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Mostly-cosmetic improvements in memory chunk header alignment coding.

Browse Source 
Add commentary about what we're doing and why.  Apply the method used for
padding in GenerationChunk to AllocChunkData, replacing the rather ad-hoc
solution used in commit 7e3aa03b4.  Reorder fields in GenerationChunk so
that the padding calculation will work even if sizeof(size_t) is different
from sizeof(void *) --- likely that will never happen, but we don't need
the assumption if we do it like this.  Improve static assertions about
alignment.

In passing, fix a couple of oversights in the "large chunk" path in
GenerationAlloc().

Discussion: https://postgr.es/m/E1eHa4J-0006hI-Q8@gemulon.postgresql.org
pull/31/head^2
Tom Lane 8 years ago
parent cc3c4af4a9
commit f65d21b258
3 changed files with 62 additions and 42 deletions
Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL
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  1. 25
      src/backend/utils/mmgr/aset.c
  2. 58
      src/backend/utils/mmgr/generation.c
  3. 21
      src/backend/utils/mmgr/slab.c

25
src/backend/utils/mmgr/aset.c
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@ -157,6 +157,14 @@ typedef struct AllocBlockData
/*
* AllocChunk
* The prefix of each piece of memory in an AllocBlock
*
* Note: to meet the memory context APIs, the payload area of the chunk must
* be maxaligned, and the "aset" link must be immediately adjacent to the
* payload area (cf. GetMemoryChunkContext). We simplify matters for this
* module by requiring sizeof(AllocChunkData) to be maxaligned, and then
* we can ensure things work by adding any required alignment padding before
* the "aset" field. There is a static assertion below that the alignment
* is done correctly.
*/
typedef struct AllocChunkData
{
@ -166,15 +174,19 @@ typedef struct AllocChunkData
/* when debugging memory usage, also store actual requested size */
/* this is zero in a free chunk */
Size requested_size;
#if MAXIMUM_ALIGNOF > 4 && SIZEOF_VOID_P == 4
Size padding;
#endif
#define ALLOCCHUNK_RAWSIZE (SIZEOF_SIZE_T * 2 + SIZEOF_VOID_P)
#else
#define ALLOCCHUNK_RAWSIZE (SIZEOF_SIZE_T + SIZEOF_VOID_P)
#endif /* MEMORY_CONTEXT_CHECKING */
/* ensure proper alignment by adding padding if needed */
#if (ALLOCCHUNK_RAWSIZE % MAXIMUM_ALIGNOF) != 0
char padding[MAXIMUM_ALIGNOF - ALLOCCHUNK_RAWSIZE % MAXIMUM_ALIGNOF];
#endif
/* aset is the owning aset if allocated, or the freelist link if free */
void *aset;
/* there must not be any padding to reach a MAXALIGN boundary here! */
} AllocChunkData;
@ -327,8 +339,11 @@ AllocSetContextCreate(MemoryContext parent,
{
AllocSet set;
/* Assert we padded AllocChunkData properly */
StaticAssertStmt(ALLOC_CHUNKHDRSZ == MAXALIGN(ALLOC_CHUNKHDRSZ),
"sizeof(AllocChunkData) is not maxaligned");
StaticAssertStmt(offsetof(AllocChunkData, aset) + sizeof(MemoryContext) ==
MAXALIGN(sizeof(AllocChunkData)),
ALLOC_CHUNKHDRSZ,
"padding calculation in AllocChunkData is wrong");
/*

58
src/backend/utils/mmgr/generation.c
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@ -46,19 +46,6 @@
#define Generation_BLOCKHDRSZ MAXALIGN(sizeof(GenerationBlock))
#define Generation_CHUNKHDRSZ sizeof(GenerationChunk)
/* Portion of Generation_CHUNKHDRSZ examined outside generation.c. */
#define Generation_CHUNK_PUBLIC \
(offsetof(GenerationChunk, size) + sizeof(Size))
/* Portion of Generation_CHUNKHDRSZ excluding trailing padding. */
#ifdef MEMORY_CONTEXT_CHECKING
#define Generation_CHUNK_USED \
(offsetof(GenerationChunk, requested_size) + sizeof(Size))
#else
#define Generation_CHUNK_USED \
(offsetof(GenerationChunk, size) + sizeof(Size))
#endif
typedef struct GenerationBlock GenerationBlock; /* forward reference */
typedef struct GenerationChunk GenerationChunk;
@ -103,28 +90,35 @@ struct GenerationBlock
/*
* 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
{
/* block owning this chunk */
void *block;
/* 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_VOID_P * 2 + SIZEOF_SIZE_T * 2)
#define GENERATIONCHUNK_RAWSIZE (SIZEOF_SIZE_T * 2 + SIZEOF_VOID_P * 2)
#else
#define GENERATIONCHUNK_RAWSIZE (SIZEOF_VOID_P * 2 + SIZEOF_SIZE_T)
#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)];
char padding[MAXIMUM_ALIGNOF - GENERATIONCHUNK_RAWSIZE % MAXIMUM_ALIGNOF];
#endif
GenerationBlock *block; /* block owning this chunk */
GenerationContext *context; /* owning context */
/* there must not be any padding to reach a MAXALIGN boundary here! */
};
@ -210,8 +204,11 @@ GenerationContextCreate(MemoryContext parent,
{
GenerationContext *set;
/* Assert we padded GenerationChunk properly */
StaticAssertStmt(Generation_CHUNKHDRSZ == MAXALIGN(Generation_CHUNKHDRSZ),
"sizeof(GenerationChunk) is not maxaligned");
StaticAssertStmt(offsetof(GenerationChunk, context) + sizeof(MemoryContext) ==
MAXALIGN(sizeof(GenerationChunk)),
Generation_CHUNKHDRSZ,
"padding calculation in GenerationChunk is wrong");
/*
@ -318,7 +315,6 @@ 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 */
@ -338,6 +334,7 @@ GenerationAlloc(MemoryContext context, Size size)
block->freeptr = block->endptr = ((char *) block) + blksize;
chunk = (GenerationChunk *) (((char *) block) + Generation_BLOCKHDRSZ);
chunk->block = block;
chunk->context = set;
chunk->size = chunk_size;
@ -356,17 +353,16 @@ GenerationAlloc(MemoryContext context, Size size)
/* add the block to the list of allocated blocks */
dlist_push_head(&set->blocks, &block->node);
GenerationAllocInfo(set, chunk);
/*
* Chunk header public fields remain DEFINED. The requested
* allocation itself can be NOACCESS or UNDEFINED; our caller will
* soon make it UNDEFINED. Make extra space at the end of the chunk,
* if any, NOACCESS.
* Chunk header fields remain DEFINED. The requested allocation
* itself can be NOACCESS or UNDEFINED; our caller will soon make it
* UNDEFINED. Make extra space at the end of the chunk, if any,
* NOACCESS.
*/
VALGRIND_MAKE_MEM_NOACCESS((char *) chunk + Generation_CHUNK_PUBLIC,
chunk_size + Generation_CHUNKHDRSZ - Generation_CHUNK_PUBLIC);
VALGRIND_MAKE_MEM_NOACCESS((char *) chunk + Generation_CHUNKHDRSZ + size,
chunk_size - size);
GenerationAllocInfo(set, chunk);
return GenerationChunkGetPointer(chunk);
}
@ -442,8 +438,8 @@ GenerationAlloc(MemoryContext context, Size size)
/*
* GenerationFree
* Update number of chunks on the block, and if all chunks on the block
* are freeed then discard the block.
* 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)

21
src/backend/utils/mmgr/slab.c
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@ -91,12 +91,18 @@ typedef struct SlabBlock
/*
* SlabChunk
* The prefix of each piece of memory in an SlabBlock
* The prefix of each piece of memory in a SlabBlock
*
* Note: to meet the memory context APIs, the payload area of the chunk must
* be maxaligned, and the "slab" link must be immediately adjacent to the
* payload area (cf. GetMemoryChunkContext). Since we support no machines on
* which MAXALIGN is more than twice sizeof(void *), this happens without any
* special hacking in this struct declaration. But there is a static
* assertion below that the alignment is done correctly.
*/
typedef struct SlabChunk
{
/* block owning this chunk */
void *block;
SlabBlock *block; /* block owning this chunk */
SlabContext *slab; /* owning context */
/* there must not be any padding to reach a MAXALIGN boundary here! */
} SlabChunk;
@ -190,8 +196,11 @@ SlabContextCreate(MemoryContext parent,
Size freelistSize;
SlabContext *slab;
/* Assert we padded SlabChunk properly */
StaticAssertStmt(sizeof(SlabChunk) == MAXALIGN(sizeof(SlabChunk)),
"sizeof(SlabChunk) is not maxaligned");
StaticAssertStmt(offsetof(SlabChunk, slab) + sizeof(MemoryContext) ==
MAXALIGN(sizeof(SlabChunk)),
sizeof(SlabChunk),
"padding calculation in SlabChunk is wrong");
/* Make sure the linked list node fits inside a freed chunk */
@ -199,7 +208,7 @@ SlabContextCreate(MemoryContext parent,
chunkSize = sizeof(int);
/* chunk, including SLAB header (both addresses nicely aligned) */
fullChunkSize = MAXALIGN(sizeof(SlabChunk) + MAXALIGN(chunkSize));
fullChunkSize = sizeof(SlabChunk) + MAXALIGN(chunkSize);
/* Make sure the block can store at least one chunk. */
if (blockSize - sizeof(SlabBlock) < fullChunkSize)
@ -443,7 +452,7 @@ SlabAlloc(MemoryContext context, Size size)
/* Prepare to initialize the chunk header. */
VALGRIND_MAKE_MEM_UNDEFINED(chunk, sizeof(SlabChunk));
chunk->block = (void *) block;
chunk->block = block;
chunk->slab = slab;
#ifdef MEMORY_CONTEXT_CHECKING

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