Update copyright for 2022

[pgsql.git] / src / backend / access / heap / visibilitymap.c

/*-------------------------------------------------------------------------
 *
 * visibilitymap.c
 *        bitmap for tracking visibility of heap tuples
 *
 * Portions Copyright (c) 1996-2022, 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/xlogutils.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

        /*
         * If no visibility map has been created yet for this relation, there's
         * nothing to truncate.
         */
        if (!smgrexists(RelationGetSmgr(rel), 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(RelationGetSmgr(rel), 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;
        SMgrRelation reln;

        /*
         * Caution: re-using this smgr pointer could fail if the relcache entry
         * gets closed.  It's safe as long as we only do smgr-level operations
         * between here and the last use of the pointer.
         */
        reln = RelationGetSmgr(rel);

        /*
         * If we haven't cached the size of the visibility map fork yet, check it
         * first.
         */
        if (reln->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] == InvalidBlockNumber)
        {
                if (smgrexists(reln, VISIBILITYMAP_FORKNUM))
                        smgrnblocks(reln, VISIBILITYMAP_FORKNUM);
                else
                        reln->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] = 0;
        }

        /* Handle requests beyond EOF */
        if (blkno >= reln->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM])
        {
                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;
        SMgrRelation reln;

        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);

        /*
         * Caution: re-using this smgr pointer could fail if the relcache entry
         * gets closed.  It's safe as long as we only do smgr-level operations
         * between here and the last use of the pointer.
         */
        reln = RelationGetSmgr(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 ((reln->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] == 0 ||
                 reln->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] == InvalidBlockNumber) &&
                !smgrexists(reln, VISIBILITYMAP_FORKNUM))
                smgrcreate(reln, VISIBILITYMAP_FORKNUM, false);

        /* Invalidate cache so that smgrnblocks() asks the kernel. */
        reln->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] = InvalidBlockNumber;
        vm_nblocks_now = smgrnblocks(reln, VISIBILITYMAP_FORKNUM);

        /* Now extend the file */
        while (vm_nblocks_now < vm_nblocks)
        {
                PageSetChecksumInplace((Page) pg.data, vm_nblocks_now);

                smgrextend(reln, 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(reln->smgr_rnode);

        UnlockRelationForExtension(rel, ExclusiveLock);
}