diff options
| author | Tom Lane <tgl@sss.pgh.pa.us> | 2017-11-24 15:50:22 -0500 |
|---|---|---|
| committer | Tom Lane <tgl@sss.pgh.pa.us> | 2017-11-24 15:50:22 -0500 |
| commit | f65d21b258085bdc8ef2cc282ab1ff12da9c595c (patch) | |
| tree | 1084f9d500d2c2f098108e69fe37738e9096c139 /src/backend/utils/mmgr/slab.c | |
| parent | cc3c4af4a948e5c5be22afe769bab41235c574e5 (diff) | |
| download | postgresql-f65d21b258085bdc8ef2cc282ab1ff12da9c595c.tar.gz postgresql-f65d21b258085bdc8ef2cc282ab1ff12da9c595c.zip | |
Mostly-cosmetic improvements in memory chunk header alignment coding.
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
Diffstat (limited to 'src/backend/utils/mmgr/slab.c')
| -rw-r--r-- | src/backend/utils/mmgr/slab.c | 21 |
1 files changed, 15 insertions, 6 deletions
diff --git a/src/backend/utils/mmgr/slab.c b/src/backend/utils/mmgr/slab.c index 35de6b6d82a..ee2175278d2 100644 --- a/src/backend/utils/mmgr/slab.c +++ b/src/backend/utils/mmgr/slab.c @@ -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 |