/src/postgres/src/backend/storage/freespace/freespace.c

Source
/*-------------------------------------------------------------------------
 *
 * freespace.c
 *    POSTGRES free space map for quickly finding free space in relations
 *
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/storage/freespace/freespace.c
 *
 *
 * NOTES:
 *
 *  Free Space Map keeps track of the amount of free space on pages, and
 *  allows quickly searching for a page with enough free space. The FSM is
 *  stored in a dedicated relation fork of all heap relations, and those
 *  index access methods that need it (see also indexfsm.c). See README for
 *  more information.
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/htup_details.h"
#include "access/xloginsert.h"
#include "access/xlogutils.h"
#include "miscadmin.h"
#include "storage/freespace.h"
#include "storage/fsm_internals.h"
#include "storage/smgr.h"
#include "utils/rel.h"


/*
 * We use just one byte to store the amount of free space on a page, so we
 * divide the amount of free space a page can have into 256 different
 * categories. The highest category, 255, represents a page with at least
 * MaxFSMRequestSize bytes of free space, and the second highest category
 * represents the range from 254 * FSM_CAT_STEP, inclusive, to
 * MaxFSMRequestSize, exclusive.
 *
 * MaxFSMRequestSize depends on the architecture and BLCKSZ, but assuming
 * default 8k BLCKSZ, and that MaxFSMRequestSize is 8164 bytes, the
 * categories look like this:
 *
 *
 * Range   Category
 * 0  - 31   0
 * 32 - 63   1
 * ...    ...  ...
 * 8096 - 8127 253
 * 8128 - 8163 254
 * 8164 - 8192 255
 *
 * The reason that MaxFSMRequestSize is special is that if MaxFSMRequestSize
 * isn't equal to a range boundary, a page with exactly MaxFSMRequestSize
 * bytes of free space wouldn't satisfy a request for MaxFSMRequestSize
 * bytes. If there isn't more than MaxFSMRequestSize bytes of free space on a
 * completely empty page, that would mean that we could never satisfy a
 * request of exactly MaxFSMRequestSize bytes.
 */
#define FSM_CATEGORIES  256
#define FSM_CAT_STEP  (BLCKSZ / FSM_CATEGORIES)
#define MaxFSMRequestSize MaxHeapTupleSize

/*
 * Depth of the on-disk tree. We need to be able to address 2^32-1 blocks,
 * and 1626 is the smallest number that satisfies X^3 >= 2^32-1. Likewise,
 * 256 is the smallest number that satisfies X^4 >= 2^32-1. In practice,
 * this means that 4096 bytes is the smallest BLCKSZ that we can get away
 * with a 3-level tree, and 512 is the smallest we support.
 */
#define FSM_TREE_DEPTH  ((SlotsPerFSMPage >= 1626) ? 3 : 4)

#define FSM_ROOT_LEVEL  (FSM_TREE_DEPTH - 1)
#define FSM_BOTTOM_LEVEL 0

/*
 * The internal FSM routines work on a logical addressing scheme. Each
 * level of the tree can be thought of as a separately addressable file.
 */
typedef struct
{
  int     level;      /* level */
  int     logpageno;    /* page number within the level */
} FSMAddress;

/* Address of the root page. */
static const FSMAddress FSM_ROOT_ADDRESS = {FSM_ROOT_LEVEL, 0};

/* functions to navigate the tree */
static FSMAddress fsm_get_child(FSMAddress parent, uint16 slot);
static FSMAddress fsm_get_parent(FSMAddress child, uint16 *slot);
static FSMAddress fsm_get_location(BlockNumber heapblk, uint16 *slot);
static BlockNumber fsm_get_heap_blk(FSMAddress addr, uint16 slot);
static BlockNumber fsm_logical_to_physical(FSMAddress addr);

static Buffer fsm_readbuf(Relation rel, FSMAddress addr, bool extend);
static Buffer fsm_extend(Relation rel, BlockNumber fsm_nblocks);

/* functions to convert amount of free space to a FSM category */
static uint8 fsm_space_avail_to_cat(Size avail);
static uint8 fsm_space_needed_to_cat(Size needed);
static Size fsm_space_cat_to_avail(uint8 cat);

/* workhorse functions for various operations */
static int  fsm_set_and_search(Relation rel, FSMAddress addr, uint16 slot,
                 uint8 newValue, uint8 minValue);
static BlockNumber fsm_search(Relation rel, uint8 min_cat);
static uint8 fsm_vacuum_page(Relation rel, FSMAddress addr,
               BlockNumber start, BlockNumber end,
               bool *eof_p);
static bool fsm_does_block_exist(Relation rel, BlockNumber blknumber);


/******** Public API ********/

/*
 * GetPageWithFreeSpace - try to find a page in the given relation with
 *    at least the specified amount of free space.
 *
 * If successful, return the block number; if not, return InvalidBlockNumber.
 *
 * The caller must be prepared for the possibility that the returned page
 * will turn out to have too little space available by the time the caller
 * gets a lock on it.  In that case, the caller should report the actual
 * amount of free space available on that page and then try again (see
 * RecordAndGetPageWithFreeSpace).  If InvalidBlockNumber is returned,
 * extend the relation.
 *
 * This can trigger FSM updates if any FSM entry is found to point to a block
 * past the end of the relation.
 */
BlockNumber
GetPageWithFreeSpace(Relation rel, Size spaceNeeded)
{
  uint8   min_cat = fsm_space_needed_to_cat(spaceNeeded);

  return fsm_search(rel, min_cat);
}

/*
 * RecordAndGetPageWithFreeSpace - update info about a page and try again.
 *
 * We provide this combo form to save some locking overhead, compared to
 * separate RecordPageWithFreeSpace + GetPageWithFreeSpace calls. There's
 * also some effort to return a page close to the old page; if there's a
 * page with enough free space on the same FSM page where the old one page
 * is located, it is preferred.
 */
BlockNumber
RecordAndGetPageWithFreeSpace(Relation rel, BlockNumber oldPage,
                Size oldSpaceAvail, Size spaceNeeded)
{
  int     old_cat = fsm_space_avail_to_cat(oldSpaceAvail);
  int     search_cat = fsm_space_needed_to_cat(spaceNeeded);
  FSMAddress  addr;
  uint16    slot;
  int     search_slot;

  /* Get the location of the FSM byte representing the heap block */
  addr = fsm_get_location(oldPage, &slot);

  search_slot = fsm_set_and_search(rel, addr, slot, old_cat, search_cat);

  /*
   * If fsm_set_and_search found a suitable new block, return that.
   * Otherwise, search as usual.
   */
  if (search_slot != -1)
  {
    BlockNumber blknum = fsm_get_heap_blk(addr, search_slot);

    /*
     * Check that the blknum is actually in the relation. Don't try to
     * update the FSM in that case, just fall back to the other case
     */
    if (fsm_does_block_exist(rel, blknum))
      return blknum;
  }
  return fsm_search(rel, search_cat);
}

/*
 * RecordPageWithFreeSpace - update info about a page.
 *
 * Note that if the new spaceAvail value is higher than the old value stored
 * in the FSM, the space might not become visible to searchers until the next
 * FreeSpaceMapVacuum call, which updates the upper level pages.
 */
void
RecordPageWithFreeSpace(Relation rel, BlockNumber heapBlk, Size spaceAvail)
{
  int     new_cat = fsm_space_avail_to_cat(spaceAvail);
  FSMAddress  addr;
  uint16    slot;

  /* Get the location of the FSM byte representing the heap block */
  addr = fsm_get_location(heapBlk, &slot);

  fsm_set_and_search(rel, addr, slot, new_cat, 0);
}

/*
 * XLogRecordPageWithFreeSpace - like RecordPageWithFreeSpace, for use in
 *    WAL replay
 */
void
XLogRecordPageWithFreeSpace(RelFileLocator rlocator, BlockNumber heapBlk,
              Size spaceAvail)
{
  int     new_cat = fsm_space_avail_to_cat(spaceAvail);
  FSMAddress  addr;
  uint16    slot;
  BlockNumber blkno;
  Buffer    buf;
  Page    page;

  /* Get the location of the FSM byte representing the heap block */
  addr = fsm_get_location(heapBlk, &slot);
  blkno = fsm_logical_to_physical(addr);

  /* If the page doesn't exist already, extend */
  buf = XLogReadBufferExtended(rlocator, FSM_FORKNUM, blkno,
                 RBM_ZERO_ON_ERROR, InvalidBuffer);
  LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);

  page = BufferGetPage(buf);
  if (PageIsNew(page))
    PageInit(page, BLCKSZ, 0);

  if (fsm_set_avail(page, slot, new_cat))
    MarkBufferDirtyHint(buf, false);
  UnlockReleaseBuffer(buf);
}

/*
 * GetRecordedFreeSpace - return the amount of free space on a particular page,
 *    according to the FSM.
 */
Size
GetRecordedFreeSpace(Relation rel, BlockNumber heapBlk)
{
  FSMAddress  addr;
  uint16    slot;
  Buffer    buf;
  uint8   cat;

  /* Get the location of the FSM byte representing the heap block */
  addr = fsm_get_location(heapBlk, &slot);

  buf = fsm_readbuf(rel, addr, false);
  if (!BufferIsValid(buf))
    return 0;
  cat = fsm_get_avail(BufferGetPage(buf), slot);
  ReleaseBuffer(buf);

  return fsm_space_cat_to_avail(cat);
}

/*
 * FreeSpaceMapPrepareTruncateRel - prepare for truncation of a relation.
 *
 * nblocks is the new size of the heap.
 *
 * Return the number of blocks of new FSM.
 * If it's InvalidBlockNumber, there is nothing to truncate;
 * otherwise the caller is responsible for calling smgrtruncate()
 * to truncate the FSM pages, and FreeSpaceMapVacuumRange()
 * to update upper-level pages in the FSM.
 */
BlockNumber
FreeSpaceMapPrepareTruncateRel(Relation rel, BlockNumber nblocks)
{
  BlockNumber new_nfsmblocks;
  FSMAddress  first_removed_address;
  uint16    first_removed_slot;
  Buffer    buf;

  /*
   * If no FSM has been created yet for this relation, there's nothing to
   * truncate.
   */
  if (!smgrexists(RelationGetSmgr(rel), FSM_FORKNUM))
    return InvalidBlockNumber;

  /* Get the location in the FSM of the first removed heap block */
  first_removed_address = fsm_get_location(nblocks, &first_removed_slot);

  /*
   * Zero out the tail of the last remaining FSM page. If the slot
   * representing the first removed heap block is at a page boundary, as the
   * first slot on the FSM page that first_removed_address points to, we can
   * just truncate that page altogether.
   */
  if (first_removed_slot > 0)
  {
    buf = fsm_readbuf(rel, first_removed_address, false);
    if (!BufferIsValid(buf))
      return InvalidBlockNumber; /* nothing to do; the FSM was already
                     * smaller */
    LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);

    /* NO EREPORT(ERROR) from here till changes are logged */
    START_CRIT_SECTION();

    fsm_truncate_avail(BufferGetPage(buf), first_removed_slot);

    /*
     * This change is non-critical, because fsm_does_block_exist() would
     * stop us from returning a truncated-away block.  However, since this
     * may remove up to SlotsPerFSMPage slots, it's nice to avoid the cost
     * of that many fsm_does_block_exist() rejections.  Use a full
     * MarkBufferDirty(), not MarkBufferDirtyHint().
     */
    MarkBufferDirty(buf);

    /*
     * WAL-log like MarkBufferDirtyHint() might have done, just to avoid
     * differing from the rest of the file in this respect.  This is
     * optional; see README mention of full page images.  XXX consider
     * XLogSaveBufferForHint() for even closer similarity.
     *
     * A higher-level operation calls us at WAL replay.  If we crash
     * before the XLOG_SMGR_TRUNCATE flushes to disk, main fork length has
     * not changed, and our fork remains valid.  If we crash after that
     * flush, redo will return here.
     */
    if (!InRecovery && RelationNeedsWAL(rel) && XLogHintBitIsNeeded())
      log_newpage_buffer(buf, false);

    END_CRIT_SECTION();

    UnlockReleaseBuffer(buf);

    new_nfsmblocks = fsm_logical_to_physical(first_removed_address) + 1;
  }
  else
  {
    new_nfsmblocks = fsm_logical_to_physical(first_removed_address);
    if (smgrnblocks(RelationGetSmgr(rel), FSM_FORKNUM) <= new_nfsmblocks)
      return InvalidBlockNumber; /* nothing to do; the FSM was already
                     * smaller */
  }

  return new_nfsmblocks;
}

/*
 * FreeSpaceMapVacuum - update upper-level pages in the rel's FSM
 *
 * We assume that the bottom-level pages have already been updated with
 * new free-space information.
 */
void
FreeSpaceMapVacuum(Relation rel)
{
  bool    dummy;

  /* Recursively scan the tree, starting at the root */
  (void) fsm_vacuum_page(rel, FSM_ROOT_ADDRESS,
               (BlockNumber) 0, InvalidBlockNumber,
               &dummy);
}

/*
 * FreeSpaceMapVacuumRange - update upper-level pages in the rel's FSM
 *
 * As above, but assume that only heap pages between start and end-1 inclusive
 * have new free-space information, so update only the upper-level slots
 * covering that block range.  end == InvalidBlockNumber is equivalent to
 * "all the rest of the relation".
 */
void
FreeSpaceMapVacuumRange(Relation rel, BlockNumber start, BlockNumber end)
{
  bool    dummy;

  /* Recursively scan the tree, starting at the root */
  if (end > start)
    (void) fsm_vacuum_page(rel, FSM_ROOT_ADDRESS, start, end, &dummy);
}

/******** Internal routines ********/

/*
 * Return category corresponding x bytes of free space
 */
static uint8
fsm_space_avail_to_cat(Size avail)
{
  int     cat;

  Assert(avail < BLCKSZ);

  if (avail >= MaxFSMRequestSize)
    return 255;

  cat = avail / FSM_CAT_STEP;

  /*
   * The highest category, 255, is reserved for MaxFSMRequestSize bytes or
   * more.
   */
  if (cat > 254)
    cat = 254;

  return (uint8) cat;
}

/*
 * Return the lower bound of the range of free space represented by given
 * category.
 */
static Size
fsm_space_cat_to_avail(uint8 cat)
{
  /* The highest category represents exactly MaxFSMRequestSize bytes. */
  if (cat == 255)
    return MaxFSMRequestSize;
  else
    return cat * FSM_CAT_STEP;
}

/*
 * Which category does a page need to have, to accommodate x bytes of data?
 * While fsm_space_avail_to_cat() rounds down, this needs to round up.
 */
static uint8
fsm_space_needed_to_cat(Size needed)
{
  int     cat;

  /* Can't ask for more space than the highest category represents */
  if (needed > MaxFSMRequestSize)
    elog(ERROR, "invalid FSM request size %zu", needed);

  if (needed == 0)
    return 1;

  cat = (needed + FSM_CAT_STEP - 1) / FSM_CAT_STEP;

  if (cat > 255)
    cat = 255;

  return (uint8) cat;
}

/*
 * Returns the physical block number of a FSM page
 */
static BlockNumber
fsm_logical_to_physical(FSMAddress addr)
{
  BlockNumber pages;
  int     leafno;
  int     l;

  /*
   * Calculate the logical page number of the first leaf page below the
   * given page.
   */
  leafno = addr.logpageno;
  for (l = 0; l < addr.level; l++)
    leafno *= SlotsPerFSMPage;

  /* Count upper level nodes required to address the leaf page */
  pages = 0;
  for (l = 0; l < FSM_TREE_DEPTH; l++)
  {
    pages += leafno + 1;
    leafno /= SlotsPerFSMPage;
  }

  /*
   * If the page we were asked for wasn't at the bottom level, subtract the
   * additional lower level pages we counted above.
   */
  pages -= addr.level;

  /* Turn the page count into 0-based block number */
  return pages - 1;
}

/*
 * Return the FSM location corresponding to given heap block.
 */
static FSMAddress
fsm_get_location(BlockNumber heapblk, uint16 *slot)
{
  FSMAddress  addr;

  addr.level = FSM_BOTTOM_LEVEL;
  addr.logpageno = heapblk / SlotsPerFSMPage;
  *slot = heapblk % SlotsPerFSMPage;

  return addr;
}

/*
 * Return the heap block number corresponding to given location in the FSM.
 */
static BlockNumber
fsm_get_heap_blk(FSMAddress addr, uint16 slot)
{
  Assert(addr.level == FSM_BOTTOM_LEVEL);
  return ((unsigned int) addr.logpageno) * SlotsPerFSMPage + slot;
}

/*
 * Given a logical address of a child page, get the logical page number of
 * the parent, and the slot within the parent corresponding to the child.
 */
static FSMAddress
fsm_get_parent(FSMAddress child, uint16 *slot)
{
  FSMAddress  parent;

  Assert(child.level < FSM_ROOT_LEVEL);

  parent.level = child.level + 1;
  parent.logpageno = child.logpageno / SlotsPerFSMPage;
  *slot = child.logpageno % SlotsPerFSMPage;

  return parent;
}

/*
 * Given a logical address of a parent page and a slot number, get the
 * logical address of the corresponding child page.
 */
static FSMAddress
fsm_get_child(FSMAddress parent, uint16 slot)
{
  FSMAddress  child;

  Assert(parent.level > FSM_BOTTOM_LEVEL);

  child.level = parent.level - 1;
  child.logpageno = parent.logpageno * SlotsPerFSMPage + slot;

  return child;
}

/*
 * Read a FSM page.
 *
 * If the page doesn't exist, InvalidBuffer is returned, or if 'extend' is
 * true, the FSM file is extended.
 */
static Buffer
fsm_readbuf(Relation rel, FSMAddress addr, bool extend)
{
  BlockNumber blkno = fsm_logical_to_physical(addr);
  Buffer    buf;
  SMgrRelation reln = RelationGetSmgr(rel);

  /*
   * If we haven't cached the size of the FSM yet, check it first.  Also
   * recheck if the requested block seems to be past end, since our cached
   * value might be stale.  (We send smgr inval messages on truncation, but
   * not on extension.)
   */
  if (reln->smgr_cached_nblocks[FSM_FORKNUM] == InvalidBlockNumber ||
    blkno >= reln->smgr_cached_nblocks[FSM_FORKNUM])
  {
    /* Invalidate the cache so smgrnblocks asks the kernel. */
    reln->smgr_cached_nblocks[FSM_FORKNUM] = InvalidBlockNumber;
    if (smgrexists(reln, FSM_FORKNUM))
      smgrnblocks(reln, FSM_FORKNUM);
    else
      reln->smgr_cached_nblocks[FSM_FORKNUM] = 0;
  }

  /*
   * For reading we use ZERO_ON_ERROR mode, and initialize the page if
   * necessary.  The FSM information is not accurate anyway, so it's better
   * to clear corrupt pages than error out. Since the FSM changes are not
   * WAL-logged, the so-called torn page problem on crash can lead to pages
   * with corrupt headers, for example.
   *
   * 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[FSM_FORKNUM])
  {
    if (extend)
      buf = fsm_extend(rel, blkno + 1);
    else
      return InvalidBuffer;
  }
  else
    buf = ReadBufferExtended(rel, FSM_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 FSM fork is at least fsm_nblocks long, extending
 * it if necessary with empty pages. And by empty, I mean pages filled
 * with zeros, meaning there's no free space.
 */
static Buffer
fsm_extend(Relation rel, BlockNumber fsm_nblocks)
{
  return ExtendBufferedRelTo(BMR_REL(rel), FSM_FORKNUM, NULL,
                 EB_CREATE_FORK_IF_NEEDED |
                 EB_CLEAR_SIZE_CACHE,
                 fsm_nblocks,
                 RBM_ZERO_ON_ERROR);
}

/*
 * Set value in given FSM page and slot.
 *
 * If minValue > 0, the updated page is also searched for a page with at
 * least minValue of free space. If one is found, its slot number is
 * returned, -1 otherwise.
 */
static int
fsm_set_and_search(Relation rel, FSMAddress addr, uint16 slot,
           uint8 newValue, uint8 minValue)
{
  Buffer    buf;
  Page    page;
  int     newslot = -1;

  buf = fsm_readbuf(rel, addr, true);
  LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);

  page = BufferGetPage(buf);

  if (fsm_set_avail(page, slot, newValue))
    MarkBufferDirtyHint(buf, false);

  if (minValue != 0)
  {
    /* Search while we still hold the lock */
    newslot = fsm_search_avail(buf, minValue,
                   addr.level == FSM_BOTTOM_LEVEL,
                   true);
  }

  UnlockReleaseBuffer(buf);

  return newslot;
}

/*
 * Search the tree for a heap page with at least min_cat of free space
 */
static BlockNumber
fsm_search(Relation rel, uint8 min_cat)
{
  int     restarts = 0;
  FSMAddress  addr = FSM_ROOT_ADDRESS;

  for (;;)
  {
    int     slot;
    Buffer    buf;
    uint8   max_avail = 0;

    /* Read the FSM page. */
    buf = fsm_readbuf(rel, addr, false);

    /* Search within the page */
    if (BufferIsValid(buf))
    {
      LockBuffer(buf, BUFFER_LOCK_SHARE);
      slot = fsm_search_avail(buf, min_cat,
                  (addr.level == FSM_BOTTOM_LEVEL),
                  false);
      if (slot == -1)
      {
        max_avail = fsm_get_max_avail(BufferGetPage(buf));
        UnlockReleaseBuffer(buf);
      }
      else
      {
        /* Keep the pin for possible update below */
        LockBuffer(buf, BUFFER_LOCK_UNLOCK);
      }
    }
    else
      slot = -1;

    if (slot != -1)
    {
      /*
       * Descend the tree, or return the found block if we're at the
       * bottom.
       */
      if (addr.level == FSM_BOTTOM_LEVEL)
      {
        BlockNumber blkno = fsm_get_heap_blk(addr, slot);
        Page    page;

        if (fsm_does_block_exist(rel, blkno))
        {
          ReleaseBuffer(buf);
          return blkno;
        }

        /*
         * Block is past the end of the relation.  Update FSM, and
         * restart from root.  The usual "advancenext" behavior is
         * pessimal for this rare scenario, since every later slot is
         * unusable in the same way.  We could zero all affected slots
         * on the same FSM page, but don't bet on the benefits of that
         * optimization justifying its compiled code bulk.
         */
        page = BufferGetPage(buf);
        LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
        fsm_set_avail(page, slot, 0);
        MarkBufferDirtyHint(buf, false);
        UnlockReleaseBuffer(buf);
        if (restarts++ > 10000) /* same rationale as below */
          return InvalidBlockNumber;
        addr = FSM_ROOT_ADDRESS;
      }
      else
      {
        ReleaseBuffer(buf);
      }
      addr = fsm_get_child(addr, slot);
    }
    else if (addr.level == FSM_ROOT_LEVEL)
    {
      /*
       * At the root, failure means there's no page with enough free
       * space in the FSM. Give up.
       */
      return InvalidBlockNumber;
    }
    else
    {
      uint16    parentslot;
      FSMAddress  parent;

      /*
       * At lower level, failure can happen if the value in the upper-
       * level node didn't reflect the value on the lower page. Update
       * the upper node, to avoid falling into the same trap again, and
       * start over.
       *
       * There's a race condition here, if another backend updates this
       * page right after we release it, and gets the lock on the parent
       * page before us. We'll then update the parent page with the now
       * stale information we had. It's OK, because it should happen
       * rarely, and will be fixed by the next vacuum.
       */
      parent = fsm_get_parent(addr, &parentslot);
      fsm_set_and_search(rel, parent, parentslot, max_avail, 0);

      /*
       * If the upper pages are badly out of date, we might need to loop
       * quite a few times, updating them as we go. Any inconsistencies
       * should eventually be corrected and the loop should end. Looping
       * indefinitely is nevertheless scary, so provide an emergency
       * valve.
       */
      if (restarts++ > 10000)
        return InvalidBlockNumber;

      /* Start search all over from the root */
      addr = FSM_ROOT_ADDRESS;
    }
  }
}


/*
 * Recursive guts of FreeSpaceMapVacuum
 *
 * Examine the FSM page indicated by addr, as well as its children, updating
 * upper-level nodes that cover the heap block range from start to end-1.
 * (It's okay if end is beyond the actual end of the map.)
 * Return the maximum freespace value on this page.
 *
 * If addr is past the end of the FSM, set *eof_p to true and return 0.
 *
 * This traverses the tree in depth-first order.  The tree is stored
 * physically in depth-first order, so this should be pretty I/O efficient.
 */
static uint8
fsm_vacuum_page(Relation rel, FSMAddress addr,
        BlockNumber start, BlockNumber end,
        bool *eof_p)
{
  Buffer    buf;
  Page    page;
  uint8   max_avail;

  /* Read the page if it exists, or return EOF */
  buf = fsm_readbuf(rel, addr, false);
  if (!BufferIsValid(buf))
  {
    *eof_p = true;
    return 0;
  }
  else
    *eof_p = false;

  page = BufferGetPage(buf);

  /*
   * If we're above the bottom level, recurse into children, and fix the
   * information stored about them at this level.
   */
  if (addr.level > FSM_BOTTOM_LEVEL)
  {
    FSMAddress  fsm_start,
          fsm_end;
    uint16    fsm_start_slot,
          fsm_end_slot;
    int     slot,
          start_slot,
          end_slot;
    bool    eof = false;

    /*
     * Compute the range of slots we need to update on this page, given
     * the requested range of heap blocks to consider.  The first slot to
     * update is the one covering the "start" block, and the last slot is
     * the one covering "end - 1".  (Some of this work will be duplicated
     * in each recursive call, but it's cheap enough to not worry about.)
     */
    fsm_start = fsm_get_location(start, &fsm_start_slot);
    fsm_end = fsm_get_location(end - 1, &fsm_end_slot);

    while (fsm_start.level < addr.level)
    {
      fsm_start = fsm_get_parent(fsm_start, &fsm_start_slot);
      fsm_end = fsm_get_parent(fsm_end, &fsm_end_slot);
    }
    Assert(fsm_start.level == addr.level);

    if (fsm_start.logpageno == addr.logpageno)
      start_slot = fsm_start_slot;
    else if (fsm_start.logpageno > addr.logpageno)
      start_slot = SlotsPerFSMPage; /* shouldn't get here... */
    else
      start_slot = 0;

    if (fsm_end.logpageno == addr.logpageno)
      end_slot = fsm_end_slot;
    else if (fsm_end.logpageno > addr.logpageno)
      end_slot = SlotsPerFSMPage - 1;
    else
      end_slot = -1;   /* shouldn't get here... */

    for (slot = start_slot; slot <= end_slot; slot++)
    {
      int     child_avail;

      CHECK_FOR_INTERRUPTS();

      /* After we hit end-of-file, just clear the rest of the slots */
      if (!eof)
        child_avail = fsm_vacuum_page(rel, fsm_get_child(addr, slot),
                        start, end,
                        &eof);
      else
        child_avail = 0;

      /* Update information about the child */
      if (fsm_get_avail(page, slot) != child_avail)
      {
        LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
        fsm_set_avail(page, slot, child_avail);
        MarkBufferDirtyHint(buf, false);
        LockBuffer(buf, BUFFER_LOCK_UNLOCK);
      }
    }
  }

  /* Now get the maximum value on the page, to return to caller */
  max_avail = fsm_get_max_avail(page);

  /*
   * Try to reset the next slot pointer. This encourages the use of
   * low-numbered pages, increasing the chances that a later vacuum can
   * truncate the relation. We don't bother with marking the page dirty if
   * it wasn't already, since this is just a hint.
   */
  LockBuffer(buf, BUFFER_LOCK_SHARE);
  if (BufferBeginSetHintBits(buf))
  {
    ((FSMPage) PageGetContents(page))->fp_next_slot = 0;
    BufferFinishSetHintBits(buf, false, false);
  }

  UnlockReleaseBuffer(buf);

  return max_avail;
}


/*
 * Check whether a block number is past the end of the relation.  This can
 * happen after WAL replay, if the FSM reached disk but newly-extended pages
 * it refers to did not.
 */
static bool
fsm_does_block_exist(Relation rel, BlockNumber blknumber)
{
  SMgrRelation smgr = RelationGetSmgr(rel);

  /*
   * If below the cached nblocks, the block surely exists.  Otherwise, we
   * face a trade-off.  We opt to compare to a fresh nblocks, incurring
   * lseek() overhead.  The alternative would be to assume the block does
   * not exist, but that would cause FSM to set zero space available for
   * blocks that main fork extension just recorded.
   */
  return ((BlockNumberIsValid(smgr->smgr_cached_nblocks[MAIN_FORKNUM]) &&
       blknumber < smgr->smgr_cached_nblocks[MAIN_FORKNUM]) ||
      blknumber < RelationGetNumberOfBlocks(rel));
}

Coverage Report

Created: 2026-10-16 06:48