generation.c

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/*-------------------------------------------------------------------------
 *
 * generation.c
 *    Generational allocator definitions.
 *
 * Generation is a custom MemoryContext implementation designed for cases of
 * chunks with similar lifespan.
 *
 * Portions Copyright (c) 2017-2025, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/utils/mmgr/generation.c
 *
 *
 *  This memory context is based on the assumption that the chunks are freed
 *  roughly in the same order as they were allocated (FIFO), or in groups with
 *  similar lifespan (generations - hence the name of the context). This is
 *  typical for various queue-like use cases, i.e. when tuples are constructed,
 *  processed and then thrown away.
 *
 *  The memory context uses a very simple approach to free space management.
 *  Instead of a complex global freelist, each block tracks a number
 *  of allocated and freed chunks.  The block is classed as empty when the
 *  number of free chunks is equal to the number of allocated chunks.  When
 *  this occurs, instead of freeing the block, we try to "recycle" it, i.e.
 *  reuse it for new allocations.  This is done by setting the block in the
 *  context's 'freeblock' field.  If the freeblock field is already occupied
 *  by another free block we simply return the newly empty block to malloc.
 *
 *  This approach to free blocks requires fewer malloc/free calls for truly
 *  first allocated, first free'd allocation patterns.
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "lib/ilist.h"
#include "port/pg_bitutils.h"
#include "utils/memdebug.h"
#include "utils/memutils.h"
#include "utils/memutils_internal.h"
#include "utils/memutils_memorychunk.h"
 
 
#define Generation_BLOCKHDRSZ   MAXALIGN(sizeof(GenerationBlock))
#define Generation_CHUNKHDRSZ   sizeof(MemoryChunk)
 
#define Generation_CHUNK_FRACTION   8
 
typedef struct GenerationBlock GenerationBlock; /* forward reference */
 
typedef void *GenerationPointer;
 
/*
 * GenerationContext is a simple memory context not reusing allocated chunks,
 * and freeing blocks once all chunks are freed.
 */
typedef struct GenerationContext
{
    MemoryContextData header;   /* Standard memory-context fields */
 
    /* Generational context parameters */
    uint32      initBlockSize;  /* initial block size */
    uint32      maxBlockSize;   /* maximum block size */
    uint32      nextBlockSize;  /* next block size to allocate */
    uint32      allocChunkLimit;    /* effective chunk size limit */
 
    GenerationBlock *block;     /* current (most recently allocated) block */
    GenerationBlock *freeblock; /* pointer to an empty block that's being
                                 * recycled, or NULL if there's no such block. */
    dlist_head  blocks;         /* list of blocks */
} GenerationContext;
 
/*
 * GenerationBlock
 *      GenerationBlock is the unit of memory that is obtained by generation.c
 *      from malloc().  It contains zero or more MemoryChunks, which are the
 *      units requested by palloc() and freed by pfree().  MemoryChunks cannot
 *      be returned to malloc() individually, instead pfree() updates the free
 *      counter of the block and when all chunks in a block are free the whole
 *      block can be returned to malloc().
 *
 *      GenerationBlock is the header data for a block --- the usable space
 *      within the block begins at the next alignment boundary.
 */
struct GenerationBlock
{
    dlist_node  node;           /* doubly-linked list of blocks */
    GenerationContext *context; /* pointer back to the owning context */
    Size        blksize;        /* allocated size of this block */
    int         nchunks;        /* number of chunks in the block */
    int         nfree;          /* number of free chunks */
    char       *freeptr;        /* start of free space in this block */
    char       *endptr;         /* end of space in this block */
};
 
/*
 * GenerationIsValid
 *      True iff set is valid generation set.
 */
#define GenerationIsValid(set) \
    (PointerIsValid(set) && IsA(set, GenerationContext))
 
/*
 * GenerationBlockIsValid
 *      True iff block is valid block of generation set.
 */
#define GenerationBlockIsValid(block) \
    (PointerIsValid(block) && GenerationIsValid((block)->context))
 
/*
 * GenerationBlockIsEmpty
 *      True iff block contains no chunks
 */
#define GenerationBlockIsEmpty(b) ((b)->nchunks == 0)
 
/*
 * We always store external chunks on a dedicated block.  This makes fetching
 * the block from an external chunk easy since it's always the first and only
 * chunk on the block.
 */
#define ExternalChunkGetBlock(chunk) \
    (GenerationBlock *) ((char *) chunk - Generation_BLOCKHDRSZ)
 
/* Obtain the keeper block for a generation context */
#define KeeperBlock(set) \
    ((GenerationBlock *) (((char *) set) + \
    MAXALIGN(sizeof(GenerationContext))))
 
/* Check if the block is the keeper block of the given generation context */
#define IsKeeperBlock(set, block) ((block) == (KeeperBlock(set)))
 
/* Inlined helper functions */
static inline void GenerationBlockInit(GenerationContext *context,
                                       GenerationBlock *block,
                                       Size blksize);
static inline void GenerationBlockMarkEmpty(GenerationBlock *block);
static inline Size GenerationBlockFreeBytes(GenerationBlock *block);
static inline void GenerationBlockFree(GenerationContext *set,
                                       GenerationBlock *block);
 
 
/*
 * Public routines
 */
 
 
/*
 * GenerationContextCreate
 *      Create a new Generation context.
 *
 * parent: parent context, or NULL if top-level context
 * name: name of context (must be statically allocated)
 * minContextSize: minimum context size
 * initBlockSize: initial allocation block size
 * maxBlockSize: maximum allocation block size
 */
MemoryContext
GenerationContextCreate(MemoryContext parent,
                        const char *name,
                        Size minContextSize,
                        Size initBlockSize,
                        Size maxBlockSize)
{
    Size        firstBlockSize;
    Size        allocSize;
    GenerationContext *set;
    GenerationBlock *block;
 
    /* ensure MemoryChunk's size is properly maxaligned */
    StaticAssertDecl(Generation_CHUNKHDRSZ == MAXALIGN(Generation_CHUNKHDRSZ),
                     "sizeof(MemoryChunk) is not maxaligned");
 
    /*
     * First, validate allocation parameters.  Asserts seem sufficient because
     * nobody varies their parameters at runtime.  We somewhat arbitrarily
     * enforce a minimum 1K block size.  We restrict the maximum block size to
     * MEMORYCHUNK_MAX_BLOCKOFFSET as MemoryChunks are limited to this in
     * regards to addressing the offset between the chunk and the block that
     * the chunk is stored on.  We would be unable to store the offset between
     * the chunk and block for any chunks that were beyond
     * MEMORYCHUNK_MAX_BLOCKOFFSET bytes into the block if the block was to be
     * larger than this.
     */
    Assert(initBlockSize == MAXALIGN(initBlockSize) &&
           initBlockSize >= 1024);
    Assert(maxBlockSize == MAXALIGN(maxBlockSize) &&
           maxBlockSize >= initBlockSize &&
           AllocHugeSizeIsValid(maxBlockSize)); /* must be safe to double */
    Assert(minContextSize == 0 ||
           (minContextSize == MAXALIGN(minContextSize) &&
            minContextSize >= 1024 &&
            minContextSize <= maxBlockSize));
    Assert(maxBlockSize <= MEMORYCHUNK_MAX_BLOCKOFFSET);
 
    /* Determine size of initial block */
    allocSize = MAXALIGN(sizeof(GenerationContext)) +
        Generation_BLOCKHDRSZ + Generation_CHUNKHDRSZ;
    if (minContextSize != 0)
        allocSize = Max(allocSize, minContextSize);
    else
        allocSize = Max(allocSize, initBlockSize);
 
    /*
     * Allocate the initial block.  Unlike other generation.c blocks, it
     * starts with the context header and its block header follows that.
     */
    set = (GenerationContext *) malloc(allocSize);
    if (set == 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.
     */
    dlist_init(&set->blocks);
 
    /* Fill in the initial block's block header */
    block = KeeperBlock(set);
    /* determine the block size and initialize it */
    firstBlockSize = allocSize - MAXALIGN(sizeof(GenerationContext));
    GenerationBlockInit(set, block, firstBlockSize);
 
    /* add it to the doubly-linked list of blocks */
    dlist_push_head(&set->blocks, &block->node);
 
    /* use it as the current allocation block */
    set->block = block;
 
    /* No free block, yet */
    set->freeblock = NULL;
 
    /* Fill in GenerationContext-specific header fields */
    set->initBlockSize = (uint32) initBlockSize;
    set->maxBlockSize = (uint32) maxBlockSize;
    set->nextBlockSize = (uint32) initBlockSize;
 
    /*
     * Compute the allocation chunk size limit for this context.
     *
     * Limit the maximum size a non-dedicated chunk can be so that we can fit
     * at least Generation_CHUNK_FRACTION of chunks this big onto the maximum
     * sized block.  We must further limit this value so that it's no more
     * than MEMORYCHUNK_MAX_VALUE.  We're unable to have non-external chunks
     * larger than that value as we store the chunk size in the MemoryChunk
     * 'value' field in the call to MemoryChunkSetHdrMask().
     */
    set->allocChunkLimit = Min(maxBlockSize, MEMORYCHUNK_MAX_VALUE);
    while ((Size) (set->allocChunkLimit + Generation_CHUNKHDRSZ) >
           (Size) ((Size) (maxBlockSize - Generation_BLOCKHDRSZ) / Generation_CHUNK_FRACTION))
        set->allocChunkLimit >>= 1;
 
    /* Finally, do the type-independent part of context creation */
    MemoryContextCreate((MemoryContext) set,
                        T_GenerationContext,
                        MCTX_GENERATION_ID,
                        parent,
                        name);
 
    ((MemoryContext) set)->mem_allocated = firstBlockSize;
 
    return (MemoryContext) set;
}
 
/*
 * GenerationReset
 *      Frees all memory which is allocated in the given set.
 *
 * The initial "keeper" block (which shares a malloc chunk with the context
 * header) is not given back to the operating system though.  In this way, we
 * don't thrash malloc() when a context is repeatedly reset after small
 * allocations.
 */
void
GenerationReset(MemoryContext context)
{
    GenerationContext *set = (GenerationContext *) context;
    dlist_mutable_iter miter;
 
    Assert(GenerationIsValid(set));
 
#ifdef MEMORY_CONTEXT_CHECKING
    /* Check for corruption and leaks before freeing */
    GenerationCheck(context);
#endif
 
    /*
     * NULLify the free block pointer.  We must do this before calling
     * GenerationBlockFree as that function never expects to free the
     * freeblock.
     */
    set->freeblock = NULL;
 
    dlist_foreach_modify(miter, &set->blocks)
    {
        GenerationBlock *block = dlist_container(GenerationBlock, node, miter.cur);
 
        if (IsKeeperBlock(set, block))
            GenerationBlockMarkEmpty(block);
        else
            GenerationBlockFree(set, block);
    }
 
    /* set it so new allocations to make use of the keeper block */
    set->block = KeeperBlock(set);
 
    /* Reset block size allocation sequence, too */
    set->nextBlockSize = set->initBlockSize;
 
    /* Ensure there is only 1 item in the dlist */
    Assert(!dlist_is_empty(&set->blocks));
    Assert(!dlist_has_next(&set->blocks, dlist_head_node(&set->blocks)));
}
 
/*
 * GenerationDelete
 *      Free all memory which is allocated in the given context.
 */
void
GenerationDelete(MemoryContext context)
{
    /* Reset to release all releasable GenerationBlocks */
    GenerationReset(context);
    /* And free the context header and keeper block */
    free(context);
}
 
/*
 * Helper for GenerationAlloc() that allocates an entire block for the chunk.
 *
 * GenerationAlloc()'s comment explains why this is separate.
 */
pg_noinline
static void *
GenerationAllocLarge(MemoryContext context, Size size, int flags)
{
    GenerationContext *set = (GenerationContext *) context;
    GenerationBlock *block;
    MemoryChunk *chunk;
    Size        chunk_size;
    Size        required_size;
    Size        blksize;
 
    /* validate 'size' is within the limits for the given 'flags' */
    MemoryContextCheckSize(context, size, flags);
 
#ifdef MEMORY_CONTEXT_CHECKING
    /* ensure there's always space for the sentinel byte */
    chunk_size = MAXALIGN(size + 1);
#else
    chunk_size = MAXALIGN(size);
#endif
    required_size = chunk_size + Generation_CHUNKHDRSZ;
    blksize = required_size + Generation_BLOCKHDRSZ;
 
    block = (GenerationBlock *) malloc(blksize);
    if (block == NULL)
        return MemoryContextAllocationFailure(context, size, flags);
 
    context->mem_allocated += blksize;
 
    /* block with a single (used) chunk */
    block->context = set;
    block->blksize = blksize;
    block->nchunks = 1;
    block->nfree = 0;
 
    /* the block is completely full */
    block->freeptr = block->endptr = ((char *) block) + blksize;
 
    chunk = (MemoryChunk *) (((char *) block) + Generation_BLOCKHDRSZ);
 
    /* mark the MemoryChunk as externally managed */
    MemoryChunkSetHdrMaskExternal(chunk, MCTX_GENERATION_ID);
 
#ifdef MEMORY_CONTEXT_CHECKING
    chunk->requested_size = size;
    /* set mark to catch clobber of "unused" space */
    Assert(size < chunk_size);
    set_sentinel(MemoryChunkGetPointer(chunk), size);
#endif
#ifdef RANDOMIZE_ALLOCATED_MEMORY
    /* fill the allocated space with junk */
    randomize_mem((char *) MemoryChunkGetPointer(chunk), size);
#endif
 
    /* add the block to the list of allocated blocks */
    dlist_push_head(&set->blocks, &block->node);
 
    /* Ensure any padding bytes are marked NOACCESS. */
    VALGRIND_MAKE_MEM_NOACCESS((char *) MemoryChunkGetPointer(chunk) + size,
                               chunk_size - size);
 
    /* Disallow access to the chunk header. */
    VALGRIND_MAKE_MEM_NOACCESS(chunk, Generation_CHUNKHDRSZ);
 
    return MemoryChunkGetPointer(chunk);
}
 
/*
 * Small helper for allocating a new chunk from a chunk, to avoid duplicating
 * the code between GenerationAlloc() and GenerationAllocFromNewBlock().
 */
static inline void *
GenerationAllocChunkFromBlock(MemoryContext context, GenerationBlock *block,
                              Size size, Size chunk_size)
{
    MemoryChunk *chunk = (MemoryChunk *) (block->freeptr);
 
    /* validate we've been given a block with enough free space */
    Assert(block != NULL);
    Assert((block->endptr - block->freeptr) >=
           Generation_CHUNKHDRSZ + chunk_size);
 
    /* Prepare to initialize the chunk header. */
    VALGRIND_MAKE_MEM_UNDEFINED(chunk, Generation_CHUNKHDRSZ);
 
    block->nchunks += 1;
    block->freeptr += (Generation_CHUNKHDRSZ + chunk_size);
 
    Assert(block->freeptr <= block->endptr);
 
    MemoryChunkSetHdrMask(chunk, block, chunk_size, MCTX_GENERATION_ID);
#ifdef MEMORY_CONTEXT_CHECKING
    chunk->requested_size = size;
    /* set mark to catch clobber of "unused" space */
    Assert(size < chunk_size);
    set_sentinel(MemoryChunkGetPointer(chunk), size);
#endif
#ifdef RANDOMIZE_ALLOCATED_MEMORY
    /* fill the allocated space with junk */
    randomize_mem((char *) MemoryChunkGetPointer(chunk), size);
#endif
 
    /* Ensure any padding bytes are marked NOACCESS. */
    VALGRIND_MAKE_MEM_NOACCESS((char *) MemoryChunkGetPointer(chunk) + size,
                               chunk_size - size);
 
    /* Disallow access to the chunk header. */
    VALGRIND_MAKE_MEM_NOACCESS(chunk, Generation_CHUNKHDRSZ);
 
    return MemoryChunkGetPointer(chunk);
}
 
/*
 * Helper for GenerationAlloc() that allocates a new block and returns a chunk
 * allocated from it.
 *
 * GenerationAlloc()'s comment explains why this is separate.
 */
pg_noinline
static void *
GenerationAllocFromNewBlock(MemoryContext context, Size size, int flags,
                            Size chunk_size)
{
    GenerationContext *set = (GenerationContext *) context;
    GenerationBlock *block;
    Size        blksize;
    Size        required_size;
 
    /*
     * The first such block has size initBlockSize, and we double the space in
     * each succeeding block, but not more than maxBlockSize.
     */
    blksize = set->nextBlockSize;
    set->nextBlockSize <<= 1;
    if (set->nextBlockSize > set->maxBlockSize)
        set->nextBlockSize = set->maxBlockSize;
 
    /* we'll need space for the chunk, chunk hdr and block hdr */
    required_size = chunk_size + Generation_CHUNKHDRSZ + Generation_BLOCKHDRSZ;
 
    /* round the size up to the next power of 2 */
    if (blksize < required_size)
        blksize = pg_nextpower2_size_t(required_size);
 
    block = (GenerationBlock *) malloc(blksize);
 
    if (block == NULL)
        return MemoryContextAllocationFailure(context, size, flags);
 
    context->mem_allocated += blksize;
 
    /* initialize the new block */
    GenerationBlockInit(set, block, blksize);
 
    /* add it to the doubly-linked list of blocks */
    dlist_push_head(&set->blocks, &block->node);
 
    /* make this the current block */
    set->block = block;
 
    return GenerationAllocChunkFromBlock(context, block, size, chunk_size);
}
 
/*
 * GenerationAlloc
 *      Returns a pointer to allocated memory of given size or raises an ERROR
 *      on allocation failure, or returns NULL when flags contains
 *      MCXT_ALLOC_NO_OOM.
 *
 * No request may exceed:
 *      MAXALIGN_DOWN(SIZE_MAX) - Generation_BLOCKHDRSZ - Generation_CHUNKHDRSZ
 * All callers use a much-lower limit.
 *
 * Note: when using valgrind, it doesn't matter how the returned allocation
 * is marked, as mcxt.c will set it to UNDEFINED.  In some paths we will
 * return space that is marked NOACCESS - GenerationRealloc has to beware!
 *
 * This function should only contain the most common code paths.  Everything
 * else should be in pg_noinline helper functions, thus avoiding the overhead
 * of creating a stack frame for the common cases.  Allocating memory is often
 * a bottleneck in many workloads, so avoiding stack frame setup is
 * worthwhile.  Helper functions should always directly return the newly
 * allocated memory so that we can just return that address directly as a tail
 * call.
 */
void *
GenerationAlloc(MemoryContext context, Size size, int flags)
{
    GenerationContext *set = (GenerationContext *) context;
    GenerationBlock *block;
    Size        chunk_size;
    Size        required_size;
 
    Assert(GenerationIsValid(set));
 
#ifdef MEMORY_CONTEXT_CHECKING
    /* ensure there's always space for the sentinel byte */
    chunk_size = MAXALIGN(size + 1);
#else
    chunk_size = MAXALIGN(size);
#endif
 
    /*
     * If requested size exceeds maximum for chunks we hand the request off to
     * GenerationAllocLarge().
     */
    if (chunk_size > set->allocChunkLimit)
        return GenerationAllocLarge(context, size, flags);
 
    required_size = chunk_size + Generation_CHUNKHDRSZ;
 
    /*
     * Not an oversized chunk.  We try to first make use of the current block,
     * but if there's not enough space in it, instead of allocating a new
     * block, we look to see if the empty freeblock has enough space.  We
     * don't try reusing the keeper block.  If it's become empty we'll reuse
     * that again only if the context is reset.
     *
     * We only try reusing the freeblock if we've no space for this allocation
     * on the current block.  When a freeblock exists, we'll switch to it once
     * the first time we can't fit an allocation in the current block.  We
     * avoid ping-ponging between the two as we need to be careful not to
     * fragment differently sized consecutive allocations between several
     * blocks.  Going between the two could cause fragmentation for FIFO
     * workloads, which generation is meant to be good at.
     */
    block = set->block;
 
    if (unlikely(GenerationBlockFreeBytes(block) < required_size))
    {
        GenerationBlock *freeblock = set->freeblock;
 
        /* freeblock, if set, must be empty */
        Assert(freeblock == NULL || GenerationBlockIsEmpty(freeblock));
 
        /* check if we have a freeblock and if it's big enough */
        if (freeblock != NULL &&
            GenerationBlockFreeBytes(freeblock) >= required_size)
        {
            /* make the freeblock the current block */
            set->freeblock = NULL;
            set->block = freeblock;
 
            return GenerationAllocChunkFromBlock(context,
                                                 freeblock,
                                                 size,
                                                 chunk_size);
        }
        else
        {
            /*
             * No freeblock, or it's not big enough for this allocation.  Make
             * a new block.
             */
            return GenerationAllocFromNewBlock(context, size, flags, chunk_size);
        }
    }
 
    /* The current block has space, so just allocate chunk there. */
    return GenerationAllocChunkFromBlock(context, block, size, chunk_size);
}
 
/*
 * GenerationBlockInit
 *      Initializes 'block' assuming 'blksize'.  Does not update the context's
 *      mem_allocated field.
 */
static inline void
GenerationBlockInit(GenerationContext *context, GenerationBlock *block,
                    Size blksize)
{
    block->context = context;
    block->blksize = blksize;
    block->nchunks = 0;
    block->nfree = 0;
 
    block->freeptr = ((char *) block) + Generation_BLOCKHDRSZ;
    block->endptr = ((char *) block) + blksize;
 
    /* Mark unallocated space NOACCESS. */
    VALGRIND_MAKE_MEM_NOACCESS(block->freeptr,
                               blksize - Generation_BLOCKHDRSZ);
}
 
/*
 * GenerationBlockMarkEmpty
 *      Set a block as empty.  Does not free the block.
 */
static inline void
GenerationBlockMarkEmpty(GenerationBlock *block)
{
#if defined(USE_VALGRIND) || defined(CLOBBER_FREED_MEMORY)
    char       *datastart = ((char *) block) + Generation_BLOCKHDRSZ;
#endif
 
#ifdef CLOBBER_FREED_MEMORY
    wipe_mem(datastart, block->freeptr - datastart);
#else
    /* wipe_mem() would have done this */
    VALGRIND_MAKE_MEM_NOACCESS(datastart, block->freeptr - datastart);
#endif
 
    /* Reset the block, but don't return it to malloc */
    block->nchunks = 0;
    block->nfree = 0;
    block->freeptr = ((char *) block) + Generation_BLOCKHDRSZ;
}
 
/*
 * GenerationBlockFreeBytes
 *      Returns the number of bytes free in 'block'
 */
static inline Size
GenerationBlockFreeBytes(GenerationBlock *block)
{
    return (block->endptr - block->freeptr);
}
 
/*
 * GenerationBlockFree
 *      Remove 'block' from 'set' and release the memory consumed by it.
 */
static inline void
GenerationBlockFree(GenerationContext *set, GenerationBlock *block)
{
    /* Make sure nobody tries to free the keeper block */
    Assert(!IsKeeperBlock(set, block));
    /* We shouldn't be freeing the freeblock either */
    Assert(block != set->freeblock);
 
    /* release the block from the list of blocks */
    dlist_delete(&block->node);
 
    ((MemoryContext) set)->mem_allocated -= block->blksize;
 
#ifdef CLOBBER_FREED_MEMORY
    wipe_mem(block, block->blksize);
#endif
 
    free(block);
}
 
/*
 * GenerationFree
 *      Update number of chunks in the block, and consider freeing the block
 *      if it's become empty.
 */
void
GenerationFree(void *pointer)
{
    MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
    GenerationBlock *block;
    GenerationContext *set;
#if (defined(MEMORY_CONTEXT_CHECKING) && defined(USE_ASSERT_CHECKING)) \
    || defined(CLOBBER_FREED_MEMORY)
    Size        chunksize;
#endif
 
    /* Allow access to the chunk header. */
    VALGRIND_MAKE_MEM_DEFINED(chunk, Generation_CHUNKHDRSZ);
 
    if (MemoryChunkIsExternal(chunk))
    {
        block = ExternalChunkGetBlock(chunk);
 
        /*
         * Try to verify that we have a sane block pointer: the block header
         * should reference a generation context.
         */
        if (!GenerationBlockIsValid(block))
            elog(ERROR, "could not find block containing chunk %p", chunk);
 
#if (defined(MEMORY_CONTEXT_CHECKING) && defined(USE_ASSERT_CHECKING)) \
    || defined(CLOBBER_FREED_MEMORY)
        chunksize = block->endptr - (char *) pointer;
#endif
    }
    else
    {
        block = MemoryChunkGetBlock(chunk);
 
        /*
         * In this path, 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(GenerationBlockIsValid(block));
 
#if (defined(MEMORY_CONTEXT_CHECKING) && defined(USE_ASSERT_CHECKING)) \
    || defined(CLOBBER_FREED_MEMORY)
        chunksize = MemoryChunkGetValue(chunk);
#endif
    }
 
#ifdef MEMORY_CONTEXT_CHECKING
    /* Test for someone scribbling on unused space in chunk */
    Assert(chunk->requested_size < chunksize);
    if (!sentinel_ok(pointer, chunk->requested_size))
        elog(WARNING, "detected write past chunk end in %s %p",
             ((MemoryContext) block->context)->name, chunk);
#endif
 
#ifdef CLOBBER_FREED_MEMORY
    wipe_mem(pointer, chunksize);
#endif
 
#ifdef MEMORY_CONTEXT_CHECKING
    /* Reset requested_size to InvalidAllocSize in freed chunks */
    chunk->requested_size = InvalidAllocSize;
#endif
 
    block->nfree += 1;
 
    Assert(block->nchunks > 0);
    Assert(block->nfree <= block->nchunks);
    Assert(block != block->context->freeblock);
 
    /* If there are still allocated chunks in the block, we're done. */
    if (likely(block->nfree < block->nchunks))
        return;
 
    set = block->context;
 
    /*-----------------------
     * The block this allocation was on has now become completely empty of
     * chunks.  In the general case, we can now return the memory for this
     * block back to malloc.  However, there are cases where we don't want to
     * do that:
     *
     * 1)   If it's the keeper block.  This block was malloc'd in the same
     *      allocation as the context itself and can't be free'd without
     *      freeing the context.
     * 2)   If it's the current block.  We could free this, but doing so would
     *      leave us nothing to set the current block to, so we just mark the
     *      block as empty so new allocations can reuse it again.
     * 3)   If we have no "freeblock" set, then we save a single block for
     *      future allocations to avoid having to malloc a new block again.
     *      This is useful for FIFO workloads as it avoids continual
     *      free/malloc cycles.
     */
    if (IsKeeperBlock(set, block) || set->block == block)
        GenerationBlockMarkEmpty(block);    /* case 1 and 2 */
    else if (set->freeblock == NULL)
    {
        /* case 3 */
        GenerationBlockMarkEmpty(block);
        set->freeblock = block;
    }
    else
        GenerationBlockFree(set, block);    /* Otherwise, free it */
}
 
/*
 * GenerationRealloc
 *      When handling repalloc, we simply allocate a new chunk, copy the data
 *      and discard the old one. The only exception is when the new size fits
 *      into the old chunk - in that case we just update chunk header.
 */
void *
GenerationRealloc(void *pointer, Size size, int flags)
{
    MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
    GenerationContext *set;
    GenerationBlock *block;
    GenerationPointer newPointer;
    Size        oldsize;
 
    /* Allow access to the chunk header. */
    VALGRIND_MAKE_MEM_DEFINED(chunk, Generation_CHUNKHDRSZ);
 
    if (MemoryChunkIsExternal(chunk))
    {
        block = ExternalChunkGetBlock(chunk);
 
        /*
         * Try to verify that we have a sane block pointer: the block header
         * should reference a generation context.
         */
        if (!GenerationBlockIsValid(block))
            elog(ERROR, "could not find block containing chunk %p", chunk);
 
        oldsize = block->endptr - (char *) pointer;
    }
    else
    {
        block = MemoryChunkGetBlock(chunk);
 
        /*
         * In this path, 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(GenerationBlockIsValid(block));
 
        oldsize = MemoryChunkGetValue(chunk);
    }
 
    set = block->context;
 
#ifdef MEMORY_CONTEXT_CHECKING
    /* Test for someone scribbling on unused space in chunk */
    Assert(chunk->requested_size < oldsize);
    if (!sentinel_ok(pointer, chunk->requested_size))
        elog(WARNING, "detected write past chunk end in %s %p",
             ((MemoryContext) set)->name, chunk);
#endif
 
    /*
     * Maybe the allocated area already big enough.  (In particular, we always
     * fall out here if the requested size is a decrease.)
     *
     * This memory context does not use power-of-2 chunk sizing and instead
     * carves the chunks to be as small as possible, so most repalloc() calls
     * will end up in the palloc/memcpy/pfree branch.
     *
     * XXX Perhaps we should annotate this condition with unlikely()?
     */
#ifdef MEMORY_CONTEXT_CHECKING
    /* With MEMORY_CONTEXT_CHECKING, we need an extra byte for the sentinel */
    if (oldsize > size)
#else
    if (oldsize >= size)
#endif
    {
#ifdef MEMORY_CONTEXT_CHECKING
        Size        oldrequest = chunk->requested_size;
 
#ifdef RANDOMIZE_ALLOCATED_MEMORY
        /* We can only fill the extra space if we know the prior request */
        if (size > oldrequest)
            randomize_mem((char *) pointer + oldrequest,
                          size - oldrequest);
#endif
 
        chunk->requested_size = size;
 
        /*
         * If this is an increase, mark any newly-available part UNDEFINED.
         * Otherwise, mark the obsolete part NOACCESS.
         */
        if (size > oldrequest)
            VALGRIND_MAKE_MEM_UNDEFINED((char *) pointer + oldrequest,
                                        size - oldrequest);
        else
            VALGRIND_MAKE_MEM_NOACCESS((char *) pointer + size,
                                       oldsize - size);
 
        /* set mark to catch clobber of "unused" space */
        set_sentinel(pointer, size);
#else                           /* !MEMORY_CONTEXT_CHECKING */
 
        /*
         * We don't have the information to determine whether we're growing
         * the old request or shrinking it, so we conservatively mark the
         * entire new allocation DEFINED.
         */
        VALGRIND_MAKE_MEM_NOACCESS(pointer, oldsize);
        VALGRIND_MAKE_MEM_DEFINED(pointer, size);
#endif
 
        /* Disallow access to the chunk header. */
        VALGRIND_MAKE_MEM_NOACCESS(chunk, Generation_CHUNKHDRSZ);
 
        return pointer;
    }
 
    /* allocate new chunk (this also checks size is valid) */
    newPointer = GenerationAlloc((MemoryContext) set, size, flags);
 
    /* leave immediately if request was not completed */
    if (newPointer == NULL)
    {
        /* Disallow access to the chunk header. */
        VALGRIND_MAKE_MEM_NOACCESS(chunk, Generation_CHUNKHDRSZ);
        return MemoryContextAllocationFailure((MemoryContext) set, size, flags);
    }
 
    /*
     * GenerationAlloc() may have returned a region that is still NOACCESS.
     * Change it to UNDEFINED for the moment; memcpy() will then transfer
     * definedness from the old allocation to the new.  If we know the old
     * allocation, copy just that much.  Otherwise, make the entire old chunk
     * defined to avoid errors as we copy the currently-NOACCESS trailing
     * bytes.
     */
    VALGRIND_MAKE_MEM_UNDEFINED(newPointer, size);
#ifdef MEMORY_CONTEXT_CHECKING
    oldsize = chunk->requested_size;
#else
    VALGRIND_MAKE_MEM_DEFINED(pointer, oldsize);
#endif
 
    /* transfer existing data (certain to fit) */
    memcpy(newPointer, pointer, oldsize);
 
    /* free old chunk */
    GenerationFree(pointer);
 
    return newPointer;
}
 
/*
 * GenerationGetChunkContext
 *      Return the MemoryContext that 'pointer' belongs to.
 */
MemoryContext
GenerationGetChunkContext(void *pointer)
{
    MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
    GenerationBlock *block;
 
    /* Allow access to the chunk header. */
    VALGRIND_MAKE_MEM_DEFINED(chunk, Generation_CHUNKHDRSZ);
 
    if (MemoryChunkIsExternal(chunk))
        block = ExternalChunkGetBlock(chunk);
    else
        block = (GenerationBlock *) MemoryChunkGetBlock(chunk);
 
    /* Disallow access to the chunk header. */
    VALGRIND_MAKE_MEM_NOACCESS(chunk, Generation_CHUNKHDRSZ);
 
    Assert(GenerationBlockIsValid(block));
    return &block->context->header;
}
 
/*
 * GenerationGetChunkSpace
 *      Given a currently-allocated chunk, determine the total space
 *      it occupies (including all memory-allocation overhead).
 */
Size
GenerationGetChunkSpace(void *pointer)
{
    MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
    Size        chunksize;
 
    /* Allow access to the chunk header. */
    VALGRIND_MAKE_MEM_DEFINED(chunk, Generation_CHUNKHDRSZ);
 
    if (MemoryChunkIsExternal(chunk))
    {
        GenerationBlock *block = ExternalChunkGetBlock(chunk);
 
        Assert(GenerationBlockIsValid(block));
        chunksize = block->endptr - (char *) pointer;
    }
    else
        chunksize = MemoryChunkGetValue(chunk);
 
    /* Disallow access to the chunk header. */
    VALGRIND_MAKE_MEM_NOACCESS(chunk, Generation_CHUNKHDRSZ);
 
    return Generation_CHUNKHDRSZ + chunksize;
}
 
/*
 * GenerationIsEmpty
 *      Is a GenerationContext empty of any allocated space?
 */
bool
GenerationIsEmpty(MemoryContext context)
{
    GenerationContext *set = (GenerationContext *) context;
    dlist_iter  iter;
 
    Assert(GenerationIsValid(set));
 
    dlist_foreach(iter, &set->blocks)
    {
        GenerationBlock *block = dlist_container(GenerationBlock, node, iter.cur);
 
        if (block->nchunks > 0)
            return false;
    }
 
    return true;
}
 
/*
 * GenerationStats
 *      Compute stats about memory consumption of a Generation 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.
 *
 * XXX freespace only accounts for empty space at the end of the block, not
 * space of freed chunks (which is unknown).
 */
void
GenerationStats(MemoryContext context,
                MemoryStatsPrintFunc printfunc, void *passthru,
                MemoryContextCounters *totals, bool print_to_stderr)
{
    GenerationContext *set = (GenerationContext *) context;
    Size        nblocks = 0;
    Size        nchunks = 0;
    Size        nfreechunks = 0;
    Size        totalspace;
    Size        freespace = 0;
    dlist_iter  iter;
 
    Assert(GenerationIsValid(set));
 
    /* Include context header in totalspace */
    totalspace = MAXALIGN(sizeof(GenerationContext));
 
    dlist_foreach(iter, &set->blocks)
    {
        GenerationBlock *block = dlist_container(GenerationBlock, node, iter.cur);
 
        nblocks++;
        nchunks += block->nchunks;
        nfreechunks += block->nfree;
        totalspace += block->blksize;
        freespace += (block->endptr - block->freeptr);
    }
 
    if (printfunc)
    {
        char        stats_string[200];
 
        snprintf(stats_string, sizeof(stats_string),
                 "%zu total in %zu blocks (%zu chunks); %zu free (%zu chunks); %zu used",
                 totalspace, nblocks, nchunks, freespace,
                 nfreechunks, totalspace - freespace);
        printfunc(context, passthru, stats_string, print_to_stderr);
    }
 
    if (totals)
    {
        totals->nblocks += nblocks;
        totals->freechunks += nfreechunks;
        totals->totalspace += totalspace;
        totals->freespace += freespace;
    }
}
 
 
#ifdef MEMORY_CONTEXT_CHECKING
 
/*
 * GenerationCheck
 *      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
GenerationCheck(MemoryContext context)
{
    GenerationContext *gen = (GenerationContext *) context;
    const char *name = context->name;
    dlist_iter  iter;
    Size        total_allocated = 0;
 
    /* walk all blocks in this context */
    dlist_foreach(iter, &gen->blocks)
    {
        GenerationBlock *block = dlist_container(GenerationBlock, node, iter.cur);
        int         nfree,
                    nchunks;
        char       *ptr;
        bool        has_external_chunk = false;
 
        total_allocated += block->blksize;
 
        /*
         * nfree > nchunks is surely wrong.  Equality is allowed as the block
         * might completely empty if it's the freeblock.
         */
        if (block->nfree > block->nchunks)
            elog(WARNING, "problem in Generation %s: number of free chunks %d in block %p exceeds %d allocated",
                 name, block->nfree, block, block->nchunks);
 
        /* check block belongs to the correct context */
        if (block->context != gen)
            elog(WARNING, "problem in Generation %s: bogus context link in block %p",
                 name, block);
 
        /* Now walk through the chunks and count them. */
        nfree = 0;
        nchunks = 0;
        ptr = ((char *) block) + Generation_BLOCKHDRSZ;
 
        while (ptr < block->freeptr)
        {
            MemoryChunk *chunk = (MemoryChunk *) ptr;
            GenerationBlock *chunkblock;
            Size        chunksize;
 
            /* Allow access to the chunk header. */
            VALGRIND_MAKE_MEM_DEFINED(chunk, Generation_CHUNKHDRSZ);
 
            if (MemoryChunkIsExternal(chunk))
            {
                chunkblock = ExternalChunkGetBlock(chunk);
                chunksize = block->endptr - (char *) MemoryChunkGetPointer(chunk);
                has_external_chunk = true;
            }
            else
            {
                chunkblock = MemoryChunkGetBlock(chunk);
                chunksize = MemoryChunkGetValue(chunk);
            }
 
            /* move to the next chunk */
            ptr += (chunksize + Generation_CHUNKHDRSZ);
 
            nchunks += 1;
 
            /* chunks have both block and context pointers, so check both */
            if (chunkblock != block)
                elog(WARNING, "problem in Generation %s: bogus block link in block %p, chunk %p",
                     name, block, chunk);
 
 
            /* is chunk allocated? */
            if (chunk->requested_size != InvalidAllocSize)
            {
                /* now make sure the chunk size is correct */
                if (chunksize < chunk->requested_size ||
                    chunksize != MAXALIGN(chunksize))
                    elog(WARNING, "problem in Generation %s: bogus chunk size in block %p, chunk %p",
                         name, block, chunk);
 
                /* check sentinel */
                Assert(chunk->requested_size < chunksize);
                if (!sentinel_ok(chunk, Generation_CHUNKHDRSZ + chunk->requested_size))
                    elog(WARNING, "problem in Generation %s: detected write past chunk end in block %p, chunk %p",
                         name, block, chunk);
            }
            else
                nfree += 1;
 
            /* if chunk is allocated, disallow access to the chunk header */
            if (chunk->requested_size != InvalidAllocSize)
                VALGRIND_MAKE_MEM_NOACCESS(chunk, Generation_CHUNKHDRSZ);
        }
 
        /*
         * Make sure we got the expected number of allocated and free chunks
         * (as tracked in the block header).
         */
        if (nchunks != block->nchunks)
            elog(WARNING, "problem in Generation %s: number of allocated chunks %d in block %p does not match header %d",
                 name, nchunks, block, block->nchunks);
 
        if (nfree != block->nfree)
            elog(WARNING, "problem in Generation %s: number of free chunks %d in block %p does not match header %d",
                 name, nfree, block, block->nfree);
 
        if (has_external_chunk && nchunks > 1)
            elog(WARNING, "problem in Generation %s: external chunk on non-dedicated block %p",
                 name, block);
 
    }
 
    Assert(total_allocated == context->mem_allocated);
}
 
#endif                          /* MEMORY_CONTEXT_CHECKING */