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Mostly-cosmetic improvements in memory chunk header alignment coding.

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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
This commit is contained in:
Tom Lane
2017-11-24 15:50:22 -05:00
parent cc3c4af4a9
commit f65d21b258
3 changed files with 62 additions and 42 deletions
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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)