On 02/27/2017 06:42 PM, Andres Freund wrote:
On 2017-02-27 12:27:48 -0500, Tom Lane wrote:Andres Freund <and...@anarazel.de> writes:The best theory I have so far that I have is that slab.c's idea of StandardChunkHeader's size doesn't match what mcxt.c think it is (because slab.c simply embeds StandardChunkHeader, but mcxt uses MAXALIGN(sizeof(StandardChunkHeader))). That's not good, but I don't quite see how that'd cause the issue, since StandardChunkHeader's size should always be properly sized.Uh, wrong. On a 32-bit machine with debug enabled, StandardChunkHeader will contain 3 4-byte fields. However, there are some such machines on which MAXALIGN is 8. For example, looking at termite's configure output: checking size of void *... 4 checking size of size_t... 4 checking size of long... 4 checking alignment of short... 2 checking alignment of int... 4 checking alignment of long... 4 checking alignment of long long int... 8 checking alignment of double... 8 axolotl's output looks similar. I expect my old HPPA dinosaur will show the failure as well.Yea, I hadn't yet realized when writing that that termite actually, despite running on ppc64, compiles a 32bit postgres. Will thus duplicate StandardChunkHeader's contents in to slab.c :( - I don't see an easy way around that...
I've tried this - essentially copying the StandardChunkHeader's contents into SlabChunk, but that does not seem to do the trick, sadly. Per pahole, the structures then (at least on armv7l) look like this:
struct SlabChunk {
void * block; /* 0 4 */
MemoryContext context; /* 4 4 */
Size size; /* 8 4 */
Size requested_size; /* 12 4 */
/* size: 16, cachelines: 1, members: 4 */
/* last cacheline: 16 bytes */
};
struct StandardChunkHeader {
MemoryContext context; /* 0 4 */
Size size; /* 4 4 */
Size requested_size; /* 8 4 */
/* size: 12, cachelines: 1, members: 3 */
/* last cacheline: 12 bytes */
};
So SlabChunk happens to be perfectly aligned (MAXIMUM_ALIGNOF=8), and so
pfree() grabs the block pointer but thinks it's the context :-(
Not sure what to do about this - the only thing I can think about is splitting SlabChunk into two separate structures, and align them independently.
The attached patch does that - it probably needs a bit more work on the comments to make it commit-ready, but it fixes the test_deconding tests on the rpi3 board I'm using for testing.
regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
slab-fix.patch
Description: binary/octet-stream
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