visibilitymap.c File Reference

#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"

Include dependency graph for visibilitymap.c:

Go to the source code of this file.

Macros
#define	MAPSIZE   (BLCKSZ - MAXALIGN(SizeOfPageHeaderData))
#define	HEAPBLOCKS_PER_BYTE   (BITS_PER_BYTE / BITS_PER_HEAPBLOCK)
#define	HEAPBLOCKS_PER_PAGE   (MAPSIZE * HEAPBLOCKS_PER_BYTE)
#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)
#define	VISIBLE_MASK64
#define	FROZEN_MASK64

Functions
static Buffer	vm_readbuf (Relation rel, BlockNumber blkno, bool extend)
static void	vm_extend (Relation rel, BlockNumber vm_nblocks)
bool	visibilitymap_clear (Relation rel, BlockNumber heapBlk, Buffer buf, uint8 flags)
void	visibilitymap_pin (Relation rel, BlockNumber heapBlk, Buffer *buf)
bool	visibilitymap_pin_ok (BlockNumber heapBlk, Buffer buf)
void	visibilitymap_set (Relation rel, BlockNumber heapBlk, Buffer heapBuf, XLogRecPtr recptr, Buffer vmBuf, TransactionId cutoff_xid, uint8 flags)
uint8	visibilitymap_get_status (Relation rel, BlockNumber heapBlk, Buffer *buf)
void	visibilitymap_count (Relation rel, BlockNumber *all_visible, BlockNumber *all_frozen)
void	visibilitymap_truncate (Relation rel, BlockNumber nheapblocks)

Macro Definition Documentation

FROZEN_MASK64

#define FROZEN_MASK64

Value:

UINT64CONST(0xaaaaaaaaaaaaaaaa) /* The upper bit of each
                                                         * bit pair */

Definition at line 122 of file visibilitymap.c.

Referenced by visibilitymap_count().

HEAPBLK_TO_MAPBLOCK

#define HEAPBLK_TO_MAPBLOCK

(

x

)

((x) / HEAPBLOCKS_PER_PAGE)

Definition at line 115 of file visibilitymap.c.

Referenced by visibilitymap_clear(), visibilitymap_get_status(), visibilitymap_pin(), visibilitymap_pin_ok(), visibilitymap_set(), and visibilitymap_truncate().

HEAPBLK_TO_MAPBYTE

#define HEAPBLK_TO_MAPBYTE

(

x

)

(((x) % HEAPBLOCKS_PER_PAGE) / HEAPBLOCKS_PER_BYTE)

Definition at line 116 of file visibilitymap.c.

Referenced by visibilitymap_clear(), visibilitymap_get_status(), visibilitymap_set(), and visibilitymap_truncate().

HEAPBLK_TO_OFFSET

#define HEAPBLK_TO_OFFSET

(

x

)

(((x) % HEAPBLOCKS_PER_BYTE) * BITS_PER_HEAPBLOCK)

Definition at line 117 of file visibilitymap.c.

Referenced by visibilitymap_clear(), visibilitymap_get_status(), visibilitymap_set(), and visibilitymap_truncate().

HEAPBLOCKS_PER_BYTE

#define HEAPBLOCKS_PER_BYTE   (BITS_PER_BYTE / BITS_PER_HEAPBLOCK)

Definition at line 109 of file visibilitymap.c.

HEAPBLOCKS_PER_PAGE

#define HEAPBLOCKS_PER_PAGE   (MAPSIZE * HEAPBLOCKS_PER_BYTE)

Definition at line 112 of file visibilitymap.c.

MAPSIZE

#define MAPSIZE   (BLCKSZ - MAXALIGN(SizeOfPageHeaderData))

Definition at line 106 of file visibilitymap.c.

Referenced by visibilitymap_count(), and visibilitymap_truncate().

VISIBLE_MASK64

#define VISIBLE_MASK64

Value:

UINT64CONST(0x5555555555555555) /* The lower bit of each
                                                         * bit pair */

Definition at line 120 of file visibilitymap.c.

Referenced by visibilitymap_count().

Function Documentation

visibilitymap_clear()

bool visibilitymap_clear	(	Relation	rel,
		BlockNumber	heapBlk,
		Buffer	buf,
		uint8	flags
	)

Definition at line 138 of file visibilitymap.c.

References Assert, BUFFER_LOCK_EXCLUSIVE, BUFFER_LOCK_UNLOCK, BufferGetBlockNumber(), BufferGetPage, BufferIsValid, DEBUG1, elog, ERROR, HEAPBLK_TO_MAPBLOCK, HEAPBLK_TO_MAPBYTE, HEAPBLK_TO_OFFSET, LockBuffer(), MarkBufferDirty(), PageGetContents, RelationGetRelationName, and VISIBILITYMAP_VALID_BITS.

Referenced by heap_delete(), heap_insert(), heap_lock_tuple(), heap_lock_updated_tuple_rec(), heap_multi_insert(), heap_update(), heap_xlog_delete(), heap_xlog_insert(), heap_xlog_lock(), heap_xlog_lock_updated(), heap_xlog_multi_insert(), heap_xlog_update(), and lazy_scan_heap().

{
    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_count()

void visibilitymap_count	(	Relation	rel,
		BlockNumber *	all_visible,
		BlockNumber *	all_frozen
	)

Definition at line 377 of file visibilitymap.c.

References Assert, BufferGetPage, BufferIsValid, FROZEN_MASK64, i, MAPSIZE, PageGetContents, pg_popcount64, ReleaseBuffer(), StaticAssertStmt, VISIBLE_MASK64, and vm_readbuf().

Referenced by do_analyze_rel(), heap_vacuum_rel(), and index_update_stats().

{
    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_get_status()

uint8 visibilitymap_get_status	(	Relation	rel,
		BlockNumber	heapBlk,
		Buffer *	buf
	)

Definition at line 329 of file visibilitymap.c.

References BufferGetBlockNumber(), BufferGetPage, BufferIsValid, DEBUG1, elog, HEAPBLK_TO_MAPBLOCK, HEAPBLK_TO_MAPBYTE, HEAPBLK_TO_OFFSET, InvalidBuffer, PageGetContents, RelationGetRelationName, ReleaseBuffer(), VISIBILITYMAP_VALID_BITS, and vm_readbuf().

Referenced by collect_visibility_data(), lazy_scan_heap(), lazy_vacuum_page(), pg_visibility(), pg_visibility_map(), and pg_visibility_map_summary().

{
    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_pin()

void visibilitymap_pin	(	Relation	rel,
		BlockNumber	heapBlk,
		Buffer *	buf
	)

Definition at line 189 of file visibilitymap.c.

References BufferGetBlockNumber(), BufferIsValid, HEAPBLK_TO_MAPBLOCK, ReleaseBuffer(), and vm_readbuf().

Referenced by GetVisibilityMapPins(), heap_delete(), heap_lock_tuple(), heap_lock_updated_tuple_rec(), heap_update(), heap_xlog_delete(), heap_xlog_insert(), heap_xlog_lock(), heap_xlog_lock_updated(), heap_xlog_multi_insert(), heap_xlog_update(), heap_xlog_visible(), lazy_scan_heap(), and RelationGetBufferForTuple().

{
    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()

bool visibilitymap_pin_ok	(	BlockNumber	heapBlk,
		Buffer	buf
	)

Definition at line 213 of file visibilitymap.c.

References BufferGetBlockNumber(), BufferIsValid, and HEAPBLK_TO_MAPBLOCK.

Referenced by GetVisibilityMapPins().

{
    BlockNumber mapBlock = HEAPBLK_TO_MAPBLOCK(heapBlk);

    return BufferIsValid(buf) && BufferGetBlockNumber(buf) == mapBlock;
}

visibilitymap_set()

void visibilitymap_set	(	Relation	rel,
		BlockNumber	heapBlk,
		Buffer	heapBuf,
		XLogRecPtr	recptr,
		Buffer	vmBuf,
		TransactionId	cutoff_xid,
		uint8	flags
	)

Definition at line 242 of file visibilitymap.c.

References Assert, BUFFER_LOCK_EXCLUSIVE, BUFFER_LOCK_UNLOCK, BufferGetBlockNumber(), BufferGetPage, BufferIsValid, DEBUG1, elog, END_CRIT_SECTION, ERROR, HEAPBLK_TO_MAPBLOCK, HEAPBLK_TO_MAPBYTE, HEAPBLK_TO_OFFSET, InRecovery, LockBuffer(), log_heap_visible(), MarkBufferDirty(), PageGetContents, PageIsAllVisible, PageSetLSN, RelationData::rd_node, RelationGetRelationName, RelationNeedsWAL, START_CRIT_SECTION, VISIBILITYMAP_VALID_BITS, XLogHintBitIsNeeded, and XLogRecPtrIsInvalid.

Referenced by heap_xlog_visible(), lazy_scan_heap(), and lazy_vacuum_page().

{
    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_truncate()

void visibilitymap_truncate	(	Relation	rel,
		BlockNumber	nheapblocks
	)

Definition at line 442 of file visibilitymap.c.

References BUFFER_LOCK_EXCLUSIVE, BufferGetPage, BufferIsValid, DEBUG1, elog, END_CRIT_SECTION, HEAPBLK_TO_MAPBLOCK, HEAPBLK_TO_MAPBYTE, HEAPBLK_TO_OFFSET, InRecovery, LockBuffer(), log_newpage_buffer(), MAPSIZE, MarkBufferDirty(), MemSet, PageGetContents, RelationData::rd_smgr, RelationGetRelationName, RelationNeedsWAL, RelationOpenSmgr, SMgrRelationData::smgr_vm_nblocks, smgrexists(), smgrnblocks(), smgrtruncate(), START_CRIT_SECTION, UnlockReleaseBuffer(), VISIBILITYMAP_FORKNUM, vm_readbuf(), and XLogHintBitIsNeeded.

Referenced by pg_truncate_visibility_map(), RelationTruncate(), and smgr_redo().

{
    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;

    /*
     * 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;
        }

        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;
    }

    /* Truncate the unused VM pages, and send smgr inval message */
    smgrtruncate(rel->rd_smgr, VISIBILITYMAP_FORKNUM, newnblocks);

    /*
     * We might as well update the local smgr_vm_nblocks setting. smgrtruncate
     * sent an smgr cache inval message, which will cause other backends to
     * invalidate their copy of smgr_vm_nblocks, and this one too at the next
     * command boundary.  But this ensures it isn't outright wrong until then.
     */
    if (rel->rd_smgr)
        rel->rd_smgr->smgr_vm_nblocks = newnblocks;
}

vm_extend()

static void vm_extend ( Relation  rel, BlockNumber  vm_nblocks  )

static

Definition at line 625 of file visibilitymap.c.

References CacheInvalidateSmgr(), PGAlignedBlock::data, ExclusiveLock, InvalidBlockNumber, LockRelationForExtension(), PageInit(), PageSetChecksumInplace(), RelationData::rd_smgr, RelationOpenSmgr, SMgrRelationData::smgr_rnode, SMgrRelationData::smgr_vm_nblocks, smgrcreate(), smgrexists(), smgrextend(), smgrnblocks(), UnlockRelationForExtension(), and VISIBILITYMAP_FORKNUM.

Referenced by vm_readbuf().

{
    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);
}

vm_readbuf()

static Buffer vm_readbuf ( Relation  rel, BlockNumber  blkno, bool  extend  )

static

Definition at line 554 of file visibilitymap.c.

References buf, BUFFER_LOCK_EXCLUSIVE, BUFFER_LOCK_UNLOCK, BufferGetPage, InvalidBlockNumber, InvalidBuffer, LockBuffer(), PageInit(), PageIsNew, RBM_ZERO_ON_ERROR, RelationData::rd_smgr, ReadBufferExtended(), RelationOpenSmgr, SMgrRelationData::smgr_vm_nblocks, smgrexists(), smgrnblocks(), VISIBILITYMAP_FORKNUM, and vm_extend().

Referenced by visibilitymap_count(), visibilitymap_get_status(), visibilitymap_pin(), and visibilitymap_truncate().

{
    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;
}