visibilitymap.c

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/*-------------------------------------------------------------------------
 *
 * visibilitymap.c
 *    bitmap for tracking visibility of heap tuples
 *
 * Portions Copyright (c) 1996-2024, 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/xloginsert.h"
#include "access/xlogutils.h"
#include "miscadmin.h"
#include "port/pg_bitutils.h"
#include "storage/bufmgr.h"
#include "storage/smgr.h"
#include "utils/inval.h"
#include "utils/rel.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_MASK8   (0x55)  /* The lower bit of each bit pair */
#define FROZEN_MASK8    (0xaa)  /* The upper bit of each bit pair */
 
/* prototypes for internal routines */
static Buffer vm_readbuf(Relation rel, BlockNumber blkno, bool extend);
static Buffer 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 vmbuf, 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;
 
    /* Must never clear all_visible bit while leaving all_frozen bit set */
    Assert(flags & VISIBILITYMAP_VALID_BITS);
    Assert(flags != VISIBILITYMAP_ALL_VISIBLE);
 
#ifdef TRACE_VISIBILITYMAP
    elog(DEBUG1, "vm_clear %s %d", RelationGetRelationName(rel), heapBlk);
#endif
 
    if (!BufferIsValid(vmbuf) || BufferGetBlockNumber(vmbuf) != mapBlock)
        elog(ERROR, "wrong buffer passed to visibilitymap_clear");
 
    LockBuffer(vmbuf, BUFFER_LOCK_EXCLUSIVE);
    map = PageGetContents(BufferGetPage(vmbuf));
 
    if (map[mapByte] & mask)
    {
        map[mapByte] &= ~mask;
 
        MarkBufferDirty(vmbuf);
        cleared = true;
    }
 
    LockBuffer(vmbuf, 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, *vmbuf 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, *vmbuf 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 *vmbuf)
{
    BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
 
    /* Reuse the old pinned buffer if possible */
    if (BufferIsValid(*vmbuf))
    {
        if (BufferGetBlockNumber(*vmbuf) == mapBlock)
            return;
 
        ReleaseBuffer(*vmbuf);
    }
    *vmbuf = vm_readbuf(rel, mapBlock, true);
}
 
/*
 *  visibilitymap_pin_ok - do we already have the correct page pinned?
 *
 * On entry, vmbuf 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 vmbuf)
{
    BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);
 
    return BufferIsValid(vmbuf) && BufferGetBlockNumber(vmbuf) == 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 VM 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 (though the heap page's LSN may *not* be updated;
 * see below).  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 || PageIsAllVisible((Page) BufferGetPage(heapBuf)));
    Assert((flags & VISIBILITYMAP_VALID_BITS) == flags);
 
    /* Must never set all_frozen bit without also setting all_visible bit */
    Assert(flags != VISIBILITYMAP_ALL_FROZEN);
 
    /* 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, 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 not, then we must *not* update the heap page's LSN. In
                 * this case, the FPI for the heap page was omitted from the
                 * WAL record inserted above, so it would be incorrect to
                 * update the heap page's LSN.
                 */
                if (XLogHintBitIsNeeded())
                {
                    Page        heapPage = BufferGetPage(heapBuf);
 
                    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, *vmbuf 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, *vmbuf is a valid buffer with the map page containing
 * the bit for heapBlk, or InvalidBuffer. The caller is responsible for
 * releasing *vmbuf 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 *vmbuf)
{
    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(*vmbuf))
    {
        if (BufferGetBlockNumber(*vmbuf) != mapBlock)
        {
            ReleaseBuffer(*vmbuf);
            *vmbuf = InvalidBuffer;
        }
    }
 
    if (!BufferIsValid(*vmbuf))
    {
        *vmbuf = vm_readbuf(rel, mapBlock, false);
        if (!BufferIsValid(*vmbuf))
            return false;
    }
 
    map = PageGetContents(BufferGetPage(*vmbuf));
 
    /*
     * 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;
 
        /*
         * 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));
 
        nvisible += pg_popcount_masked((const char *) map, MAPSIZE, VISIBLE_MASK8);
        if (all_frozen)
            nfrozen += pg_popcount_masked((const char *) map, MAPSIZE, FROZEN_MASK8);
 
        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;
    }
 
    /*
     * For reading we 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.
     *
     * We use the same path below to initialize pages when extending the
     * relation, as a concurrent extension can end up with vm_extend()
     * returning an already-initialized page.
     */
    if (blkno >= reln->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM])
    {
        if (extend)
            buf = vm_extend(rel, blkno + 1);
        else
            return InvalidBuffer;
    }
    else
        buf = ReadBufferExtended(rel, VISIBILITYMAP_FORKNUM, blkno,
                                 RBM_ZERO_ON_ERROR, NULL);
 
    /*
     * Initializing the page when needed 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.
     */
    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 Buffer
vm_extend(Relation rel, BlockNumber vm_nblocks)
{
    Buffer      buf;
 
    buf = ExtendBufferedRelTo(BMR_REL(rel), VISIBILITYMAP_FORKNUM, NULL,
                              EB_CREATE_FORK_IF_NEEDED |
                              EB_CLEAR_SIZE_CACHE,
                              vm_nblocks,
                              RBM_ZERO_ON_ERROR);
 
    /*
     * 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(RelationGetSmgr(rel)->smgr_rlocator);
 
    return buf;
}