Adding upstream version 14.5.upstream/14.5upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 832 insertions, 0 deletions

diff --git a/src/backend/utils/mmgr/generation.c b/src/backend/utils/mmgr/generation.c
new file mode 100644
index 0000000..584cd61
--- /dev/null
+++ b/src/backend/utils/mmgr/generation.c
@@ -0,0 +1,832 @@
+/*-------------------------------------------------------------------------
+ *
+ * generation.c
+ *	  Generational allocator definitions.
+ *
+ * Generation is a custom MemoryContext implementation designed for cases of
+ * chunks with similar lifespan.
+ *
+ * Portions Copyright (c) 2017-2021, 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. Freed chunks are not reused, and once all
+ *	chunks in a block are freed, the whole block is thrown away. When the
+ *	chunks allocated in the same block have similar lifespan, this works
+ *	very well and is very cheap.
+ *
+ *	The current implementation only uses a fixed block size - maybe it should
+ *	adapt a min/max block size range, and grow the blocks automatically.
+ *	It already uses dedicated blocks for oversized chunks.
+ *
+ *	XXX It might be possible to improve this by keeping a small freelist for
+ *	only a small number of recent blocks, but it's not clear it's worth the
+ *	additional complexity.
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "lib/ilist.h"
+#include "utils/memdebug.h"
+#include "utils/memutils.h"
+
+
+#define Generation_BLOCKHDRSZ	MAXALIGN(sizeof(GenerationBlock))
+#define Generation_CHUNKHDRSZ	sizeof(GenerationChunk)
+
+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		blockSize;		/* standard block size */
+
+	GenerationBlock *block;		/* current (most recently allocated) 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 one 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 is 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))
+
+/*
+ * 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)
+ * blockSize: generation block size
+ */
+MemoryContext
+GenerationContextCreate(MemoryContext parent,
+						const char *name,
+						Size blockSize)
+{
+	GenerationContext *set;
+
+	/* 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.  (If we're going to throw an
+	 * error, we should do so before the context is created, not after.)  We
+	 * somewhat arbitrarily enforce a minimum 1K block size, mostly because
+	 * that's what AllocSet does.
+	 */
+	if (blockSize != MAXALIGN(blockSize) ||
+		blockSize < 1024 ||
+		!AllocHugeSizeIsValid(blockSize))
+		elog(ERROR, "invalid blockSize for memory context: %zu",
+			 blockSize);
+
+	/*
+	 * Allocate the context header.  Unlike aset.c, we never try to combine
+	 * this with the first regular block, since that would prevent us from
+	 * freeing the first generation of allocations.
+	 */
+
+	set = (GenerationContext *) malloc(MAXALIGN(sizeof(GenerationContext)));
+	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.
+	 */
+
+	/* Fill in GenerationContext-specific header fields */
+	set->blockSize = blockSize;
+	set->block = NULL;
+	dlist_init(&set->blocks);
+
+	/* Finally, do the type-independent part of context creation */
+	MemoryContextCreate((MemoryContext) set,
+						T_GenerationContext,
+						&GenerationMethods,
+						parent,
+						name);
+
+	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
+
+	dlist_foreach_modify(miter, &set->blocks)
+	{
+		GenerationBlock *block = dlist_container(GenerationBlock, node, miter.cur);
+
+		dlist_delete(miter.cur);
+
+		context->mem_allocated -= block->blksize;
+
+#ifdef CLOBBER_FREED_MEMORY
+		wipe_mem(block, block->blksize);
+#endif
+
+		free(block);
+	}
+
+	set->block = NULL;
+
+	Assert(dlist_is_empty(&set->blocks));
+}
+
+/*
+ * GenerationDelete
+ *		Free all memory which is allocated in the given context.
+ */
+static void
+GenerationDelete(MemoryContext context)
+{
+	/* Reset to release all the GenerationBlocks */
+	GenerationReset(context);
+	/* And free the context header */
+	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);
+
+	/* is it an over-sized chunk? if yes, allocate special block */
+	if (chunk_size > set->blockSize / 8)
+	{
+		Size		blksize = chunk_size + Generation_BLOCKHDRSZ + Generation_CHUNKHDRSZ;
+
+		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 over-sized chunk. Is there enough space in the current block? If
+	 * not, allocate a new "regular" block.
+	 */
+	block = set->block;
+
+	if ((block == NULL) ||
+		(block->endptr - block->freeptr) < Generation_CHUNKHDRSZ + chunk_size)
+	{
+		Size		blksize = set->blockSize;
+
+		block = (GenerationBlock *) malloc(blksize);
+
+		if (block == NULL)
+			return NULL;
+
+		context->mem_allocated += 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);
+
+		/* add it to the doubly-linked list of blocks */
+		dlist_push_head(&set->blocks, &block->node);
+
+		/* 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);
+}
+
+/*
+ * 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;
+
+	/*
+	 * 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);
+
+	/* Also make sure the block is not marked as the current block. */
+	if (set->block == block)
+		set->block = NULL;
+
+	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;
+
+	return dlist_is_empty(&set->blocks);
+}
+
+/*
+ * 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 %zd blocks (%zd chunks); %zu free (%zd 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, and we don't expect to see
+		 * equality either, because such a block should have gotten freed.
+		 */
+		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 */