slab.c

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/*-------------------------------------------------------------------------
 *
 * slab.c
 *    SLAB allocator definitions.
 *
 * SLAB is a MemoryContext implementation designed for cases where large
 * numbers of equally-sized objects are allocated (and freed).
 *
 *
 * Portions Copyright (c) 2017-2022, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/utils/mmgr/slab.c
 *
 *
 * NOTE:
 *  The constant allocation size allows significant simplification and various
 *  optimizations over more general purpose allocators. The blocks are carved
 *  into chunks of exactly the right size (plus alignment), not wasting any
 *  memory.
 *
 *  The information about free chunks is maintained both at the block level and
 *  global (context) level. This is possible as the chunk size (and thus also
 *  the number of chunks per block) is fixed.
 *
 *  On each block, free chunks are tracked in a simple linked list. Contents
 *  of free chunks is replaced with an index of the next free chunk, forming
 *  a very simple linked list. Each block also contains a counter of free
 *  chunks. Combined with the local block-level freelist, it makes it trivial
 *  to eventually free the whole block.
 *
 *  At the context level, we use 'freelist' to track blocks ordered by number
 *  of free chunks, starting with blocks having a single allocated chunk, and
 *  with completely full blocks on the tail.
 *
 *  This also allows various optimizations - for example when searching for
 *  free chunk, the allocator reuses space from the fullest blocks first, in
 *  the hope that some of the less full blocks will get completely empty (and
 *  returned back to the OS).
 *
 *  For each block, we maintain pointer to the first free chunk - this is quite
 *  cheap and allows us to skip all the preceding used chunks, eliminating
 *  a significant number of lookups in many common usage patterns. In the worst
 *  case this performs as if the pointer was not maintained.
 *
 *  We cache the freelist index for the blocks with the fewest free chunks
 *  (minFreeChunks), so that we don't have to search the freelist on every
 *  SlabAlloc() call, which is quite expensive.
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "lib/ilist.h"
#include "utils/memdebug.h"
#include "utils/memutils.h"
#include "utils/memutils_memorychunk.h"
#include "utils/memutils_internal.h"
 
#define Slab_BLOCKHDRSZ MAXALIGN(sizeof(SlabBlock))
 
/*
 * SlabContext is a specialized implementation of MemoryContext.
 */
typedef struct SlabContext
{
    MemoryContextData header;   /* Standard memory-context fields */
    /* Allocation parameters for this context: */
    Size        chunkSize;      /* chunk size */
    Size        fullChunkSize;  /* chunk size including header and alignment */
    Size        blockSize;      /* block size */
    Size        headerSize;     /* allocated size of context header */
    int         chunksPerBlock; /* number of chunks per block */
    int         minFreeChunks;  /* min number of free chunks in any block */
    int         nblocks;        /* number of blocks allocated */
#ifdef MEMORY_CONTEXT_CHECKING
    bool       *freechunks;     /* bitmap of free chunks in a block */
#endif
    /* blocks with free space, grouped by number of free chunks: */
    dlist_head  freelist[FLEXIBLE_ARRAY_MEMBER];
} SlabContext;
 
/*
 * SlabBlock
 *      Structure of a single block in SLAB allocator.
 *
 * node: doubly-linked list of blocks in global freelist
 * nfree: number of free chunks in this block
 * firstFreeChunk: index of the first free chunk
 */
typedef struct SlabBlock
{
    dlist_node  node;           /* doubly-linked list */
    int         nfree;          /* number of free chunks */
    int         firstFreeChunk; /* index of the first free chunk in the block */
    SlabContext *slab;          /* owning context */
} SlabBlock;
 
 
#define Slab_CHUNKHDRSZ sizeof(MemoryChunk)
#define SlabPointerGetChunk(ptr)    \
    ((MemoryChunk *)(((char *)(ptr)) - sizeof(MemoryChunk)))
#define SlabChunkGetPointer(chk)    \
    ((void *)(((char *)(chk)) + sizeof(MemoryChunk)))
#define SlabBlockGetChunk(slab, block, idx) \
    ((MemoryChunk *) ((char *) (block) + Slab_BLOCKHDRSZ    \
                    + (idx * slab->fullChunkSize)))
#define SlabBlockStart(block)   \
    ((char *) block + Slab_BLOCKHDRSZ)
#define SlabChunkIndex(slab, block, chunk)  \
    (((char *) chunk - SlabBlockStart(block)) / slab->fullChunkSize)
 
/*
 * SlabIsValid
 *      True iff set is valid slab allocation set.
 */
#define SlabIsValid(set) \
    (PointerIsValid(set) && IsA(set, SlabContext))
 
/*
 * SlabBlockIsValid
 *      True iff block is valid block of slab allocation set.
 */
#define SlabBlockIsValid(block) \
    (PointerIsValid(block) && SlabIsValid((block)->slab))
 
 
/*
 * SlabContextCreate
 *      Create a new Slab context.
 *
 * parent: parent context, or NULL if top-level context
 * name: name of context (must be statically allocated)
 * blockSize: allocation block size
 * chunkSize: allocation chunk size
 *
 * The MAXALIGN(chunkSize) may not exceed MEMORYCHUNK_MAX_VALUE
 */
MemoryContext
SlabContextCreate(MemoryContext parent,
                  const char *name,
                  Size blockSize,
                  Size chunkSize)
{
    int         chunksPerBlock;
    Size        fullChunkSize;
    Size        freelistSize;
    Size        headerSize;
    SlabContext *slab;
    int         i;
 
    /* ensure MemoryChunk's size is properly maxaligned */
    StaticAssertStmt(Slab_CHUNKHDRSZ == MAXALIGN(Slab_CHUNKHDRSZ),
                     "sizeof(MemoryChunk) is not maxaligned");
    Assert(MAXALIGN(chunkSize) <= MEMORYCHUNK_MAX_VALUE);
 
    /* Make sure the linked list node fits inside a freed chunk */
    if (chunkSize < sizeof(int))
        chunkSize = sizeof(int);
 
    /* chunk, including SLAB header (both addresses nicely aligned) */
#ifdef MEMORY_CONTEXT_CHECKING
    /* ensure there's always space for the sentinel byte */
    fullChunkSize = Slab_CHUNKHDRSZ + MAXALIGN(chunkSize + 1);
#else
    fullChunkSize = Slab_CHUNKHDRSZ + MAXALIGN(chunkSize);
#endif
 
    /* Make sure the block can store at least one chunk. */
    if (blockSize < fullChunkSize + Slab_BLOCKHDRSZ)
        elog(ERROR, "block size %zu for slab is too small for %zu chunks",
             blockSize, chunkSize);
 
    /* Compute maximum number of chunks per block */
    chunksPerBlock = (blockSize - Slab_BLOCKHDRSZ) / fullChunkSize;
 
    /* The freelist starts with 0, ends with chunksPerBlock. */
    freelistSize = sizeof(dlist_head) * (chunksPerBlock + 1);
 
    /*
     * Allocate the context header.  Unlike aset.c, we never try to combine
     * this with the first regular block; not worth the extra complication.
     */
 
    /* Size of the memory context header */
    headerSize = offsetof(SlabContext, freelist) + freelistSize;
 
#ifdef MEMORY_CONTEXT_CHECKING
 
    /*
     * With memory checking, we need to allocate extra space for the bitmap of
     * free chunks. The bitmap is an array of bools, so we don't need to worry
     * about alignment.
     */
    headerSize += chunksPerBlock * sizeof(bool);
#endif
 
    slab = (SlabContext *) malloc(headerSize);
    if (slab == NULL)
    {
        MemoryContextStats(TopMemoryContext);
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of memory"),
                 errdetail("Failed while creating memory context \"%s\".",
                           name)));
    }
 
    /*
     * Avoid writing code that can fail between here and MemoryContextCreate;
     * we'd leak the header if we ereport in this stretch.
     */
 
    /* Fill in SlabContext-specific header fields */
    slab->chunkSize = chunkSize;
    slab->fullChunkSize = fullChunkSize;
    slab->blockSize = blockSize;
    slab->headerSize = headerSize;
    slab->chunksPerBlock = chunksPerBlock;
    slab->minFreeChunks = 0;
    slab->nblocks = 0;
 
    /* initialize the freelist slots */
    for (i = 0; i < (slab->chunksPerBlock + 1); i++)
        dlist_init(&slab->freelist[i]);
 
#ifdef MEMORY_CONTEXT_CHECKING
    /* set the freechunks pointer right after the freelists array */
    slab->freechunks
        = (bool *) slab + offsetof(SlabContext, freelist) + freelistSize;
#endif
 
    /* Finally, do the type-independent part of context creation */
    MemoryContextCreate((MemoryContext) slab,
                        T_SlabContext,
                        MCTX_SLAB_ID,
                        parent,
                        name);
 
    return (MemoryContext) slab;
}
 
/*
 * SlabReset
 *      Frees all memory which is allocated in the given set.
 *
 * The code simply frees all the blocks in the context - we don't keep any
 * keeper blocks or anything like that.
 */
void
SlabReset(MemoryContext context)
{
    SlabContext *slab = (SlabContext *) context;
    int         i;
 
    Assert(SlabIsValid(slab));
 
#ifdef MEMORY_CONTEXT_CHECKING
    /* Check for corruption and leaks before freeing */
    SlabCheck(context);
#endif
 
    /* walk over freelists and free the blocks */
    for (i = 0; i <= slab->chunksPerBlock; i++)
    {
        dlist_mutable_iter miter;
 
        dlist_foreach_modify(miter, &slab->freelist[i])
        {
            SlabBlock  *block = dlist_container(SlabBlock, node, miter.cur);
 
            dlist_delete(miter.cur);
 
#ifdef CLOBBER_FREED_MEMORY
            wipe_mem(block, slab->blockSize);
#endif
            free(block);
            slab->nblocks--;
            context->mem_allocated -= slab->blockSize;
        }
    }
 
    slab->minFreeChunks = 0;
 
    Assert(slab->nblocks == 0);
    Assert(context->mem_allocated == 0);
}
 
/*
 * SlabDelete
 *      Free all memory which is allocated in the given context.
 */
void
SlabDelete(MemoryContext context)
{
    /* Reset to release all the SlabBlocks */
    SlabReset(context);
    /* And free the context header */
    free(context);
}
 
/*
 * SlabAlloc
 *      Returns pointer to allocated memory of given size or NULL if
 *      request could not be completed; memory is added to the slab.
 */
void *
SlabAlloc(MemoryContext context, Size size)
{
    SlabContext *slab = (SlabContext *) context;
    SlabBlock  *block;
    MemoryChunk *chunk;
    int         idx;
 
    Assert(SlabIsValid(slab));
 
    Assert((slab->minFreeChunks >= 0) &&
           (slab->minFreeChunks < slab->chunksPerBlock));
 
    /* make sure we only allow correct request size */
    if (size != slab->chunkSize)
        elog(ERROR, "unexpected alloc chunk size %zu (expected %zu)",
             size, slab->chunkSize);
 
    /*
     * If there are no free chunks in any existing block, create a new block
     * and put it to the last freelist bucket.
     *
     * slab->minFreeChunks == 0 means there are no blocks with free chunks,
     * thanks to how minFreeChunks is updated at the end of SlabAlloc().
     */
    if (slab->minFreeChunks == 0)
    {
        block = (SlabBlock *) malloc(slab->blockSize);
 
        if (block == NULL)
            return NULL;
 
        block->nfree = slab->chunksPerBlock;
        block->firstFreeChunk = 0;
        block->slab = slab;
 
        /*
         * Put all the chunks on a freelist. Walk the chunks and point each
         * one to the next one.
         */
        for (idx = 0; idx < slab->chunksPerBlock; idx++)
        {
            chunk = SlabBlockGetChunk(slab, block, idx);
            *(int32 *) MemoryChunkGetPointer(chunk) = (idx + 1);
        }
 
        /*
         * And add it to the last freelist with all chunks empty.
         *
         * We know there are no blocks in the freelist, otherwise we wouldn't
         * need a new block.
         */
        Assert(dlist_is_empty(&slab->freelist[slab->chunksPerBlock]));
 
        dlist_push_head(&slab->freelist[slab->chunksPerBlock], &block->node);
 
        slab->minFreeChunks = slab->chunksPerBlock;
        slab->nblocks += 1;
        context->mem_allocated += slab->blockSize;
    }
 
    /* grab the block from the freelist (even the new block is there) */
    block = dlist_head_element(SlabBlock, node,
                               &slab->freelist[slab->minFreeChunks]);
 
    /* make sure we actually got a valid block, with matching nfree */
    Assert(block != NULL);
    Assert(slab->minFreeChunks == block->nfree);
    Assert(block->nfree > 0);
 
    /* we know index of the first free chunk in the block */
    idx = block->firstFreeChunk;
 
    /* make sure the chunk index is valid, and that it's marked as empty */
    Assert((idx >= 0) && (idx < slab->chunksPerBlock));
 
    /* compute the chunk location block start (after the block header) */
    chunk = SlabBlockGetChunk(slab, block, idx);
 
    /*
     * Update the block nfree count, and also the minFreeChunks as we've
     * decreased nfree for a block with the minimum number of free chunks
     * (because that's how we chose the block).
     */
    block->nfree--;
    slab->minFreeChunks = block->nfree;
 
    /*
     * Remove the chunk from the freelist head. The index of the next free
     * chunk is stored in the chunk itself.
     */
    VALGRIND_MAKE_MEM_DEFINED(MemoryChunkGetPointer(chunk), sizeof(int32));
    block->firstFreeChunk = *(int32 *) MemoryChunkGetPointer(chunk);
 
    Assert(block->firstFreeChunk >= 0);
    Assert(block->firstFreeChunk <= slab->chunksPerBlock);
 
    Assert((block->nfree != 0 &&
            block->firstFreeChunk < slab->chunksPerBlock) ||
           (block->nfree == 0 &&
            block->firstFreeChunk == slab->chunksPerBlock));
 
    /* move the whole block to the right place in the freelist */
    dlist_delete(&block->node);
    dlist_push_head(&slab->freelist[block->nfree], &block->node);
 
    /*
     * And finally update minFreeChunks, i.e. the index to the block with the
     * lowest number of free chunks. We only need to do that when the block
     * got full (otherwise we know the current block is the right one). We'll
     * simply walk the freelist until we find a non-empty entry.
     */
    if (slab->minFreeChunks == 0)
    {
        for (idx = 1; idx <= slab->chunksPerBlock; idx++)
        {
            if (dlist_is_empty(&slab->freelist[idx]))
                continue;
 
            /* found a non-empty freelist */
            slab->minFreeChunks = idx;
            break;
        }
    }
 
    if (slab->minFreeChunks == slab->chunksPerBlock)
        slab->minFreeChunks = 0;
 
    /* Prepare to initialize the chunk header. */
    VALGRIND_MAKE_MEM_UNDEFINED(chunk, Slab_CHUNKHDRSZ);
 
    MemoryChunkSetHdrMask(chunk, block, MAXALIGN(slab->chunkSize),
                          MCTX_SLAB_ID);
#ifdef MEMORY_CONTEXT_CHECKING
    /* slab mark to catch clobber of "unused" space */
    Assert(slab->chunkSize < (slab->fullChunkSize - Slab_CHUNKHDRSZ));
    set_sentinel(MemoryChunkGetPointer(chunk), size);
    VALGRIND_MAKE_MEM_NOACCESS(((char *) chunk) +
                               Slab_CHUNKHDRSZ + slab->chunkSize,
                               slab->fullChunkSize -
                               (slab->chunkSize + Slab_CHUNKHDRSZ));
#endif
 
#ifdef RANDOMIZE_ALLOCATED_MEMORY
    /* fill the allocated space with junk */
    randomize_mem((char *) MemoryChunkGetPointer(chunk), size);
#endif
 
    Assert(slab->nblocks * slab->blockSize == context->mem_allocated);
 
    return MemoryChunkGetPointer(chunk);
}
 
/*
 * SlabFree
 *      Frees allocated memory; memory is removed from the slab.
 */
void
SlabFree(void *pointer)
{
    MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
    SlabBlock  *block = MemoryChunkGetBlock(chunk);
    SlabContext *slab;
    int         idx;
 
    /*
     * For speed reasons we just Assert that the referenced block is good.
     * Future field experience may show that this Assert had better become a
     * regular runtime test-and-elog check.
     */
    Assert(SlabBlockIsValid(block));
    slab = block->slab;
 
#ifdef MEMORY_CONTEXT_CHECKING
    /* Test for someone scribbling on unused space in chunk */
    Assert(slab->chunkSize < (slab->fullChunkSize - Slab_CHUNKHDRSZ));
    if (!sentinel_ok(pointer, slab->chunkSize))
        elog(WARNING, "detected write past chunk end in %s %p",
             slab->header.name, chunk);
#endif
 
    /* compute index of the chunk with respect to block start */
    idx = SlabChunkIndex(slab, block, chunk);
 
    /* add chunk to freelist, and update block nfree count */
    *(int32 *) pointer = block->firstFreeChunk;
    block->firstFreeChunk = idx;
    block->nfree++;
 
    Assert(block->nfree > 0);
    Assert(block->nfree <= slab->chunksPerBlock);
 
#ifdef CLOBBER_FREED_MEMORY
    /* XXX don't wipe the int32 index, used for block-level freelist */
    wipe_mem((char *) pointer + sizeof(int32),
             slab->chunkSize - sizeof(int32));
#endif
 
    /* remove the block from a freelist */
    dlist_delete(&block->node);
 
    /*
     * See if we need to update the minFreeChunks field for the slab - we only
     * need to do that if there the block had that number of free chunks
     * before we freed one. In that case, we check if there still are blocks
     * in the original freelist and we either keep the current value (if there
     * still are blocks) or increment it by one (the new block is still the
     * one with minimum free chunks).
     *
     * The one exception is when the block will get completely free - in that
     * case we will free it, se we can't use it for minFreeChunks. It however
     * means there are no more blocks with free chunks.
     */
    if (slab->minFreeChunks == (block->nfree - 1))
    {
        /* Have we removed the last chunk from the freelist? */
        if (dlist_is_empty(&slab->freelist[slab->minFreeChunks]))
        {
            /* but if we made the block entirely free, we'll free it */
            if (block->nfree == slab->chunksPerBlock)
                slab->minFreeChunks = 0;
            else
                slab->minFreeChunks++;
        }
    }
 
    /* If the block is now completely empty, free it. */
    if (block->nfree == slab->chunksPerBlock)
    {
        free(block);
        slab->nblocks--;
        slab->header.mem_allocated -= slab->blockSize;
    }
    else
        dlist_push_head(&slab->freelist[block->nfree], &block->node);
 
    Assert(slab->nblocks >= 0);
    Assert(slab->nblocks * slab->blockSize == slab->header.mem_allocated);
}
 
/*
 * SlabRealloc
 *      Change the allocated size of a chunk.
 *
 * As Slab is designed for allocating equally-sized chunks of memory, it can't
 * do an actual chunk size change.  We try to be gentle and allow calls with
 * exactly the same size, as in that case we can simply return the same
 * chunk.  When the size differs, we throw an error.
 *
 * We could also allow requests with size < chunkSize.  That however seems
 * rather pointless - Slab is meant for chunks of constant size, and moreover
 * realloc is usually used to enlarge the chunk.
 */
void *
SlabRealloc(void *pointer, Size size)
{
    MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
    SlabBlock  *block = MemoryChunkGetBlock(chunk);
    SlabContext *slab;
 
    /*
     * Try to verify that we have a sane block pointer: the block header
     * should reference a slab context.  (We use a test-and-elog, not just
     * Assert, because it seems highly likely that we're here in error in the
     * first place.)
     */
    if (!SlabBlockIsValid(block))
        elog(ERROR, "could not find block containing chunk %p", chunk);
    slab = block->slab;
 
    /* can't do actual realloc with slab, but let's try to be gentle */
    if (size == slab->chunkSize)
        return pointer;
 
    elog(ERROR, "slab allocator does not support realloc()");
    return NULL;                /* keep compiler quiet */
}
 
/*
 * SlabGetChunkContext
 *      Return the MemoryContext that 'pointer' belongs to.
 */
MemoryContext
SlabGetChunkContext(void *pointer)
{
    MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
    SlabBlock  *block = MemoryChunkGetBlock(chunk);
 
    Assert(SlabBlockIsValid(block));
    return &block->slab->header;
}
 
/*
 * SlabGetChunkSpace
 *      Given a currently-allocated chunk, determine the total space
 *      it occupies (including all memory-allocation overhead).
 */
Size
SlabGetChunkSpace(void *pointer)
{
    MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
    SlabBlock  *block = MemoryChunkGetBlock(chunk);
    SlabContext *slab;
 
    Assert(SlabBlockIsValid(block));
    slab = block->slab;
 
    return slab->fullChunkSize;
}
 
/*
 * SlabIsEmpty
 *      Is an Slab empty of any allocated space?
 */
bool
SlabIsEmpty(MemoryContext context)
{
    SlabContext *slab = (SlabContext *) context;
 
    Assert(SlabIsValid(slab));
 
    return (slab->nblocks == 0);
}
 
/*
 * SlabStats
 *      Compute stats about memory consumption of a Slab context.
 *
 * printfunc: if not NULL, pass a human-readable stats string to this.
 * passthru: pass this pointer through to printfunc.
 * totals: if not NULL, add stats about this context into *totals.
 * print_to_stderr: print stats to stderr if true, elog otherwise.
 */
void
SlabStats(MemoryContext context,
          MemoryStatsPrintFunc printfunc, void *passthru,
          MemoryContextCounters *totals,
          bool print_to_stderr)
{
    SlabContext *slab = (SlabContext *) context;
    Size        nblocks = 0;
    Size        freechunks = 0;
    Size        totalspace;
    Size        freespace = 0;
    int         i;
 
    Assert(SlabIsValid(slab));
 
    /* Include context header in totalspace */
    totalspace = slab->headerSize;
 
    for (i = 0; i <= slab->chunksPerBlock; i++)
    {
        dlist_iter  iter;
 
        dlist_foreach(iter, &slab->freelist[i])
        {
            SlabBlock  *block = dlist_container(SlabBlock, node, iter.cur);
 
            nblocks++;
            totalspace += slab->blockSize;
            freespace += slab->fullChunkSize * block->nfree;
            freechunks += block->nfree;
        }
    }
 
    if (printfunc)
    {
        char        stats_string[200];
 
        snprintf(stats_string, sizeof(stats_string),
                 "%zu total in %zu blocks; %zu free (%zu chunks); %zu used",
                 totalspace, nblocks, freespace, freechunks,
                 totalspace - freespace);
        printfunc(context, passthru, stats_string, print_to_stderr);
    }
 
    if (totals)
    {
        totals->nblocks += nblocks;
        totals->freechunks += freechunks;
        totals->totalspace += totalspace;
        totals->freespace += freespace;
    }
}
 
 
#ifdef MEMORY_CONTEXT_CHECKING
 
/*
 * SlabCheck
 *      Walk through chunks and check consistency of memory.
 *
 * NOTE: report errors as WARNING, *not* ERROR or FATAL.  Otherwise you'll
 * find yourself in an infinite loop when trouble occurs, because this
 * routine will be entered again when elog cleanup tries to release memory!
 */
void
SlabCheck(MemoryContext context)
{
    SlabContext *slab = (SlabContext *) context;
    int         i;
    const char *name = slab->header.name;
 
    Assert(SlabIsValid(slab));
    Assert(slab->chunksPerBlock > 0);
 
    /* walk all the freelists */
    for (i = 0; i <= slab->chunksPerBlock; i++)
    {
        int         j,
                    nfree;
        dlist_iter  iter;
 
        /* walk all blocks on this freelist */
        dlist_foreach(iter, &slab->freelist[i])
        {
            int         idx;
            SlabBlock  *block = dlist_container(SlabBlock, node, iter.cur);
 
            /*
             * Make sure the number of free chunks (in the block header)
             * matches position in the freelist.
             */
            if (block->nfree != i)
                elog(WARNING, "problem in slab %s: number of free chunks %d in block %p does not match freelist %d",
                     name, block->nfree, block, i);
 
            /* make sure the slab pointer correctly points to this context */
            if (block->slab != slab)
                elog(WARNING, "problem in slab %s: bogus slab link in block %p",
                     name, block);
 
            /* reset the bitmap of free chunks for this block */
            memset(slab->freechunks, 0, (slab->chunksPerBlock * sizeof(bool)));
            idx = block->firstFreeChunk;
 
            /*
             * Now walk through the chunks, count the free ones and also
             * perform some additional checks for the used ones. As the chunk
             * freelist is stored within the chunks themselves, we have to
             * walk through the chunks and construct our own bitmap.
             */
 
            nfree = 0;
            while (idx < slab->chunksPerBlock)
            {
                MemoryChunk *chunk;
 
                /* count the chunk as free, add it to the bitmap */
                nfree++;
                slab->freechunks[idx] = true;
 
                /* read index of the next free chunk */
                chunk = SlabBlockGetChunk(slab, block, idx);
                VALGRIND_MAKE_MEM_DEFINED(MemoryChunkGetPointer(chunk), sizeof(int32));
                idx = *(int32 *) MemoryChunkGetPointer(chunk);
            }
 
            for (j = 0; j < slab->chunksPerBlock; j++)
            {
                /* non-zero bit in the bitmap means chunk the chunk is used */
                if (!slab->freechunks[j])
                {
                    MemoryChunk *chunk = SlabBlockGetChunk(slab, block, j);
                    SlabBlock  *chunkblock = (SlabBlock *) MemoryChunkGetBlock(chunk);
 
                    /*
                     * check the chunk's blockoffset correctly points back to
                     * the block
                     */
                    if (chunkblock != block)
                        elog(WARNING, "problem in slab %s: bogus block link in block %p, chunk %p",
                             name, block, chunk);
 
                    /* check the sentinel byte is intact */
                    Assert(slab->chunkSize < (slab->fullChunkSize - Slab_CHUNKHDRSZ));
                    if (!sentinel_ok(chunk, Slab_CHUNKHDRSZ + slab->chunkSize))
                        elog(WARNING, "problem in slab %s: detected write past chunk end in block %p, chunk %p",
                             name, block, chunk);
                }
            }
 
            /*
             * Make sure we got the expected number of free chunks (as tracked
             * in the block header).
             */
            if (nfree != block->nfree)
                elog(WARNING, "problem in slab %s: number of free chunks %d in block %p does not match bitmap %d",
                     name, block->nfree, block, nfree);
        }
    }
 
    Assert(slab->nblocks * slab->blockSize == context->mem_allocated);
}
 
#endif                          /* MEMORY_CONTEXT_CHECKING */