Horton

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

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Tomas Vondra <tomas.vondra@enterprisedb.com>, Amit Kapila <amit.kapila16@gmail.com>, David Rowley <dgrowleyml@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Yura Sokolov <y.sokolov@postgrespro.ru>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2022-08-29T15:57:45Z
Lists: pgsql-hackers 
Hi,

On 2022-08-29 11:43:14 -0400, Tom Lane wrote:
> Tomas Vondra <tomas.vondra@enterprisedb.com> writes:
> > 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.
> 
> Right, same conclusion I just came to.  But it's not a "pre-existing"
> bug, because sizeof(SlabBlock) *was* maxaligned until David added
> another field to it.
> 
> I think adding a padding field to SlabBlock would be a less messy
> solution than your patch.

That just seems to invite the same problem happening again later and it's
harder to ensure that the padding is correct across platforms.

Greetings,

Andres Freund

Commits

  1. Harden memory context allocators against bogus chunk pointers.

  2. Improve our ability to detect bogus pointers passed to pfree et al.

  3. Remove MemoryContextContains().

  4. Remove uses of MemoryContextContains in nodeAgg.c and nodeWindowAgg.c.

  5. Temporarily make MemoryContextContains return false

  6. Make MemoryContextContains work correctly again

  7. Make more effort to put a sentinel at the end of allocated memory

  8. Fix some possibly latent bugs in slab.c

  9. Various cleanups of the new memory context header code

  10. Revert "Add missing padding from MemoryChunk struct"

  11. Use MAXALIGN() in calculations using sizeof(SlabBlock)

  12. Add missing padding from MemoryChunk struct

  13. Improve performance of and reduce overheads of memory management