Re: Reducing the chunk header sizes on all memory context types

On 8/29/22 17:27, Tomas Vondra wrote:
> ...
>
> I suspect it's a pre-existing bug in Slab allocator, because it does this:
>
> #define SlabBlockGetChunk(slab, block, idx) \
> ((MemoryChunk *) ((char *) (block) + sizeof(SlabBlock) \
> + (idx * slab->fullChunkSize)))
>
> and SlabBlock is only 20B, i.e. not a multiple of 8B. Which would mean
> that even if we allocate block and size the chunks carefully (with all
> the MAXALIGN things), we ultimately slice the block incorrectly.
>

The attached patch seems to fix the issue for me - at least it seems
like that. This probably will need to get backpatched, I guess. Maybe we
should add an assert to MemoryChunkGetPointer to check alignment?

> This would explain the 4B difference I reported before, I think. But I'm
> just as astonished we got this far in the tests - regular regression
> tests don't do much logical decoding, and we only use slab for changes,
> but I see the failure in 006 test in src/test/recovery, so the first
> five completed fine.
>

I got confused - the first 5 tests in src/test/recovery don't do any
logical decoding, so it's not surprising it's the 006 that fails.

regards

--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company