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

On 8/30/22 02:45, David Rowley wrote:
> On Tue, 30 Aug 2022 at 03:39, Tomas Vondra
> <tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>> 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?
>
> Hi Tomas,
>
> I just wanted to check with you if you ran the full make check-world
> with this patch?
>
> I don't yet have a working ARM 32-bit environment to test, but on
> trying it with x86 32-bit and adjusting MAXIMUM_ALIGNOF to 8, I'm
> getting failures in test_decoding. Namely:
>
> test twophase ... FAILED 51 ms
> test twophase_stream ... FAILED 25 ms
>
> INSERT INTO test_prepared2 VALUES (5);
> SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL,
> NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
> +WARNING: problem in slab Change: detected write past chunk end in
> block 0x58b8ecf0, chunk 0x58b8ed08
> +WARNING: problem in slab Change: detected write past chunk end in
> block 0x58b8ecf0, chunk 0x58b8ed58
> +WARNING: problem in slab Change: detected write past chunk end in
> block 0x58b8ecf0, chunk 0x58b8eda8
> +WARNING: problem in slab Change: detected write past chunk end in
> block 0x58b8ecf0, chunk 0x58b8edf8
> +WARNING: problem in slab Change: detected write past chunk end in
> block 0x58b8ecf0, chunk 0x58b8ee48
>
> I think the existing sentinel check looks wrong:
>
> if (!sentinel_ok(chunk, slab->chunkSize))
>
> shouldn't that be passing the pointer rather than the chunk?
>

I agree the check in SlabCheck() looks wrong, as it's ignoring the chunk
header (unlike the other contexts).

But yeah, I ran "make check-world" and it passed just fine, so my only
explanation is that the check never actually executes because there's no
space for the sentinel thanks to alignment, and the tweak you did breaks
that. Strange ...

regards

--
Tomas Vondra
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