Adding upstream version 13.4.upstream/13.4upstream

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

1 files changed, 674 insertions, 0 deletions

diff --git a/src/backend/access/heap/visibilitymap.c b/src/backend/access/heap/visibilitymap.c
new file mode 100644
index 0000000..0a51678
--- /dev/null
+++ b/src/backend/access/heap/visibilitymap.c
@@ -0,0 +1,674 @@
+/*-------------------------------------------------------------------------
+ *
+ * visibilitymap.c
+ *	  bitmap for tracking visibility of heap tuples
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/access/heap/visibilitymap.c
+ *
+ * INTERFACE ROUTINES
+ *		visibilitymap_clear  - clear bits for one page in the visibility map
+ *		visibilitymap_pin	 - pin a map page for setting a bit
+ *		visibilitymap_pin_ok - check whether correct map page is already pinned
+ *		visibilitymap_set	 - set a bit in a previously pinned page
+ *		visibilitymap_get_status - get status of bits
+ *		visibilitymap_count  - count number of bits set in visibility map
+ *		visibilitymap_prepare_truncate -
+ *			prepare for truncation of the visibility map
+ *
+ * NOTES
+ *
+ * The visibility map is a bitmap with two bits (all-visible and all-frozen)
+ * per heap page. A set all-visible bit means that all tuples on the page are
+ * known visible to all transactions, and therefore the page doesn't need to
+ * be vacuumed. A set all-frozen bit means that all tuples on the page are
+ * completely frozen, and therefore the page doesn't need to be vacuumed even
+ * if whole table scanning vacuum is required (e.g. anti-wraparound vacuum).
+ * The all-frozen bit must be set only when the page is already all-visible.
+ *
+ * The map is conservative in the sense that we make sure that whenever a bit
+ * is set, we know the condition is true, but if a bit is not set, it might or
+ * might not be true.
+ *
+ * Clearing visibility map bits is not separately WAL-logged.  The callers
+ * must make sure that whenever a bit is cleared, the bit is cleared on WAL
+ * replay of the updating operation as well.
+ *
+ * When we *set* a visibility map during VACUUM, we must write WAL.  This may
+ * seem counterintuitive, since the bit is basically a hint: if it is clear,
+ * it may still be the case that every tuple on the page is visible to all
+ * transactions; we just don't know that for certain.  The difficulty is that
+ * there are two bits which are typically set together: the PD_ALL_VISIBLE bit
+ * on the page itself, and the visibility map bit.  If a crash occurs after the
+ * visibility map page makes it to disk and before the updated heap page makes
+ * it to disk, redo must set the bit on the heap page.  Otherwise, the next
+ * insert, update, or delete on the heap page will fail to realize that the
+ * visibility map bit must be cleared, possibly causing index-only scans to
+ * return wrong answers.
+ *
+ * VACUUM will normally skip pages for which the visibility map bit is set;
+ * such pages can't contain any dead tuples and therefore don't need vacuuming.
+ *
+ * LOCKING
+ *
+ * In heapam.c, whenever a page is modified so that not all tuples on the
+ * page are visible to everyone anymore, the corresponding bit in the
+ * visibility map is cleared. In order to be crash-safe, we need to do this
+ * while still holding a lock on the heap page and in the same critical
+ * section that logs the page modification. However, we don't want to hold
+ * the buffer lock over any I/O that may be required to read in the visibility
+ * map page.  To avoid this, we examine the heap page before locking it;
+ * if the page-level PD_ALL_VISIBLE bit is set, we pin the visibility map
+ * bit.  Then, we lock the buffer.  But this creates a race condition: there
+ * is a possibility that in the time it takes to lock the buffer, the
+ * PD_ALL_VISIBLE bit gets set.  If that happens, we have to unlock the
+ * buffer, pin the visibility map page, and relock the buffer.  This shouldn't
+ * happen often, because only VACUUM currently sets visibility map bits,
+ * and the race will only occur if VACUUM processes a given page at almost
+ * exactly the same time that someone tries to further modify it.
+ *
+ * To set a bit, you need to hold a lock on the heap page. That prevents
+ * the race condition where VACUUM sees that all tuples on the page are
+ * visible to everyone, but another backend modifies the page before VACUUM
+ * sets the bit in the visibility map.
+ *
+ * When a bit is set, the LSN of the visibility map page is updated to make
+ * sure that the visibility map update doesn't get written to disk before the
+ * WAL record of the changes that made it possible to set the bit is flushed.
+ * But when a bit is cleared, we don't have to do that because it's always
+ * safe to clear a bit in the map from correctness point of view.
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "access/heapam_xlog.h"
+#include "access/visibilitymap.h"
+#include "access/xlog.h"
+#include "miscadmin.h"
+#include "port/pg_bitutils.h"
+#include "storage/bufmgr.h"
+#include "storage/lmgr.h"
+#include "storage/smgr.h"
+#include "utils/inval.h"
+
+
+/*#define TRACE_VISIBILITYMAP */
+
+/*
+ * Size of the bitmap on each visibility map page, in bytes. There's no
+ * extra headers, so the whole page minus the standard page header is
+ * used for the bitmap.
+ */
+#define MAPSIZE (BLCKSZ - MAXALIGN(SizeOfPageHeaderData))
+
+/* Number of heap blocks we can represent in one byte */
+#define HEAPBLOCKS_PER_BYTE (BITS_PER_BYTE / BITS_PER_HEAPBLOCK)
+
+/* Number of heap blocks we can represent in one visibility map page. */
+#define HEAPBLOCKS_PER_PAGE (MAPSIZE * HEAPBLOCKS_PER_BYTE)
+
+/* Mapping from heap block number to the right bit in the visibility map */
+#define HEAPBLK_TO_MAPBLOCK(x) ((x) / HEAPBLOCKS_PER_PAGE)
+#define HEAPBLK_TO_MAPBYTE(x) (((x) % HEAPBLOCKS_PER_PAGE) / HEAPBLOCKS_PER_BYTE)
+#define HEAPBLK_TO_OFFSET(x) (((x) % HEAPBLOCKS_PER_BYTE) * BITS_PER_HEAPBLOCK)
+
+/* Masks for counting subsets of bits in the visibility map. */
+#define VISIBLE_MASK64	UINT64CONST(0x5555555555555555) /* The lower bit of each
+														 * bit pair */
+#define FROZEN_MASK64	UINT64CONST(0xaaaaaaaaaaaaaaaa) /* The upper bit of each
+														 * bit pair */
+
+/* prototypes for internal routines */
+static Buffer vm_readbuf(Relation rel, BlockNumber blkno, bool extend);
+static void vm_extend(Relation rel, BlockNumber vm_nblocks);
+
+
+/*
+ *	visibilitymap_clear - clear specified bits for one page in visibility map
+ *
+ * You must pass a buffer containing the correct map page to this function.
+ * Call visibilitymap_pin first to pin the right one. This function doesn't do
+ * any I/O.  Returns true if any bits have been cleared and false otherwise.
+ */
+bool
+visibilitymap_clear(Relation rel, BlockNumber heapBlk, Buffer buf, uint8 flags)
+{
+	BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
+	int			mapByte = HEAPBLK_TO_MAPBYTE(heapBlk);
+	int			mapOffset = HEAPBLK_TO_OFFSET(heapBlk);
+	uint8		mask = flags << mapOffset;
+	char	   *map;
+	bool		cleared = false;
+
+	Assert(flags & VISIBILITYMAP_VALID_BITS);
+
+#ifdef TRACE_VISIBILITYMAP
+	elog(DEBUG1, "vm_clear %s %d", RelationGetRelationName(rel), heapBlk);
+#endif
+
+	if (!BufferIsValid(buf) || BufferGetBlockNumber(buf) != mapBlock)
+		elog(ERROR, "wrong buffer passed to visibilitymap_clear");
+
+	LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
+	map = PageGetContents(BufferGetPage(buf));
+
+	if (map[mapByte] & mask)
+	{
+		map[mapByte] &= ~mask;
+
+		MarkBufferDirty(buf);
+		cleared = true;
+	}
+
+	LockBuffer(buf, BUFFER_LOCK_UNLOCK);
+
+	return cleared;
+}
+
+/*
+ *	visibilitymap_pin - pin a map page for setting a bit
+ *
+ * Setting a bit in the visibility map is a two-phase operation. First, call
+ * visibilitymap_pin, to pin the visibility map page containing the bit for
+ * the heap page. Because that can require I/O to read the map page, you
+ * shouldn't hold a lock on the heap page while doing that. Then, call
+ * visibilitymap_set to actually set the bit.
+ *
+ * On entry, *buf should be InvalidBuffer or a valid buffer returned by
+ * an earlier call to visibilitymap_pin or visibilitymap_get_status on the same
+ * relation. On return, *buf is a valid buffer with the map page containing
+ * the bit for heapBlk.
+ *
+ * If the page doesn't exist in the map file yet, it is extended.
+ */
+void
+visibilitymap_pin(Relation rel, BlockNumber heapBlk, Buffer *buf)
+{
+	BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
+
+	/* Reuse the old pinned buffer if possible */
+	if (BufferIsValid(*buf))
+	{
+		if (BufferGetBlockNumber(*buf) == mapBlock)
+			return;
+
+		ReleaseBuffer(*buf);
+	}
+	*buf = vm_readbuf(rel, mapBlock, true);
+}
+
+/*
+ *	visibilitymap_pin_ok - do we already have the correct page pinned?
+ *
+ * On entry, buf should be InvalidBuffer or a valid buffer returned by
+ * an earlier call to visibilitymap_pin or visibilitymap_get_status on the same
+ * relation.  The return value indicates whether the buffer covers the
+ * given heapBlk.
+ */
+bool
+visibilitymap_pin_ok(BlockNumber heapBlk, Buffer buf)
+{
+	BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
+
+	return BufferIsValid(buf) && BufferGetBlockNumber(buf) == mapBlock;
+}
+
+/*
+ *	visibilitymap_set - set bit(s) on a previously pinned page
+ *
+ * recptr is the LSN of the XLOG record we're replaying, if we're in recovery,
+ * or InvalidXLogRecPtr in normal running.  The page LSN is advanced to the
+ * one provided; in normal running, we generate a new XLOG record and set the
+ * page LSN to that value.  cutoff_xid is the largest xmin on the page being
+ * marked all-visible; it is needed for Hot Standby, and can be
+ * InvalidTransactionId if the page contains no tuples.  It can also be set
+ * to InvalidTransactionId when a page that is already all-visible is being
+ * marked all-frozen.
+ *
+ * Caller is expected to set the heap page's PD_ALL_VISIBLE bit before calling
+ * this function. Except in recovery, caller should also pass the heap
+ * buffer. When checksums are enabled and we're not in recovery, we must add
+ * the heap buffer to the WAL chain to protect it from being torn.
+ *
+ * You must pass a buffer containing the correct map page to this function.
+ * Call visibilitymap_pin first to pin the right one. This function doesn't do
+ * any I/O.
+ */
+void
+visibilitymap_set(Relation rel, BlockNumber heapBlk, Buffer heapBuf,
+				  XLogRecPtr recptr, Buffer vmBuf, TransactionId cutoff_xid,
+				  uint8 flags)
+{
+	BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
+	uint32		mapByte = HEAPBLK_TO_MAPBYTE(heapBlk);
+	uint8		mapOffset = HEAPBLK_TO_OFFSET(heapBlk);
+	Page		page;
+	uint8	   *map;
+
+#ifdef TRACE_VISIBILITYMAP
+	elog(DEBUG1, "vm_set %s %d", RelationGetRelationName(rel), heapBlk);
+#endif
+
+	Assert(InRecovery || XLogRecPtrIsInvalid(recptr));
+	Assert(InRecovery || BufferIsValid(heapBuf));
+	Assert(flags & VISIBILITYMAP_VALID_BITS);
+
+	/* Check that we have the right heap page pinned, if present */
+	if (BufferIsValid(heapBuf) && BufferGetBlockNumber(heapBuf) != heapBlk)
+		elog(ERROR, "wrong heap buffer passed to visibilitymap_set");
+
+	/* Check that we have the right VM page pinned */
+	if (!BufferIsValid(vmBuf) || BufferGetBlockNumber(vmBuf) != mapBlock)
+		elog(ERROR, "wrong VM buffer passed to visibilitymap_set");
+
+	page = BufferGetPage(vmBuf);
+	map = (uint8 *) PageGetContents(page);
+	LockBuffer(vmBuf, BUFFER_LOCK_EXCLUSIVE);
+
+	if (flags != (map[mapByte] >> mapOffset & VISIBILITYMAP_VALID_BITS))
+	{
+		START_CRIT_SECTION();
+
+		map[mapByte] |= (flags << mapOffset);
+		MarkBufferDirty(vmBuf);
+
+		if (RelationNeedsWAL(rel))
+		{
+			if (XLogRecPtrIsInvalid(recptr))
+			{
+				Assert(!InRecovery);
+				recptr = log_heap_visible(rel->rd_node, heapBuf, vmBuf,
+										  cutoff_xid, flags);
+
+				/*
+				 * If data checksums are enabled (or wal_log_hints=on), we
+				 * need to protect the heap page from being torn.
+				 */
+				if (XLogHintBitIsNeeded())
+				{
+					Page		heapPage = BufferGetPage(heapBuf);
+
+					/* caller is expected to set PD_ALL_VISIBLE first */
+					Assert(PageIsAllVisible(heapPage));
+					PageSetLSN(heapPage, recptr);
+				}
+			}
+			PageSetLSN(page, recptr);
+		}
+
+		END_CRIT_SECTION();
+	}
+
+	LockBuffer(vmBuf, BUFFER_LOCK_UNLOCK);
+}
+
+/*
+ *	visibilitymap_get_status - get status of bits
+ *
+ * Are all tuples on heapBlk visible to all or are marked frozen, according
+ * to the visibility map?
+ *
+ * On entry, *buf should be InvalidBuffer or a valid buffer returned by an
+ * earlier call to visibilitymap_pin or visibilitymap_get_status on the same
+ * relation. On return, *buf is a valid buffer with the map page containing
+ * the bit for heapBlk, or InvalidBuffer. The caller is responsible for
+ * releasing *buf after it's done testing and setting bits.
+ *
+ * NOTE: This function is typically called without a lock on the heap page,
+ * so somebody else could change the bit just after we look at it.  In fact,
+ * since we don't lock the visibility map page either, it's even possible that
+ * someone else could have changed the bit just before we look at it, but yet
+ * we might see the old value.  It is the caller's responsibility to deal with
+ * all concurrency issues!
+ */
+uint8
+visibilitymap_get_status(Relation rel, BlockNumber heapBlk, Buffer *buf)
+{
+	BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
+	uint32		mapByte = HEAPBLK_TO_MAPBYTE(heapBlk);
+	uint8		mapOffset = HEAPBLK_TO_OFFSET(heapBlk);
+	char	   *map;
+	uint8		result;
+
+#ifdef TRACE_VISIBILITYMAP
+	elog(DEBUG1, "vm_get_status %s %d", RelationGetRelationName(rel), heapBlk);
+#endif
+
+	/* Reuse the old pinned buffer if possible */
+	if (BufferIsValid(*buf))
+	{
+		if (BufferGetBlockNumber(*buf) != mapBlock)
+		{
+			ReleaseBuffer(*buf);
+			*buf = InvalidBuffer;
+		}
+	}
+
+	if (!BufferIsValid(*buf))
+	{
+		*buf = vm_readbuf(rel, mapBlock, false);
+		if (!BufferIsValid(*buf))
+			return false;
+	}
+
+	map = PageGetContents(BufferGetPage(*buf));
+
+	/*
+	 * A single byte read is atomic.  There could be memory-ordering effects
+	 * here, but for performance reasons we make it the caller's job to worry
+	 * about that.
+	 */
+	result = ((map[mapByte] >> mapOffset) & VISIBILITYMAP_VALID_BITS);
+	return result;
+}
+
+/*
+ *	visibilitymap_count  - count number of bits set in visibility map
+ *
+ * Note: we ignore the possibility of race conditions when the table is being
+ * extended concurrently with the call.  New pages added to the table aren't
+ * going to be marked all-visible or all-frozen, so they won't affect the result.
+ */
+void
+visibilitymap_count(Relation rel, BlockNumber *all_visible, BlockNumber *all_frozen)
+{
+	BlockNumber mapBlock;
+	BlockNumber nvisible = 0;
+	BlockNumber nfrozen = 0;
+
+	/* all_visible must be specified */
+	Assert(all_visible);
+
+	for (mapBlock = 0;; mapBlock++)
+	{
+		Buffer		mapBuffer;
+		uint64	   *map;
+		int			i;
+
+		/*
+		 * Read till we fall off the end of the map.  We assume that any extra
+		 * bytes in the last page are zeroed, so we don't bother excluding
+		 * them from the count.
+		 */
+		mapBuffer = vm_readbuf(rel, mapBlock, false);
+		if (!BufferIsValid(mapBuffer))
+			break;
+
+		/*
+		 * We choose not to lock the page, since the result is going to be
+		 * immediately stale anyway if anyone is concurrently setting or
+		 * clearing bits, and we only really need an approximate value.
+		 */
+		map = (uint64 *) PageGetContents(BufferGetPage(mapBuffer));
+
+		StaticAssertStmt(MAPSIZE % sizeof(uint64) == 0,
+						 "unsupported MAPSIZE");
+		if (all_frozen == NULL)
+		{
+			for (i = 0; i < MAPSIZE / sizeof(uint64); i++)
+				nvisible += pg_popcount64(map[i] & VISIBLE_MASK64);
+		}
+		else
+		{
+			for (i = 0; i < MAPSIZE / sizeof(uint64); i++)
+			{
+				nvisible += pg_popcount64(map[i] & VISIBLE_MASK64);
+				nfrozen += pg_popcount64(map[i] & FROZEN_MASK64);
+			}
+		}
+
+		ReleaseBuffer(mapBuffer);
+	}
+
+	*all_visible = nvisible;
+	if (all_frozen)
+		*all_frozen = nfrozen;
+}
+
+/*
+ *	visibilitymap_prepare_truncate -
+ *			prepare for truncation of the visibility map
+ *
+ * nheapblocks is the new size of the heap.
+ *
+ * Return the number of blocks of new visibility map.
+ * If it's InvalidBlockNumber, there is nothing to truncate;
+ * otherwise the caller is responsible for calling smgrtruncate()
+ * to truncate the visibility map pages.
+ */
+BlockNumber
+visibilitymap_prepare_truncate(Relation rel, BlockNumber nheapblocks)
+{
+	BlockNumber newnblocks;
+
+	/* last remaining block, byte, and bit */
+	BlockNumber truncBlock = HEAPBLK_TO_MAPBLOCK(nheapblocks);
+	uint32		truncByte = HEAPBLK_TO_MAPBYTE(nheapblocks);
+	uint8		truncOffset = HEAPBLK_TO_OFFSET(nheapblocks);
+
+#ifdef TRACE_VISIBILITYMAP
+	elog(DEBUG1, "vm_truncate %s %d", RelationGetRelationName(rel), nheapblocks);
+#endif
+
+	RelationOpenSmgr(rel);
+
+	/*
+	 * If no visibility map has been created yet for this relation, there's
+	 * nothing to truncate.
+	 */
+	if (!smgrexists(rel->rd_smgr, VISIBILITYMAP_FORKNUM))
+		return InvalidBlockNumber;
+
+	/*
+	 * Unless the new size is exactly at a visibility map page boundary, the
+	 * tail bits in the last remaining map page, representing truncated heap
+	 * blocks, need to be cleared. This is not only tidy, but also necessary
+	 * because we don't get a chance to clear the bits if the heap is extended
+	 * again.
+	 */
+	if (truncByte != 0 || truncOffset != 0)
+	{
+		Buffer		mapBuffer;
+		Page		page;
+		char	   *map;
+
+		newnblocks = truncBlock + 1;
+
+		mapBuffer = vm_readbuf(rel, truncBlock, false);
+		if (!BufferIsValid(mapBuffer))
+		{
+			/* nothing to do, the file was already smaller */
+			return InvalidBlockNumber;
+		}
+
+		page = BufferGetPage(mapBuffer);
+		map = PageGetContents(page);
+
+		LockBuffer(mapBuffer, BUFFER_LOCK_EXCLUSIVE);
+
+		/* NO EREPORT(ERROR) from here till changes are logged */
+		START_CRIT_SECTION();
+
+		/* Clear out the unwanted bytes. */
+		MemSet(&map[truncByte + 1], 0, MAPSIZE - (truncByte + 1));
+
+		/*----
+		 * Mask out the unwanted bits of the last remaining byte.
+		 *
+		 * ((1 << 0) - 1) = 00000000
+		 * ((1 << 1) - 1) = 00000001
+		 * ...
+		 * ((1 << 6) - 1) = 00111111
+		 * ((1 << 7) - 1) = 01111111
+		 *----
+		 */
+		map[truncByte] &= (1 << truncOffset) - 1;
+
+		/*
+		 * Truncation of a relation is WAL-logged at a higher-level, and we
+		 * will be called at WAL replay. But if checksums are enabled, we need
+		 * to still write a WAL record to protect against a torn page, if the
+		 * page is flushed to disk before the truncation WAL record. We cannot
+		 * use MarkBufferDirtyHint here, because that will not dirty the page
+		 * during recovery.
+		 */
+		MarkBufferDirty(mapBuffer);
+		if (!InRecovery && RelationNeedsWAL(rel) && XLogHintBitIsNeeded())
+			log_newpage_buffer(mapBuffer, false);
+
+		END_CRIT_SECTION();
+
+		UnlockReleaseBuffer(mapBuffer);
+	}
+	else
+		newnblocks = truncBlock;
+
+	if (smgrnblocks(rel->rd_smgr, VISIBILITYMAP_FORKNUM) <= newnblocks)
+	{
+		/* nothing to do, the file was already smaller than requested size */
+		return InvalidBlockNumber;
+	}
+
+	return newnblocks;
+}
+
+/*
+ * Read a visibility map page.
+ *
+ * If the page doesn't exist, InvalidBuffer is returned, or if 'extend' is
+ * true, the visibility map file is extended.
+ */
+static Buffer
+vm_readbuf(Relation rel, BlockNumber blkno, bool extend)
+{
+	Buffer		buf;
+
+	/*
+	 * We might not have opened the relation at the smgr level yet, or we
+	 * might have been forced to close it by a sinval message.  The code below
+	 * won't necessarily notice relation extension immediately when extend =
+	 * false, so we rely on sinval messages to ensure that our ideas about the
+	 * size of the map aren't too far out of date.
+	 */
+	RelationOpenSmgr(rel);
+
+	/*
+	 * If we haven't cached the size of the visibility map fork yet, check it
+	 * first.
+	 */
+	if (rel->rd_smgr->smgr_vm_nblocks == InvalidBlockNumber)
+	{
+		if (smgrexists(rel->rd_smgr, VISIBILITYMAP_FORKNUM))
+			rel->rd_smgr->smgr_vm_nblocks = smgrnblocks(rel->rd_smgr,
+														VISIBILITYMAP_FORKNUM);
+		else
+			rel->rd_smgr->smgr_vm_nblocks = 0;
+	}
+
+	/* Handle requests beyond EOF */
+	if (blkno >= rel->rd_smgr->smgr_vm_nblocks)
+	{
+		if (extend)
+			vm_extend(rel, blkno + 1);
+		else
+			return InvalidBuffer;
+	}
+
+	/*
+	 * Use ZERO_ON_ERROR mode, and initialize the page if necessary. It's
+	 * always safe to clear bits, so it's better to clear corrupt pages than
+	 * error out.
+	 *
+	 * The initialize-the-page part is trickier than it looks, because of the
+	 * possibility of multiple backends doing this concurrently, and our
+	 * desire to not uselessly take the buffer lock in the normal path where
+	 * the page is OK.  We must take the lock to initialize the page, so
+	 * recheck page newness after we have the lock, in case someone else
+	 * already did it.  Also, because we initially check PageIsNew with no
+	 * lock, it's possible to fall through and return the buffer while someone
+	 * else is still initializing the page (i.e., we might see pd_upper as set
+	 * but other page header fields are still zeroes).  This is harmless for
+	 * callers that will take a buffer lock themselves, but some callers
+	 * inspect the page without any lock at all.  The latter is OK only so
+	 * long as it doesn't depend on the page header having correct contents.
+	 * Current usage is safe because PageGetContents() does not require that.
+	 */
+	buf = ReadBufferExtended(rel, VISIBILITYMAP_FORKNUM, blkno,
+							 RBM_ZERO_ON_ERROR, NULL);
+	if (PageIsNew(BufferGetPage(buf)))
+	{
+		LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
+		if (PageIsNew(BufferGetPage(buf)))
+			PageInit(BufferGetPage(buf), BLCKSZ, 0);
+		LockBuffer(buf, BUFFER_LOCK_UNLOCK);
+	}
+	return buf;
+}
+
+/*
+ * Ensure that the visibility map fork is at least vm_nblocks long, extending
+ * it if necessary with zeroed pages.
+ */
+static void
+vm_extend(Relation rel, BlockNumber vm_nblocks)
+{
+	BlockNumber vm_nblocks_now;
+	PGAlignedBlock pg;
+
+	PageInit((Page) pg.data, BLCKSZ, 0);
+
+	/*
+	 * We use the relation extension lock to lock out other backends trying to
+	 * extend the visibility map at the same time. It also locks out extension
+	 * of the main fork, unnecessarily, but extending the visibility map
+	 * happens seldom enough that it doesn't seem worthwhile to have a
+	 * separate lock tag type for it.
+	 *
+	 * Note that another backend might have extended or created the relation
+	 * by the time we get the lock.
+	 */
+	LockRelationForExtension(rel, ExclusiveLock);
+
+	/* Might have to re-open if a cache flush happened */
+	RelationOpenSmgr(rel);
+
+	/*
+	 * Create the file first if it doesn't exist.  If smgr_vm_nblocks is
+	 * positive then it must exist, no need for an smgrexists call.
+	 */
+	if ((rel->rd_smgr->smgr_vm_nblocks == 0 ||
+		 rel->rd_smgr->smgr_vm_nblocks == InvalidBlockNumber) &&
+		!smgrexists(rel->rd_smgr, VISIBILITYMAP_FORKNUM))
+		smgrcreate(rel->rd_smgr, VISIBILITYMAP_FORKNUM, false);
+
+	vm_nblocks_now = smgrnblocks(rel->rd_smgr, VISIBILITYMAP_FORKNUM);
+
+	/* Now extend the file */
+	while (vm_nblocks_now < vm_nblocks)
+	{
+		PageSetChecksumInplace((Page) pg.data, vm_nblocks_now);
+
+		smgrextend(rel->rd_smgr, VISIBILITYMAP_FORKNUM, vm_nblocks_now,
+				   pg.data, false);
+		vm_nblocks_now++;
+	}
+
+	/*
+	 * Send a shared-inval message to force other backends to close any smgr
+	 * references they may have for this rel, which we are about to change.
+	 * This is a useful optimization because it means that backends don't have
+	 * to keep checking for creation or extension of the file, which happens
+	 * infrequently.
+	 */
+	CacheInvalidateSmgr(rel->rd_smgr->smgr_rnode);
+
+	/* Update local cache with the up-to-date size */
+	rel->rd_smgr->smgr_vm_nblocks = vm_nblocks_now;
+
+	UnlockRelationForExtension(rel, ExclusiveLock);
+}