, and will be held at exit.
+ */
+int
+SimpleLruZeroPage(SlruCtl ctl, int pageno)
+{
+	SlruShared	shared = ctl->shared;
+	int			slotno;
+
+	/* Find a suitable buffer slot for the page */
+	slotno = SlruSelectLRUPage(ctl, pageno);
+	Assert(shared->page_status[slotno] == SLRU_PAGE_EMPTY ||
+		   (shared->page_status[slotno] == SLRU_PAGE_VALID &&
+			!shared->page_dirty[slotno]) ||
+		   shared->page_number[slotno] == pageno);
+
+	/* Mark the slot as containing this page */
+	shared->page_number[slotno] = pageno;
+	shared->page_status[slotno] = SLRU_PAGE_VALID;
+	shared->page_dirty[slotno] = true;
+	SlruRecentlyUsed(shared, slotno);
+
+	/* Set the buffer to zeroes */
+	MemSet(shared->page_buffer[slotno], 0, BLCKSZ);
+
+	/* Set the LSNs for this new page to zero */
+	SimpleLruZeroLSNs(ctl, slotno);
+
+	/* Assume this page is now the latest active page */
+	shared->latest_page_number = pageno;
+
+	/* update the stats counter of zeroed pages */
+	pgstat_count_slru_page_zeroed(shared->slru_stats_idx);
+
+	return slotno;
+}
+
+/*
+ * Zero all the LSNs we store for this slru page.
+ *
+ * This should be called each time we create a new page, and each time we read
+ * in a page from disk into an existing buffer.  (Such an old page cannot
+ * have any interesting LSNs, since we'd have flushed them before writing
+ * the page in the first place.)
+ *
+ * This assumes that InvalidXLogRecPtr is bitwise-all-0.
+ */
+static void
+SimpleLruZeroLSNs(SlruCtl ctl, int slotno)
+{
+	SlruShared	shared = ctl->shared;
+
+	if (shared->lsn_groups_per_page > 0)
+		MemSet(&shared->group_lsn[slotno * shared->lsn_groups_per_page], 0,
+			   shared->lsn_groups_per_page * sizeof(XLogRecPtr));
+}
+
+/*
+ * Wait for any active I/O on a page slot to finish.  (This does not
+ * guarantee that new I/O hasn't been started before we return, though.
+ * In fact the slot might not even contain the same page anymore.)
+ *
+ * Control lock must be held at entry, and will be held at exit.
+ */
+static void
+SimpleLruWaitIO(SlruCtl ctl, int slotno)
+{
+	SlruShared	shared = ctl->shared;
+
+	/* See notes at top of file */
+	LWLockRelease(shared->ControlLock);
+	LWLockAcquire(&shared->buffer_locks[slotno].lock, LW_SHARED);
+	LWLockRelease(&shared->buffer_locks[slotno].lock);
+	LWLockAcquire(shared->ControlLock, LW_EXCLUSIVE);
+
+	/*
+	 * If the slot is still in an io-in-progress state, then either someone
+	 * already started a new I/O on the slot, or a previous I/O failed and
+	 * neglected to reset the page state.  That shouldn't happen, really, but
+	 * it seems worth a few extra cycles to check and recover from it. We can
+	 * cheaply test for failure by seeing if the buffer lock is still held (we
+	 * assume that transaction abort would release the lock).
+	 */
+	if (shared->page_status[slotno] == SLRU_PAGE_READ_IN_PROGRESS ||
+		shared->page_status[slotno] == SLRU_PAGE_WRITE_IN_PROGRESS)
+	{
+		if (LWLockConditionalAcquire(&shared->buffer_locks[slotno].lock, LW_SHARED))
+		{
+			/* indeed, the I/O must have failed */
+			if (shared->page_status[slotno] == SLRU_PAGE_READ_IN_PROGRESS)
+				shared->page_status[slotno] = SLRU_PAGE_EMPTY;
+			else				/* write_in_progress */
+			{
+				shared->page_status[slotno] = SLRU_PAGE_VALID;
+				shared->page_dirty[slotno] = true;
+			}
+			LWLockRelease(&shared->buffer_locks[slotno].lock);
+		}
+	}
+}
+
+/*
+ * Find a page in a shared buffer, reading it in if necessary.
+ * The page number must correspond to an already-initialized page.
+ *
+ * If write_ok is true then it is OK to return a page that is in
+ * WRITE_IN_PROGRESS state; it is the caller's responsibility to be sure
+ * that modification of the page is safe.  If write_ok is false then we
+ * will not return the page until it is not undergoing active I/O.
+ *
+ * The passed-in xid is used only for error reporting, and may be
+ * InvalidTransactionId if no specific xid is associated with the action.
+ *
+ * Return value is the shared-buffer slot number now holding the page.
+ * The buffer's LRU access info is updated.
+ *
+ * Control lock must be held at entry, and will be held at exit.
+ */
+int
+SimpleLruReadPage(SlruCtl ctl, int pageno, bool write_ok,
+				  TransactionId xid)
+{
+	SlruShared	shared = ctl->shared;
+
+	/* Outer loop handles restart if we must wait for someone else's I/O */
+	for (;;)
+	{
+		int			slotno;
+		bool		ok;
+
+		/* See if page already is in memory; if not, pick victim slot */
+		slotno = SlruSelectLRUPage(ctl, pageno);
+
+		/* Did we find the page in memory? */
+		if (shared->page_number[slotno] == pageno &&
+			shared->page_status[slotno] != SLRU_PAGE_EMPTY)
+		{
+			/*
+			 * If page is still being read in, we must wait for I/O.  Likewise
+			 * if the page is being written and the caller said that's not OK.
+			 */
+			if (shared->page_status[slotno] == SLRU_PAGE_READ_IN_PROGRESS ||
+				(shared->page_status[slotno] == SLRU_PAGE_WRITE_IN_PROGRESS &&
+				 !write_ok))
+			{
+				SimpleLruWaitIO(ctl, slotno);
+				/* Now we must recheck state from the top */
+				continue;
+			}
+			/* Otherwise, it's ready to use */
+			SlruRecentlyUsed(shared, slotno);
+
+			/* update the stats counter of pages found in the SLRU */
+			pgstat_count_slru_page_hit(shared->slru_stats_idx);
+
+			return slotno;
+		}
+
+		/* We found no match; assert we selected a freeable slot */
+		Assert(shared->page_status[slotno] == SLRU_PAGE_EMPTY ||
+			   (shared->page_status[slotno] == SLRU_PAGE_VALID &&
+				!shared->page_dirty[slotno]));
+
+		/* Mark the slot read-busy */
+		shared->page_number[slotno] = pageno;
+		shared->page_status[slotno] = SLRU_PAGE_READ_IN_PROGRESS;
+		shared->page_dirty[slotno] = false;
+
+		/* Acquire per-buffer lock (cannot deadlock, see notes at top) */
+		LWLockAcquire(&shared->buffer_locks[slotno].lock, LW_EXCLUSIVE);
+
+		/* Release control lock while doing I/O */
+		LWLockRelease(shared->ControlLock);
+
+		/* Do the read */
+		ok = SlruPhysicalReadPage(ctl, pageno, slotno);
+
+		/* Set the LSNs for this newly read-in page to zero */
+		SimpleLruZeroLSNs(ctl, slotno);
+
+		/* Re-acquire control lock and update page state */
+		LWLockAcquire(shared->ControlLock, LW_EXCLUSIVE);
+
+		Assert(shared->page_number[slotno] == pageno &&
+			   shared->page_status[slotno] == SLRU_PAGE_READ_IN_PROGRESS &&
+			   !shared->page_dirty[slotno]);
+
+		shared->page_status[slotno] = ok ? SLRU_PAGE_VALID : SLRU_PAGE_EMPTY;
+
+		LWLockRelease(&shared->buffer_locks[slotno].lock);
+
+		/* Now it's okay to ereport if we failed */
+		if (!ok)
+			SlruReportIOError(ctl, pageno, xid);
+
+		SlruRecentlyUsed(shared, slotno);
+
+		/* update the stats counter of pages not found in SLRU */
+		pgstat_count_slru_page_read(shared->slru_stats_idx);
+
+		return slotno;
+	}
+}
+
+/*
+ * Find a page in a shared buffer, reading it in if necessary.
+ * The page number must correspond to an already-initialized page.
+ * The caller must intend only read-only access to the page.
+ *
+ * The passed-in xid is used only for error reporting, and may be
+ * InvalidTransactionId if no specific xid is associated with the action.
+ *
+ * Return value is the shared-buffer slot number now holding the page.
+ * The buffer's LRU access info is updated.
+ *
+ * Control lock must NOT be held at entry, but will be held at exit.
+ * It is unspecified whether the lock will be shared or exclusive.
+ */
+int
+SimpleLruReadPage_ReadOnly(SlruCtl ctl, int pageno, TransactionId xid)
+{
+	SlruShared	shared = ctl->shared;
+	int			slotno;
+
+	/* Try to find the page while holding only shared lock */
+	LWLockAcquire(shared->ControlLock, LW_SHARED);
+
+	/* See if page is already in a buffer */
+	for (slotno = 0; slotno < shared->num_slots; slotno++)
+	{
+		if (shared->page_number[slotno] == pageno &&
+			shared->page_status[slotno] != SLRU_PAGE_EMPTY &&
+			shared->page_status[slotno] != SLRU_PAGE_READ_IN_PROGRESS)
+		{
+			/* See comments for SlruRecentlyUsed macro */
+			SlruRecentlyUsed(shared, slotno);
+
+			/* update the stats counter of pages found in the SLRU */
+			pgstat_count_slru_page_hit(shared->slru_stats_idx);
+
+			return slotno;
+		}
+	}
+
+	/* No luck, so switch to normal exclusive lock and do regular read */
+	LWLockRelease(shared->ControlLock);
+	LWLockAcquire(shared->ControlLock, LW_EXCLUSIVE);
+
+	return SimpleLruReadPage(ctl, pageno, true, xid);
+}
+
+/*
+ * Write a page from a shared buffer, if necessary.
+ * Does nothing if the specified slot is not dirty.
+ *
+ * NOTE: only one write attempt is made here.  Hence, it is possible that
+ * the page is still dirty at exit (if someone else re-dirtied it during
+ * the write).  However, we *do* attempt a fresh write even if the page
+ * is already being written; this is for checkpoints.
+ *
+ * Control lock must be held at entry, and will be held at exit.
+ */
+static void
+SlruInternalWritePage(SlruCtl ctl, int slotno, SlruWriteAll fdata)
+{
+	SlruShared	shared = ctl->shared;
+	int			pageno = shared->page_number[slotno];
+	bool		ok;
+
+	/* If a write is in progress, wait for it to finish */
+	while (shared->page_status[slotno] == SLRU_PAGE_WRITE_IN_PROGRESS &&
+		   shared->page_number[slotno] == pageno)
+	{
+		SimpleLruWaitIO(ctl, slotno);
+	}
+
+	/*
+	 * Do nothing if page is not dirty, or if buffer no longer contains the
+	 * same page we were called for.
+	 */
+	if (!shared->page_dirty[slotno] ||
+		shared->page_status[slotno] != SLRU_PAGE_VALID ||
+		shared->page_number[slotno] != pageno)
+		return;
+
+	/*
+	 * Mark the slot write-busy, and clear the dirtybit.  After this point, a
+	 * transaction status update on this page will mark it dirty again.
+	 */
+	shared->page_status[slotno] = SLRU_PAGE_WRITE_IN_PROGRESS;
+	shared->page_dirty[slotno] = false;
+
+	/* Acquire per-buffer lock (cannot deadlock, see notes at top) */
+	LWLockAcquire(&shared->buffer_locks[slotno].lock, LW_EXCLUSIVE);
+
+	/* Release control lock while doing I/O */
+	LWLockRelease(shared->ControlLock);
+
+	/* Do the write */
+	ok = SlruPhysicalWritePage(ctl, pageno, slotno, fdata);
+
+	/* If we failed, and we're in a flush, better close the files */
+	if (!ok && fdata)
+	{
+		int			i;
+
+		for (i = 0; i < fdata->num_files; i++)
+			CloseTransientFile(fdata->fd[i]);
+	}
+
+	/* Re-acquire control lock and update page state */
+	LWLockAcquire(shared->ControlLock, LW_EXCLUSIVE);
+
+	Assert(shared->page_number[slotno] == pageno &&
+		   shared->page_status[slotno] == SLRU_PAGE_WRITE_IN_PROGRESS);
+
+	/* If we failed to write, mark the page dirty again */
+	if (!ok)
+		shared->page_dirty[slotno] = true;
+
+	shared->page_status[slotno] = SLRU_PAGE_VALID;
+
+	LWLockRelease(&shared->buffer_locks[slotno].lock);
+
+	/* Now it's okay to ereport if we failed */
+	if (!ok)
+		SlruReportIOError(ctl, pageno, InvalidTransactionId);
+
+	/* If part of a checkpoint, count this as a buffer written. */
+	if (fdata)
+		CheckpointStats.ckpt_bufs_written++;
+}
+
+/*
+ * Wrapper of SlruInternalWritePage, for external callers.
+ * fdata is always passed a NULL here.
+ */
+void
+SimpleLruWritePage(SlruCtl ctl, int slotno)
+{
+	SlruInternalWritePage(ctl, slotno, NULL);
+}
+
+/*
+ * Return whether the given page exists on disk.
+ *
+ * A false return means that either the file does not exist, or that it's not
+ * large enough to contain the given page.
+ */
+bool
+SimpleLruDoesPhysicalPageExist(SlruCtl ctl, int pageno)
+{
+	int			segno = pageno / SLRU_PAGES_PER_SEGMENT;
+	int			rpageno = pageno % SLRU_PAGES_PER_SEGMENT;
+	int			offset = rpageno * BLCKSZ;
+	char		path[MAXPGPATH];
+	int			fd;
+	bool		result;
+	off_t		endpos;
+
+	/* update the stats counter of checked pages */
+	pgstat_count_slru_page_exists(ctl->shared->slru_stats_idx);
+
+	SlruFileName(ctl, path, segno);
+
+	fd = OpenTransientFile(path, O_RDONLY | PG_BINARY);
+	if (fd < 0)
+	{
+		/* expected: file doesn't exist */
+		if (errno == ENOENT)
+			return false;
+
+		/* report error normally */
+		slru_errcause = SLRU_OPEN_FAILED;
+		slru_errno = errno;
+		SlruReportIOError(ctl, pageno, 0);
+	}
+
+	if ((endpos = lseek(fd, 0, SEEK_END)) < 0)
+	{
+		slru_errcause = SLRU_SEEK_FAILED;
+		slru_errno = errno;
+		SlruReportIOError(ctl, pageno, 0);
+	}
+
+	result = endpos >= (off_t) (offset + BLCKSZ);
+
+	if (CloseTransientFile(fd) != 0)
+	{
+		slru_errcause = SLRU_CLOSE_FAILED;
+		slru_errno = errno;
+		return false;
+	}
+
+	return result;
+}
+
+/*
+ * Physical read of a (previously existing) page into a buffer slot
+ *
+ * On failure, we cannot just ereport(ERROR) since caller has put state in
+ * shared memory that must be undone.  So, we return false and save enough
+ * info in static variables to let SlruReportIOError make the report.
+ *
+ * For now, assume it's not worth keeping a file pointer open across
+ * read/write operations.  We could cache one virtual file pointer ...
+ */
+static bool
+SlruPhysicalReadPage(SlruCtl ctl, int pageno, int slotno)
+{
+	SlruShared	shared = ctl->shared;
+	int			segno = pageno / SLRU_PAGES_PER_SEGMENT;
+	int			rpageno = pageno % SLRU_PAGES_PER_SEGMENT;
+	off_t		offset = rpageno * BLCKSZ;
+	char		path[MAXPGPATH];
+	int			fd;
+
+	SlruFileName(ctl, path, segno);
+
+	/*
+	 * In a crash-and-restart situation, it's possible for us to receive
+	 * commands to set the commit status of transactions whose bits are in
+	 * already-truncated segments of the commit log (see notes in
+	 * SlruPhysicalWritePage).  Hence, if we are InRecovery, allow the case
+	 * where the file doesn't exist, and return zeroes instead.
+	 */
+	fd = OpenTransientFile(path, O_RDONLY | PG_BINARY);
+	if (fd < 0)
+	{
+		if (errno != ENOENT || !InRecovery)
+		{
+			slru_errcause = SLRU_OPEN_FAILED;
+			slru_errno = errno;
+			return false;
+		}
+
+		ereport(LOG,
+				(errmsg("file \"%s\" doesn't exist, reading as zeroes",
+						path)));
+		MemSet(shared->page_buffer[slotno], 0, BLCKSZ);
+		return true;
+	}
+
+	errno = 0;
+	pgstat_report_wait_start(WAIT_EVENT_SLRU_READ);
+	if (pg_pread(fd, shared->page_buffer[slotno], BLCKSZ, offset) != BLCKSZ)
+	{
+		pgstat_report_wait_end();
+		slru_errcause = SLRU_READ_FAILED;
+		slru_errno = errno;
+		CloseTransientFile(fd);
+		return false;
+	}
+	pgstat_report_wait_end();
+
+	if (CloseTransientFile(fd) != 0)
+	{
+		slru_errcause = SLRU_CLOSE_FAILED;
+		slru_errno = errno;
+		return false;
+	}
+
+	return true;
+}
+
+/*
+ * Physical write of a page from a buffer slot
+ *
+ * On failure, we cannot just ereport(ERROR) since caller has put state in
+ * shared memory that must be undone.  So, we return false and save enough
+ * info in static variables to let SlruReportIOError make the report.
+ *
+ * For now, assume it's not worth keeping a file pointer open across
+ * independent read/write operations.  We do batch operations during
+ * SimpleLruWriteAll, though.
+ *
+ * fdata is NULL for a standalone write, pointer to open-file info during
+ * SimpleLruWriteAll.
+ */
+static bool
+SlruPhysicalWritePage(SlruCtl ctl, int pageno, int slotno, SlruWriteAll fdata)
+{
+	SlruShared	shared = ctl->shared;
+	int			segno = pageno / SLRU_PAGES_PER_SEGMENT;
+	int			rpageno = pageno % SLRU_PAGES_PER_SEGMENT;
+	off_t		offset = rpageno * BLCKSZ;
+	char		path[MAXPGPATH];
+	int			fd = -1;
+
+	/* update the stats counter of written pages */
+	pgstat_count_slru_page_written(shared->slru_stats_idx);
+
+	/*
+	 * Honor the write-WAL-before-data rule, if appropriate, so that we do not
+	 * write out data before associated WAL records.  This is the same action
+	 * performed during FlushBuffer() in the main buffer manager.
+	 */
+	if (shared->group_lsn != NULL)
+	{
+		/*
+		 * We must determine the largest async-commit LSN for the page. This
+		 * is a bit tedious, but since this entire function is a slow path
+		 * anyway, it seems better to do this here than to maintain a per-page
+		 * LSN variable (which'd need an extra comparison in the
+		 * transaction-commit path).
+		 */
+		XLogRecPtr	max_lsn;
+		int			lsnindex,
+					lsnoff;
+
+		lsnindex = slotno * shared->lsn_groups_per_page;
+		max_lsn = shared->group_lsn[lsnindex++];
+		for (lsnoff = 1; lsnoff < shared->lsn_groups_per_page; lsnoff++)
+		{
+			XLogRecPtr	this_lsn = shared->group_lsn[lsnindex++];
+
+			if (max_lsn < this_lsn)
+				max_lsn = this_lsn;
+		}
+
+		if (!XLogRecPtrIsInvalid(max_lsn))
+		{
+			/*
+			 * As noted above, elog(ERROR) is not acceptable here, so if
+			 * XLogFlush were to fail, we must PANIC.  This isn't much of a
+			 * restriction because XLogFlush is just about all critical
+			 * section anyway, but let's make sure.
+			 */
+			START_CRIT_SECTION();
+			XLogFlush(max_lsn);
+			END_CRIT_SECTION();
+		}
+	}
+
+	/*
+	 * During a SimpleLruWriteAll, we may already have the desired file open.
+	 */
+	if (fdata)
+	{
+		int			i;
+
+		for (i = 0; i < fdata->num_files; i++)
+		{
+			if (fdata->segno[i] == segno)
+			{
+				fd = fdata->fd[i];
+				break;
+			}
+		}
+	}
+
+	if (fd < 0)
+	{
+		/*
+		 * If the file doesn't already exist, we should create it.  It is
+		 * possible for this to need to happen when writing a page that's not
+		 * first in its segment; we assume the OS can cope with that. (Note:
+		 * it might seem that it'd be okay to create files only when
+		 * SimpleLruZeroPage is called for the first page of a segment.
+		 * However, if after a crash and restart the REDO logic elects to
+		 * replay the log from a checkpoint before the latest one, then it's
+		 * possible that we will get commands to set transaction status of
+		 * transactions that have already been truncated from the commit log.
+		 * Easiest way to deal with that is to accept references to
+		 * nonexistent files here and in SlruPhysicalReadPage.)
+		 *
+		 * Note: it is possible for more than one backend to be executing this
+		 * code simultaneously for different pages of the same file. Hence,
+		 * don't use O_EXCL or O_TRUNC or anything like that.
+		 */
+		SlruFileName(ctl, path, segno);
+		fd = OpenTransientFile(path, O_RDWR | O_CREAT | PG_BINARY);
+		if (fd < 0)
+		{
+			slru_errcause = SLRU_OPEN_FAILED;
+			slru_errno = errno;
+			return false;
+		}
+
+		if (fdata)
+		{
+			if (fdata->num_files < MAX_WRITEALL_BUFFERS)
+			{
+				fdata->fd[fdata->num_files] = fd;
+				fdata->segno[fdata->num_files] = segno;
+				fdata->num_files++;
+			}
+			else
+			{
+				/*
+				 * In the unlikely event that we exceed MAX_WRITEALL_BUFFERS,
+				 * fall back to treating it as a standalone write.
+				 */
+				fdata = NULL;
+			}
+		}
+	}
+
+	errno = 0;
+	pgstat_report_wait_start(WAIT_EVENT_SLRU_WRITE);
+	if (pg_pwrite(fd, shared->page_buffer[slotno], BLCKSZ, offset) != BLCKSZ)
+	{
+		pgstat_report_wait_end();
+		/* if write didn't set errno, assume problem is no disk space */
+		if (errno == 0)
+			errno = ENOSPC;
+		slru_errcause = SLRU_WRITE_FAILED;
+		slru_errno = errno;
+		if (!fdata)
+			CloseTransientFile(fd);
+		return false;
+	}
+	pgstat_report_wait_end();
+
+	/* Queue up a sync request for the checkpointer. */
+	if (ctl->sync_handler != SYNC_HANDLER_NONE)
+	{
+		FileTag		tag;
+
+		INIT_SLRUFILETAG(tag, ctl->sync_handler, segno);
+		if (!RegisterSyncRequest(&tag, SYNC_REQUEST, false))
+		{
+			/* No space to enqueue sync request.  Do it synchronously. */
+			pgstat_report_wait_start(WAIT_EVENT_SLRU_SYNC);
+			if (pg_fsync(fd) != 0)
+			{
+				pgstat_report_wait_end();
+				slru_errcause = SLRU_FSYNC_FAILED;
+				slru_errno = errno;
+				CloseTransientFile(fd);
+				return false;
+			}
+			pgstat_report_wait_end();
+		}
+	}
+
+	/* Close file, unless part of flush request. */
+	if (!fdata)
+	{
+		if (CloseTransientFile(fd) != 0)
+		{
+			slru_errcause = SLRU_CLOSE_FAILED;
+			slru_errno = errno;
+			return false;
+		}
+	}
+
+	return true;
+}
+
+/*
+ * Issue the error message after failure of SlruPhysicalReadPage or
+ * SlruPhysicalWritePage.  Call this after cleaning up shared-memory state.
+ */
+static void
+SlruReportIOError(SlruCtl ctl, int pageno, TransactionId xid)
+{
+	int			segno = pageno / SLRU_PAGES_PER_SEGMENT;
+	int			rpageno = pageno % SLRU_PAGES_PER_SEGMENT;
+	int			offset = rpageno * BLCKSZ;
+	char		path[MAXPGPATH];
+
+	SlruFileName(ctl, path, segno);
+	errno = slru_errno;
+	switch (slru_errcause)
+	{
+		case SLRU_OPEN_FAILED:
+			ereport(ERROR,
+					(errcode_for_file_access(),
+					 errmsg("could not access status of transaction %u", xid),
+					 errdetail("Could not open file \"%s\": %m.", path)));
+			break;
+		case SLRU_SEEK_FAILED:
+			ereport(ERROR,
+					(errcode_for_file_access(),
+					 errmsg("could not access status of transaction %u", xid),
+					 errdetail("Could not seek in file \"%s\" to offset %d: %m.",
+							   path, offset)));
+			break;
+		case SLRU_READ_FAILED:
+			if (errno)
+				ereport(ERROR,
+						(errcode_for_file_access(),
+						 errmsg("could not access status of transaction %u", xid),
+						 errdetail("Could not read from file \"%s\" at offset %d: %m.",
+								   path, offset)));
+			else
+				ereport(ERROR,
+						(errmsg("could not access status of transaction %u", xid),
+						 errdetail("Could not read from file \"%s\" at offset %d: read too few bytes.", path, offset)));
+			break;
+		case SLRU_WRITE_FAILED:
+			if (errno)
+				ereport(ERROR,
+						(errcode_for_file_access(),
+						 errmsg("could not access status of transaction %u", xid),
+						 errdetail("Could not write to file \"%s\" at offset %d: %m.",
+								   path, offset)));
+			else
+				ereport(ERROR,
+						(errmsg("could not access status of transaction %u", xid),
+						 errdetail("Could not write to file \"%s\" at offset %d: wrote too few bytes.",
+								   path, offset)));
+			break;
+		case SLRU_FSYNC_FAILED:
+			ereport(data_sync_elevel(ERROR),
+					(errcode_for_file_access(),
+					 errmsg("could not access status of transaction %u", xid),
+					 errdetail("Could not fsync file \"%s\": %m.",
+							   path)));
+			break;
+		case SLRU_CLOSE_FAILED:
+			ereport(ERROR,
+					(errcode_for_file_access(),
+					 errmsg("could not access status of transaction %u", xid),
+					 errdetail("Could not close file \"%s\": %m.",
+							   path)));
+			break;
+		default:
+			/* can't get here, we trust */
+			elog(ERROR, "unrecognized SimpleLru error cause: %d",
+				 (int) slru_errcause);
+			break;
+	}
+}
+
+/*
+ * Select the slot to re-use when we need a free slot.
+ *
+ * The target page number is passed because we need to consider the
+ * possibility that some other process reads in the target page while
+ * we are doing I/O to free a slot.  Hence, check or recheck to see if
+ * any slot already holds the target page, and return that slot if so.
+ * Thus, the returned slot is *either* a slot already holding the pageno
+ * (could be any state except EMPTY), *or* a freeable slot (state EMPTY
+ * or CLEAN).
+ *
+ * Control lock must be held at entry, and will be held at exit.
+ */
+static int
+SlruSelectLRUPage(SlruCtl ctl, int pageno)
+{
+	SlruShared	shared = ctl->shared;
+
+	/* Outer loop handles restart after I/O */
+	for (;;)
+	{
+		int			slotno;
+		int			cur_count;
+		int			bestvalidslot = 0;	/* keep compiler quiet */
+		int			best_valid_delta = -1;
+		int			best_valid_page_number = 0; /* keep compiler quiet */
+		int			bestinvalidslot = 0;	/* keep compiler quiet */
+		int			best_invalid_delta = -1;
+		int			best_invalid_page_number = 0;	/* keep compiler quiet */
+
+		/* See if page already has a buffer assigned */
+		for (slotno = 0; slotno < shared->num_slots; slotno++)
+		{
+			if (shared->page_number[slotno] == pageno &&
+				shared->page_status[slotno] != SLRU_PAGE_EMPTY)
+				return slotno;
+		}
+
+		/*
+		 * If we find any EMPTY slot, just select that one. Else choose a
+		 * victim page to replace.  We normally take the least recently used
+		 * valid page, but we will never take the slot containing
+		 * latest_page_number, even if it appears least recently used.  We
+		 * will select a slot that is already I/O busy only if there is no
+		 * other choice: a read-busy slot will not be least recently used once
+		 * the read finishes, and waiting for an I/O on a write-busy slot is
+		 * inferior to just picking some other slot.  Testing shows the slot
+		 * we pick instead will often be clean, allowing us to begin a read at
+		 * once.
+		 *
+		 * Normally the page_lru_count values will all be different and so
+		 * there will be a well-defined LRU page.  But since we allow
+		 * concurrent execution of SlruRecentlyUsed() within
+		 * SimpleLruReadPage_ReadOnly(), it is possible that multiple pages
+		 * acquire the same lru_count values.  In that case we break ties by
+		 * choosing the furthest-back page.
+		 *
+		 * Notice that this next line forcibly advances cur_lru_count to a
+		 * value that is certainly beyond any value that will be in the
+		 * page_lru_count array after the loop finishes.  This ensures that
+		 * the next execution of SlruRecentlyUsed will mark the page newly
+		 * used, even if it's for a page that has the current counter value.
+		 * That gets us back on the path to having good data when there are
+		 * multiple pages with the same lru_count.
+		 */
+		cur_count = (shared->cur_lru_count)++;
+		for (slotno = 0; slotno < shared->num_slots; slotno++)
+		{
+			int			this_delta;
+			int			this_page_number;
+
+			if (shared->page_status[slotno] == SLRU_PAGE_EMPTY)
+				return slotno;
+			this_delta = cur_count - shared->page_lru_count[slotno];
+			if (this_delta < 0)
+			{
+				/*
+				 * Clean up in case shared updates have caused cur_count
+				 * increments to get "lost".  We back off the page counts,
+				 * rather than trying to increase cur_count, to avoid any
+				 * question of infinite loops or failure in the presence of
+				 * wrapped-around counts.
+				 */
+				shared->page_lru_count[slotno] = cur_count;
+				this_delta = 0;
+			}
+			this_page_number = shared->page_number[slotno];
+			if (this_page_number == shared->latest_page_number)
+				continue;
+			if (shared->page_status[slotno] == SLRU_PAGE_VALID)
+			{
+				if (this_delta > best_valid_delta ||
+					(this_delta == best_valid_delta &&
+					 ctl->PagePrecedes(this_page_number,
+									   best_valid_page_number)))
+				{
+					bestvalidslot = slotno;
+					best_valid_delta = this_delta;
+					best_valid_page_number = this_page_number;
+				}
+			}
+			else
+			{
+				if (this_delta > best_invalid_delta ||
+					(this_delta == best_invalid_delta &&
+					 ctl->PagePrecedes(this_page_number,
+									   best_invalid_page_number)))
+				{
+					bestinvalidslot = slotno;
+					best_invalid_delta = this_delta;
+					best_invalid_page_number = this_page_number;
+				}
+			}
+		}
+
+		/*
+		 * If all pages (except possibly the latest one) are I/O busy, we'll
+		 * have to wait for an I/O to complete and then retry.  In that
+		 * unhappy case, we choose to wait for the I/O on the least recently
+		 * used slot, on the assumption that it was likely initiated first of
+		 * all the I/Os in progress and may therefore finish first.
+		 */
+		if (best_valid_delta < 0)
+		{
+			SimpleLruWaitIO(ctl, bestinvalidslot);
+			continue;
+		}
+
+		/*
+		 * If the selected page is clean, we're set.
+		 */
+		if (!shared->page_dirty[bestvalidslot])
+			return bestvalidslot;
+
+		/*
+		 * Write the page.
+		 */
+		SlruInternalWritePage(ctl, bestvalidslot, NULL);
+
+		/*
+		 * Now loop back and try again.  This is the easiest way of dealing
+		 * with corner cases such as the victim page being re-dirtied while we
+		 * wrote it.
+		 */
+	}
+}
+
+/*
+ * Write dirty pages to disk during checkpoint or database shutdown.  Flushing
+ * is deferred until the next call to ProcessSyncRequests(), though we do fsync
+ * the containing directory here to make sure that newly created directory
+ * entries are on disk.
+ */
+void
+SimpleLruWriteAll(SlruCtl ctl, bool allow_redirtied)
+{
+	SlruShared	shared = ctl->shared;
+	SlruWriteAllData fdata;
+	int			slotno;
+	int			pageno = 0;
+	int			i;
+	bool		ok;
+
+	/* update the stats counter of flushes */
+	pgstat_count_slru_flush(shared->slru_stats_idx);
+
+	/*
+	 * Find and write dirty pages
+	 */
+	fdata.num_files = 0;
+
+	LWLockAcquire(shared->ControlLock, LW_EXCLUSIVE);
+
+	for (slotno = 0; slotno < shared->num_slots; slotno++)
+	{
+		SlruInternalWritePage(ctl, slotno, &fdata);
+
+		/*
+		 * In some places (e.g. checkpoints), we cannot assert that the slot
+		 * is clean now, since another process might have re-dirtied it
+		 * already.  That's okay.
+		 */
+		Assert(allow_redirtied ||
+			   shared->page_status[slotno] == SLRU_PAGE_EMPTY ||
+			   (shared->page_status[slotno] == SLRU_PAGE_VALID &&
+				!shared->page_dirty[slotno]));
+	}
+
+	LWLockRelease(shared->ControlLock);
+
+	/*
+	 * Now close any files that were open
+	 */
+	ok = true;
+	for (i = 0; i < fdata.num_files; i++)
+	{
+		if (CloseTransientFile(fdata.fd[i]) != 0)
+		{
+			slru_errcause = SLRU_CLOSE_FAILED;
+			slru_errno = errno;
+			pageno = fdata.segno[i] * SLRU_PAGES_PER_SEGMENT;
+			ok = false;
+		}
+	}
+	if (!ok)
+		SlruReportIOError(ctl, pageno, InvalidTransactionId);
+
+	/* Ensure that directory entries for new files are on disk. */
+	if (ctl->sync_handler != SYNC_HANDLER_NONE)
+		fsync_fname(ctl->Dir, true);
+}
+
+/*
+ * Remove all segments before the one holding the passed page number
+ *
+ * All SLRUs prevent concurrent calls to this function, either with an LWLock
+ * or by calling it only as part of a checkpoint.  Mutual exclusion must begin
+ * before computing cutoffPage.  Mutual exclusion must end after any limit
+ * update that would permit other backends to write fresh data into the
+ * segment immediately preceding the one containing cutoffPage.  Otherwise,
+ * when the SLRU is quite full, SimpleLruTruncate() might delete that segment
+ * after it has accrued freshly-written data.
+ */
+void
+SimpleLruTruncate(SlruCtl ctl, int cutoffPage)
+{
+	SlruShared	shared = ctl->shared;
+	int			slotno;
+
+	/* update the stats counter of truncates */
+	pgstat_count_slru_truncate(shared->slru_stats_idx);
+
+	/*
+	 * Scan shared memory and remove any pages preceding the cutoff page, to
+	 * ensure we won't rewrite them later.  (Since this is normally called in
+	 * or just after a checkpoint, any dirty pages should have been flushed
+	 * already ... we're just being extra careful here.)
+	 */
+	LWLockAcquire(shared->ControlLock, LW_EXCLUSIVE);
+
+restart:
+
+	/*
+	 * While we are holding the lock, make an important safety check: the
+	 * current endpoint page must not be eligible for removal.
+	 */
+	if (ctl->PagePrecedes(shared->latest_page_number, cutoffPage))
+	{
+		LWLockRelease(shared->ControlLock);
+		ereport(LOG,
+				(errmsg("could not truncate directory \"%s\": apparent wraparound",
+						ctl->Dir)));
+		return;
+	}
+
+	for (slotno = 0; slotno < shared->num_slots; slotno++)
+	{
+		if (shared->page_status[slotno] == SLRU_PAGE_EMPTY)
+			continue;
+		if (!ctl->PagePrecedes(shared->page_number[slotno], cutoffPage))
+			continue;
+
+		/*
+		 * If page is clean, just change state to EMPTY (expected case).
+		 */
+		if (shared->page_status[slotno] == SLRU_PAGE_VALID &&
+			!shared->page_dirty[slotno])
+		{
+			shared->page_status[slotno] = SLRU_PAGE_EMPTY;
+			continue;
+		}
+
+		/*
+		 * Hmm, we have (or may have) I/O operations acting on the page, so
+		 * we've got to wait for them to finish and then start again. This is
+		 * the same logic as in SlruSelectLRUPage.  (XXX if page is dirty,
+		 * wouldn't it be OK to just discard it without writing it?
+		 * SlruMayDeleteSegment() uses a stricter qualification, so we might
+		 * not delete this page in the end; even if we don't delete it, we
+		 * won't have cause to read its data again.  For now, keep the logic
+		 * the same as it was.)
+		 */
+		if (shared->page_status[slotno] == SLRU_PAGE_VALID)
+			SlruInternalWritePage(ctl, slotno, NULL);
+		else
+			SimpleLruWaitIO(ctl, slotno);
+		goto restart;
+	}
+
+	LWLockRelease(shared->ControlLock);
+
+	/* Now we can remove the old segment(s) */
+	(void) SlruScanDirectory(ctl, SlruScanDirCbDeleteCutoff, &cutoffPage);
+}
+
+/*
+ * Delete an individual SLRU segment.
+ *
+ * NB: This does not touch the SLRU buffers themselves, callers have to ensure
+ * they either can't yet contain anything, or have already been cleaned out.
+ */
+static void
+SlruInternalDeleteSegment(SlruCtl ctl, int segno)
+{
+	char		path[MAXPGPATH];
+
+	/* Forget any fsync requests queued for this segment. */
+	if (ctl->sync_handler != SYNC_HANDLER_NONE)
+	{
+		FileTag		tag;
+
+		INIT_SLRUFILETAG(tag, ctl->sync_handler, segno);
+		RegisterSyncRequest(&tag, SYNC_FORGET_REQUEST, true);
+	}
+
+	/* Unlink the file. */
+	SlruFileName(ctl, path, segno);
+	ereport(DEBUG2, (errmsg_internal("removing file \"%s\"", path)));
+	unlink(path);
+}
+
+/*
+ * Delete an individual SLRU segment, identified by the segment number.
+ */
+void
+SlruDeleteSegment(SlruCtl ctl, int segno)
+{
+	SlruShared	shared = ctl->shared;
+	int			slotno;
+	bool		did_write;
+
+	/* Clean out any possibly existing references to the segment. */
+	LWLockAcquire(shared->ControlLock, LW_EXCLUSIVE);
+restart:
+	did_write = false;
+	for (slotno = 0; slotno < shared->num_slots; slotno++)
+	{
+		int			pagesegno = shared->page_number[slotno] / SLRU_PAGES_PER_SEGMENT;
+
+		if (shared->page_status[slotno] == SLRU_PAGE_EMPTY)
+			continue;
+
+		/* not the segment we're looking for */
+		if (pagesegno != segno)
+			continue;
+
+		/* If page is clean, just change state to EMPTY (expected case). */
+		if (shared->page_status[slotno] == SLRU_PAGE_VALID &&
+			!shared->page_dirty[slotno])
+		{
+			shared->page_status[slotno] = SLRU_PAGE_EMPTY;
+			continue;
+		}
+
+		/* Same logic as SimpleLruTruncate() */
+		if (shared->page_status[slotno] == SLRU_PAGE_VALID)
+			SlruInternalWritePage(ctl, slotno, NULL);
+		else
+			SimpleLruWaitIO(ctl, slotno);
+
+		did_write = true;
+	}
+
+	/*
+	 * Be extra careful and re-check. The IO functions release the control
+	 * lock, so new pages could have been read in.
+	 */
+	if (did_write)
+		goto restart;
+
+	SlruInternalDeleteSegment(ctl, segno);
+
+	LWLockRelease(shared->ControlLock);
+}
+
+/*
+ * Determine whether a segment is okay to delete.
+ *
+ * segpage is the first page of the segment, and cutoffPage is the oldest (in
+ * PagePrecedes order) page in the SLRU containing still-useful data.  Since
+ * every core PagePrecedes callback implements "wrap around", check the
+ * segment's first and last pages:
+ *
+ * first<cutoff  && last<cutoff:  yes
+ * first<cutoff  && last>=cutoff: no; cutoff falls inside this segment
+ * first>=cutoff && last<cutoff:  no; wrap point falls inside this segment
+ * first>=cutoff && last>=cutoff: no; every page of this segment is too young
+ */
+static bool
+SlruMayDeleteSegment(SlruCtl ctl, int segpage, int cutoffPage)
+{
+	int			seg_last_page = segpage + SLRU_PAGES_PER_SEGMENT - 1;
+
+	Assert(segpage % SLRU_PAGES_PER_SEGMENT == 0);
+
+	return (ctl->PagePrecedes(segpage, cutoffPage) &&
+			ctl->PagePrecedes(seg_last_page, cutoffPage));
+}
+
+#ifdef USE_ASSERT_CHECKING
+static void
+SlruPagePrecedesTestOffset(SlruCtl ctl, int per_page, uint32 offset)
+{
+	TransactionId lhs,
+				rhs;
+	int			newestPage,
+				oldestPage;
+	TransactionId newestXact,
+				oldestXact;
+
+	/*
+	 * Compare an XID pair having undefined order (see RFC 1982), a pair at
+	 * "opposite ends" of the XID space.  TransactionIdPrecedes() treats each
+	 * as preceding the other.  If RHS is oldestXact, LHS is the first XID we
+	 * must not assign.
+	 */
+	lhs = per_page + offset;	/* skip first page to avoid non-normal XIDs */
+	rhs = lhs + (1U << 31);
+	Assert(TransactionIdPrecedes(lhs, rhs));
+	Assert(TransactionIdPrecedes(rhs, lhs));
+	Assert(!TransactionIdPrecedes(lhs - 1, rhs));
+	Assert(TransactionIdPrecedes(rhs, lhs - 1));
+	Assert(TransactionIdPrecedes(lhs + 1, rhs));
+	Assert(!TransactionIdPrecedes(rhs, lhs + 1));
+	Assert(!TransactionIdFollowsOrEquals(lhs, rhs));
+	Assert(!TransactionIdFollowsOrEquals(rhs, lhs));
+	Assert(!ctl->PagePrecedes(lhs / per_page, lhs / per_page));
+	Assert(!ctl->PagePrecedes(lhs / per_page, rhs / per_page));
+	Assert(!ctl->PagePrecedes(rhs / per_page, lhs / per_page));
+	Assert(!ctl->PagePrecedes((lhs - per_page) / per_page, rhs / per_page));
+	Assert(ctl->PagePrecedes(rhs / per_page, (lhs - 3 * per_page) / per_page));
+	Assert(ctl->PagePrecedes(rhs / per_page, (lhs - 2 * per_page) / per_page));
+	Assert(ctl->PagePrecedes(rhs / per_page, (lhs - 1 * per_page) / per_page)
+		   || (1U << 31) % per_page != 0);	/* See CommitTsPagePrecedes() */
+	Assert(ctl->PagePrecedes((lhs + 1 * per_page) / per_page, rhs / per_page)
+		   || (1U << 31) % per_page != 0);
+	Assert(ctl->PagePrecedes((lhs + 2 * per_page) / per_page, rhs / per_page));
+	Assert(ctl->PagePrecedes((lhs + 3 * per_page) / per_page, rhs / per_page));
+	Assert(!ctl->PagePrecedes(rhs / per_page, (lhs + per_page) / per_page));
+
+	/*
+	 * GetNewTransactionId() has assigned the last XID it can safely use, and
+	 * that XID is in the *LAST* page of the second segment.  We must not
+	 * delete that segment.
+	 */
+	newestPage = 2 * SLRU_PAGES_PER_SEGMENT - 1;
+	newestXact = newestPage * per_page + offset;
+	Assert(newestXact / per_page == newestPage);
+	oldestXact = newestXact + 1;
+	oldestXact -= 1U << 31;
+	oldestPage = oldestXact / per_page;
+	Assert(!SlruMayDeleteSegment(ctl,
+								 (newestPage -
+								  newestPage % SLRU_PAGES_PER_SEGMENT),
+								 oldestPage));
+
+	/*
+	 * GetNewTransactionId() has assigned the last XID it can safely use, and
+	 * that XID is in the *FIRST* page of the second segment.  We must not
+	 * delete that segment.
+	 */
+	newestPage = SLRU_PAGES_PER_SEGMENT;
+	newestXact = newestPage * per_page + offset;
+	Assert(newestXact / per_page == newestPage);
+	oldestXact = newestXact + 1;
+	oldestXact -= 1U << 31;
+	oldestPage = oldestXact / per_page;
+	Assert(!SlruMayDeleteSegment(ctl,
+								 (newestPage -
+								  newestPage % SLRU_PAGES_PER_SEGMENT),
+								 oldestPage));
+}
+
+/*
+ * Unit-test a PagePrecedes function.
+ *
+ * This assumes every uint32 >= FirstNormalTransactionId is a valid key.  It
+ * assumes each value occupies a contiguous, fixed-size region of SLRU bytes.
+ * (MultiXactMemberCtl separates flags from XIDs.  NotifyCtl has
+ * variable-length entries, no keys, and no random access.  These unit tests
+ * do not apply to them.)
+ */
+void
+SlruPagePrecedesUnitTests(SlruCtl ctl, int per_page)
+{
+	/* Test first, middle and last entries of a page. */
+	SlruPagePrecedesTestOffset(ctl, per_page, 0);
+	SlruPagePrecedesTestOffset(ctl, per_page, per_page / 2);
+	SlruPagePrecedesTestOffset(ctl, per_page, per_page - 1);
+}
+#endif
+
+/*
+ * SlruScanDirectory callback
+ *		This callback reports true if there's any segment wholly prior to the
+ *		one containing the page passed as "data".
+ */
+bool
+SlruScanDirCbReportPresence(SlruCtl ctl, char *filename, int segpage, void *data)
+{
+	int			cutoffPage = *(int *) data;
+
+	if (SlruMayDeleteSegment(ctl, segpage, cutoffPage))
+		return true;			/* found one; don't iterate any more */
+
+	return false;				/* keep going */
+}
+
+/*
+ * SlruScanDirectory callback.
+ *		This callback deletes segments prior to the one passed in as "data".
+ */
+static bool
+SlruScanDirCbDeleteCutoff(SlruCtl ctl, char *filename, int segpage, void *data)
+{
+	int			cutoffPage = *(int *) data;
+
+	if (SlruMayDeleteSegment(ctl, segpage, cutoffPage))
+		SlruInternalDeleteSegment(ctl, segpage / SLRU_PAGES_PER_SEGMENT);
+
+	return false;				/* keep going */
+}
+
+/*
+ * SlruScanDirectory callback.
+ *		This callback deletes all segments.
+ */
+bool
+SlruScanDirCbDeleteAll(SlruCtl ctl, char *filename, int segpage, void *data)
+{
+	SlruInternalDeleteSegment(ctl, segpage / SLRU_PAGES_PER_SEGMENT);
+
+	return false;				/* keep going */
+}
+
+/*
+ * Scan the SimpleLru directory and apply a callback to each file found in it.
+ *
+ * If the callback returns true, the scan is stopped.  The last return value
+ * from the callback is returned.
+ *
+ * The callback receives the following arguments: 1. the SlruCtl struct for the
+ * slru being truncated; 2. the filename being considered; 3. the page number
+ * for the first page of that file; 4. a pointer to the opaque data given to us
+ * by the caller.
+ *
+ * Note that the ordering in which the directory is scanned is not guaranteed.
+ *
+ * Note that no locking is applied.
+ */
+bool
+SlruScanDirectory(SlruCtl ctl, SlruScanCallback callback, void *data)
+{
+	bool		retval = false;
+	DIR		   *cldir;
+	struct dirent *clde;
+	int			segno;
+	int			segpage;
+
+	cldir = AllocateDir(ctl->Dir);
+	while ((clde = ReadDir(cldir, ctl->Dir)) != NULL)
+	{
+		size_t		len;
+
+		len = strlen(clde->d_name);
+
+		if ((len == 4 || len == 5 || len == 6) &&
+			strspn(clde->d_name, "0123456789ABCDEF") == len)
+		{
+			segno = (int) strtol(clde->d_name, NULL, 16);
+			segpage = segno * SLRU_PAGES_PER_SEGMENT;
+
+			elog(DEBUG2, "SlruScanDirectory invoking callback on %s/%s",
+				 ctl->Dir, clde->d_name);
+			retval = callback(ctl, clde->d_name, segpage, data);
+			if (retval)
+				break;
+		}
+	}
+	FreeDir(cldir);
+
+	return retval;
+}
+
+/*
+ * Individual SLRUs (clog, ...) have to provide a sync.c handler function so
+ * that they can provide the correct "SlruCtl" (otherwise we don't know how to
+ * build the path), but they just forward to this common implementation that
+ * performs the fsync.
+ */
+int
+SlruSyncFileTag(SlruCtl ctl, const FileTag *ftag, char *path)
+{
+	int			fd;
+	int			save_errno;
+	int			result;
+
+	SlruFileName(ctl, path, ftag->segno);
+
+	fd = OpenTransientFile(path, O_RDWR | PG_BINARY);
+	if (fd < 0)
+		return -1;
+
+	pgstat_report_wait_start(WAIT_EVENT_SLRU_FLUSH_SYNC);
+	result = pg_fsync(fd);
+	pgstat_report_wait_end();
+	save_errno = errno;
+
+	CloseTransientFile(fd);
+
+	errno = save_errno;
+	return result;
+}