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) 2017-2022, 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"
 
 
#define Generation_BLOCKHDRSZ   MAXALIGN(sizeof(GenerationBlock))
#define Generation_CHUNKHDRSZ   sizeof(GenerationChunk)
 
#define Generation_CHUNK_FRACTION   8
 
typedef struct GenerationBlock GenerationBlock; /* forward reference */
typedef struct GenerationChunk GenerationChunk;
 
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 */
    Size        initBlockSize;  /* initial block size */
    Size        maxBlockSize;   /* maximum block size */
    Size        nextBlockSize;  /* next block size to allocate */
    Size        allocChunkLimit;    /* effective chunk size limit */
 
    GenerationBlock *block;     /* current (most recently allocated) block, or
                                 * NULL if we've just freed the most recent
                                 * block */
    GenerationBlock *freeblock; /* pointer to a block that's being recycled,
                                 * or NULL if there's no such block. */
    GenerationBlock *keeper;    /* keep this block over resets */
    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 GenerationChunks, which are
 *      the units requested by palloc() and freed by pfree().  GenerationChunks
 *      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 */
    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 */
};
 
/*
 * GenerationChunk
 *      The prefix of each piece of memory in a GenerationBlock
 *
 * Note: to meet the memory context APIs, the payload area of the chunk must
 * be maxaligned, and the "context" link must be immediately adjacent to the
 * payload area (cf. GetMemoryChunkContext).  We simplify matters for this
 * module by requiring sizeof(GenerationChunk) to be maxaligned, and then
 * we can ensure things work by adding any required alignment padding before
 * the pointer fields.  There is a static assertion below that the alignment
 * is done correctly.
 */
struct GenerationChunk
{
    /* size is always the size of the usable space in the chunk */
    Size        size;
#ifdef MEMORY_CONTEXT_CHECKING
    /* when debugging memory usage, also store actual requested size */
    /* this is zero in a free chunk */
    Size        requested_size;
 
#define GENERATIONCHUNK_RAWSIZE  (SIZEOF_SIZE_T * 2 + SIZEOF_VOID_P * 2)
#else
#define GENERATIONCHUNK_RAWSIZE  (SIZEOF_SIZE_T + SIZEOF_VOID_P * 2)
#endif                          /* MEMORY_CONTEXT_CHECKING */
 
    /* ensure proper alignment by adding padding if needed */
#if (GENERATIONCHUNK_RAWSIZE % MAXIMUM_ALIGNOF) != 0
    char        padding[MAXIMUM_ALIGNOF - GENERATIONCHUNK_RAWSIZE % MAXIMUM_ALIGNOF];
#endif
 
    GenerationBlock *block;     /* block owning this chunk */
    GenerationContext *context; /* owning context, or NULL if freed chunk */
    /* there must not be any padding to reach a MAXALIGN boundary here! */
};
 
/*
 * Only the "context" field should be accessed outside this module.
 * We keep the rest of an allocated chunk's header marked NOACCESS when using
 * valgrind.  But note that freed chunk headers are kept accessible, for
 * simplicity.
 */
#define GENERATIONCHUNK_PRIVATE_LEN offsetof(GenerationChunk, context)
 
/*
 * GenerationIsValid
 *      True iff set is valid allocation set.
 */
#define GenerationIsValid(set) PointerIsValid(set)
 
#define GenerationPointerGetChunk(ptr) \
    ((GenerationChunk *)(((char *)(ptr)) - Generation_CHUNKHDRSZ))
#define GenerationChunkGetPointer(chk) \
    ((GenerationPointer *)(((char *)(chk)) + Generation_CHUNKHDRSZ))
 
/* Inlined helper functions */
static inline void GenerationBlockInit(GenerationBlock *block, Size blksize);
static inline bool GenerationBlockIsEmpty(GenerationBlock *block);
static inline void GenerationBlockMarkEmpty(GenerationBlock *block);
static inline Size GenerationBlockFreeBytes(GenerationBlock *block);
static inline void GenerationBlockFree(GenerationContext *set,
                                       GenerationBlock *block);
 
/*
 * These functions implement the MemoryContext API for Generation contexts.
 */
static void *GenerationAlloc(MemoryContext context, Size size);
static void GenerationFree(MemoryContext context, void *pointer);
static void *GenerationRealloc(MemoryContext context, void *pointer, Size size);
static void GenerationReset(MemoryContext context);
static void GenerationDelete(MemoryContext context);
static Size GenerationGetChunkSpace(MemoryContext context, void *pointer);
static bool GenerationIsEmpty(MemoryContext context);
static void GenerationStats(MemoryContext context,
                            MemoryStatsPrintFunc printfunc, void *passthru,
                            MemoryContextCounters *totals,
                            bool print_to_stderr);
 
#ifdef MEMORY_CONTEXT_CHECKING
static void GenerationCheck(MemoryContext context);
#endif
 
/*
 * This is the virtual function table for Generation contexts.
 */
static const MemoryContextMethods GenerationMethods = {
    GenerationAlloc,
    GenerationFree,
    GenerationRealloc,
    GenerationReset,
    GenerationDelete,
    GenerationGetChunkSpace,
    GenerationIsEmpty,
    GenerationStats
#ifdef MEMORY_CONTEXT_CHECKING
    ,GenerationCheck
#endif
};
 
 
/*
 * 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;
 
    /* Assert we padded GenerationChunk properly */
    StaticAssertStmt(Generation_CHUNKHDRSZ == MAXALIGN(Generation_CHUNKHDRSZ),
                     "sizeof(GenerationChunk) is not maxaligned");
    StaticAssertStmt(offsetof(GenerationChunk, context) + sizeof(MemoryContext) ==
                     Generation_CHUNKHDRSZ,
                     "padding calculation in GenerationChunk is wrong");
 
    /*
     * First, validate allocation parameters.  Asserts seem sufficient because
     * nobody varies their parameters at runtime.  We somewhat arbitrarily
     * enforce a minimum 1K block size.
     */
    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));
 
    /* 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 = (GenerationBlock *) (((char *) set) + MAXALIGN(sizeof(GenerationContext)));
    /* determine the block size and initialize it */
    firstBlockSize = allocSize - MAXALIGN(sizeof(GenerationContext));
    GenerationBlockInit(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;
 
    /* Mark block as not to be released at reset time */
    set->keeper = block;
 
    /* Fill in GenerationContext-specific header fields */
    set->initBlockSize = initBlockSize;
    set->maxBlockSize = maxBlockSize;
    set->nextBlockSize = initBlockSize;
 
    /*
     * Compute the allocation chunk size limit for this context.
     *
     * Follows similar ideas as AllocSet, see aset.c for details ...
     */
    set->allocChunkLimit = maxBlockSize;
    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,
                        &GenerationMethods,
                        parent,
                        name);
 
    ((MemoryContext) set)->mem_allocated = firstBlockSize;
 
    return (MemoryContext) set;
}
 
/*
 * GenerationReset
 *      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.
 */
static void
GenerationReset(MemoryContext context)
{
    GenerationContext *set = (GenerationContext *) context;
    dlist_mutable_iter miter;
 
    AssertArg(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 (block == set->keeper)
            GenerationBlockMarkEmpty(block);
        else
            GenerationBlockFree(set, block);
    }
 
    /* set it so new allocations to make use of the keeper block */
    set->block = set->keeper;
 
    /* 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.
 */
static void
GenerationDelete(MemoryContext context)
{
    /* Reset to release all releasable GenerationBlocks */
    GenerationReset(context);
    /* And free the context header and keeper block */
    free(context);
}
 
/*
 * GenerationAlloc
 *      Returns pointer to allocated memory of given size or NULL if
 *      request could not be completed; memory is added to the set.
 *
 * 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!
 */
static void *
GenerationAlloc(MemoryContext context, Size size)
{
    GenerationContext *set = (GenerationContext *) context;
    GenerationBlock *block;
    GenerationChunk *chunk;
    Size        chunk_size = MAXALIGN(size);
    Size        required_size = chunk_size + Generation_CHUNKHDRSZ;
 
    /* is it an over-sized chunk? if yes, allocate special block */
    if (chunk_size > set->allocChunkLimit)
    {
        Size        blksize = required_size + Generation_BLOCKHDRSZ;
 
        block = (GenerationBlock *) malloc(blksize);
        if (block == NULL)
            return NULL;
 
        context->mem_allocated += blksize;
 
        /* block with a single (used) chunk */
        block->blksize = blksize;
        block->nchunks = 1;
        block->nfree = 0;
 
        /* the block is completely full */
        block->freeptr = block->endptr = ((char *) block) + blksize;
 
        chunk = (GenerationChunk *) (((char *) block) + Generation_BLOCKHDRSZ);
        chunk->block = block;
        chunk->context = set;
        chunk->size = chunk_size;
 
#ifdef MEMORY_CONTEXT_CHECKING
        chunk->requested_size = size;
        /* set mark to catch clobber of "unused" space */
        if (size < chunk_size)
            set_sentinel(GenerationChunkGetPointer(chunk), size);
#endif
#ifdef RANDOMIZE_ALLOCATED_MEMORY
        /* fill the allocated space with junk */
        randomize_mem((char *) GenerationChunkGetPointer(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 *) GenerationChunkGetPointer(chunk) + size,
                                   chunk_size - size);
 
        /* Disallow external access to private part of chunk header. */
        VALGRIND_MAKE_MEM_NOACCESS(chunk, GENERATIONCHUNK_PRIVATE_LEN);
 
        return GenerationChunkGetPointer(chunk);
    }
 
    /*
     * 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 freeblock is empty and has enough space.
     * If not, we'll also try the same using the keeper block.  The keeper
     * block may have become empty and we have no other way to reuse it again
     * if we don't try to use it explicitly here.
     *
     * We don't want to start filling the freeblock before the current block
     * is full, otherwise we may cause fragmentation in FIFO type workloads.
     * We only switch to using the freeblock or keeper block if those blocks
     * are completely empty.  If we didn't do that we could end up fragmenting
     * consecutive allocations over multiple blocks which would be a problem
     * that would compound over time.
     */
    block = set->block;
 
    if (block == NULL ||
        GenerationBlockFreeBytes(block) < required_size)
    {
        Size        blksize;
        GenerationBlock *freeblock = set->freeblock;
 
        if (freeblock != NULL &&
            GenerationBlockIsEmpty(freeblock) &&
            GenerationBlockFreeBytes(freeblock) >= required_size)
        {
            block = freeblock;
 
            /*
             * Zero out the freeblock as we'll set this to the current block
             * below
             */
            set->freeblock = NULL;
        }
        else if (GenerationBlockIsEmpty(set->keeper) &&
                 GenerationBlockFreeBytes(set->keeper) >= required_size)
        {
            block = set->keeper;
        }
        else
        {
            /*
             * 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 a block hdr too, so add that to the required size */
            required_size += 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 NULL;
 
            context->mem_allocated += blksize;
 
            /* initialize the new block */
            GenerationBlockInit(block, blksize);
 
            /* add it to the doubly-linked list of blocks */
            dlist_push_head(&set->blocks, &block->node);
 
            /* Zero out the freeblock in case it's become full */
            set->freeblock = NULL;
        }
 
        /* and also use it as the current allocation block */
        set->block = block;
    }
 
    /* we're supposed to have a block with enough free space now */
    Assert(block != NULL);
    Assert((block->endptr - block->freeptr) >= Generation_CHUNKHDRSZ + chunk_size);
 
    chunk = (GenerationChunk *) block->freeptr;
 
    /* 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);
 
    chunk->block = block;
    chunk->context = set;
    chunk->size = chunk_size;
 
#ifdef MEMORY_CONTEXT_CHECKING
    chunk->requested_size = size;
    /* set mark to catch clobber of "unused" space */
    if (size < chunk->size)
        set_sentinel(GenerationChunkGetPointer(chunk), size);
#endif
#ifdef RANDOMIZE_ALLOCATED_MEMORY
    /* fill the allocated space with junk */
    randomize_mem((char *) GenerationChunkGetPointer(chunk), size);
#endif
 
    /* Ensure any padding bytes are marked NOACCESS. */
    VALGRIND_MAKE_MEM_NOACCESS((char *) GenerationChunkGetPointer(chunk) + size,
                               chunk_size - size);
 
    /* Disallow external access to private part of chunk header. */
    VALGRIND_MAKE_MEM_NOACCESS(chunk, GENERATIONCHUNK_PRIVATE_LEN);
 
    return GenerationChunkGetPointer(chunk);
}
 
/*
 * GenerationBlockInit
 *      Initializes 'block' assuming 'blksize'.  Does not update the context's
 *      mem_allocated field.
 */
static inline void
GenerationBlockInit(GenerationBlock *block, Size blksize)
{
    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);
}
 
/*
 * GenerationBlockIsEmpty
 *      Returns true iif 'block' contains no chunks
 */
static inline bool
GenerationBlockIsEmpty(GenerationBlock *block)
{
    return (block->nchunks == 0);
}
 
/*
 * 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(block != set->keeper);
    /* 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 if all chunks in the block
 *      are now free then discard the block.
 */
static void
GenerationFree(MemoryContext context, void *pointer)
{
    GenerationContext *set = (GenerationContext *) context;
    GenerationChunk *chunk = GenerationPointerGetChunk(pointer);
    GenerationBlock *block;
 
    /* Allow access to private part of chunk header. */
    VALGRIND_MAKE_MEM_DEFINED(chunk, GENERATIONCHUNK_PRIVATE_LEN);
 
    block = chunk->block;
 
#ifdef MEMORY_CONTEXT_CHECKING
    /* Test for someone scribbling on unused space in chunk */
    if (chunk->requested_size < chunk->size)
        if (!sentinel_ok(pointer, chunk->requested_size))
            elog(WARNING, "detected write past chunk end in %s %p",
                 ((MemoryContext) set)->name, chunk);
#endif
 
#ifdef CLOBBER_FREED_MEMORY
    wipe_mem(pointer, chunk->size);
#endif
 
    /* Reset context to NULL in freed chunks */
    chunk->context = NULL;
 
#ifdef MEMORY_CONTEXT_CHECKING
    /* Reset requested_size to 0 in freed chunks */
    chunk->requested_size = 0;
#endif
 
    block->nfree += 1;
 
    Assert(block->nchunks > 0);
    Assert(block->nfree <= block->nchunks);
 
    /* If there are still allocated chunks in the block, we're done. */
    if (block->nfree < block->nchunks)
        return;
 
    /* Don't try to free the keeper block, just mark it empty */
    if (block == set->keeper)
    {
        GenerationBlockMarkEmpty(block);
        return;
    }
 
    /*
     * If there is no freeblock set or if this is the freeblock then instead
     * of freeing this memory, we keep it around so that new allocations have
     * the option of recycling it.
     */
    if (set->freeblock == NULL || set->freeblock == block)
    {
        /* XXX should we only recycle maxBlockSize sized blocks? */
        set->freeblock = block;
        GenerationBlockMarkEmpty(block);
        return;
    }
 
    /* Also make sure the block is not marked as the current block. */
    if (set->block == block)
        set->block = NULL;
 
    /*
     * The block is empty, so let's get rid of it. First remove it from the
     * list of blocks, then return it to malloc().
     */
    dlist_delete(&block->node);
 
    context->mem_allocated -= block->blksize;
    free(block);
}
 
/*
 * 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.
 */
static void *
GenerationRealloc(MemoryContext context, void *pointer, Size size)
{
    GenerationContext *set = (GenerationContext *) context;
    GenerationChunk *chunk = GenerationPointerGetChunk(pointer);
    GenerationPointer newPointer;
    Size        oldsize;
 
    /* Allow access to private part of chunk header. */
    VALGRIND_MAKE_MEM_DEFINED(chunk, GENERATIONCHUNK_PRIVATE_LEN);
 
    oldsize = chunk->size;
 
#ifdef MEMORY_CONTEXT_CHECKING
    /* Test for someone scribbling on unused space in chunk */
    if (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 is >= the new size.  (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()?
     */
    if (oldsize >= size)
    {
#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 */
        if (size < oldsize)
            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 external access to private part of chunk header. */
        VALGRIND_MAKE_MEM_NOACCESS(chunk, GENERATIONCHUNK_PRIVATE_LEN);
 
        return pointer;
    }
 
    /* allocate new chunk */
    newPointer = GenerationAlloc((MemoryContext) set, size);
 
    /* leave immediately if request was not completed */
    if (newPointer == NULL)
    {
        /* Disallow external access to private part of chunk header. */
        VALGRIND_MAKE_MEM_NOACCESS(chunk, GENERATIONCHUNK_PRIVATE_LEN);
        return NULL;
    }
 
    /*
     * 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((MemoryContext) set, pointer);
 
    return newPointer;
}
 
/*
 * GenerationGetChunkSpace
 *      Given a currently-allocated chunk, determine the total space
 *      it occupies (including all memory-allocation overhead).
 */
static Size
GenerationGetChunkSpace(MemoryContext context, void *pointer)
{
    GenerationChunk *chunk = GenerationPointerGetChunk(pointer);
    Size        result;
 
    VALGRIND_MAKE_MEM_DEFINED(chunk, GENERATIONCHUNK_PRIVATE_LEN);
    result = chunk->size + Generation_CHUNKHDRSZ;
    VALGRIND_MAKE_MEM_NOACCESS(chunk, GENERATIONCHUNK_PRIVATE_LEN);
    return result;
}
 
/*
 * GenerationIsEmpty
 *      Is a GenerationContext empty of any allocated space?
 */
static bool
GenerationIsEmpty(MemoryContext context)
{
    GenerationContext *set = (GenerationContext *) context;
    dlist_iter  iter;
 
    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).
 */
static 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;
 
    /* 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!
 */
static 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;
 
        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);
 
        /* Now walk through the chunks and count them. */
        nfree = 0;
        nchunks = 0;
        ptr = ((char *) block) + Generation_BLOCKHDRSZ;
 
        while (ptr < block->freeptr)
        {
            GenerationChunk *chunk = (GenerationChunk *) ptr;
 
            /* Allow access to private part of chunk header. */
            VALGRIND_MAKE_MEM_DEFINED(chunk, GENERATIONCHUNK_PRIVATE_LEN);
 
            /* move to the next chunk */
            ptr += (chunk->size + Generation_CHUNKHDRSZ);
 
            nchunks += 1;
 
            /* chunks have both block and context pointers, so check both */
            if (chunk->block != block)
                elog(WARNING, "problem in Generation %s: bogus block link in block %p, chunk %p",
                     name, block, chunk);
 
            /*
             * Check for valid context pointer.  Note this is an incomplete
             * test, since palloc(0) produces an allocated chunk with
             * requested_size == 0.
             */
            if ((chunk->requested_size > 0 && chunk->context != gen) ||
                (chunk->context != gen && chunk->context != NULL))
                elog(WARNING, "problem in Generation %s: bogus context link in block %p, chunk %p",
                     name, block, chunk);
 
            /* now make sure the chunk size is correct */
            if (chunk->size < chunk->requested_size ||
                chunk->size != MAXALIGN(chunk->size))
                elog(WARNING, "problem in Generation %s: bogus chunk size in block %p, chunk %p",
                     name, block, chunk);
 
            /* is chunk allocated? */
            if (chunk->context != NULL)
            {
                /* check sentinel, but only in allocated blocks */
                if (chunk->requested_size < chunk->size &&
                    !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 external access to private part
             * of chunk header.
             */
            if (chunk->context != NULL)
                VALGRIND_MAKE_MEM_NOACCESS(chunk, GENERATIONCHUNK_PRIVATE_LEN);
        }
 
        /*
         * 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);
    }
 
    Assert(total_allocated == context->mem_allocated);
}
 
#endif                          /* MEMORY_CONTEXT_CHECKING */