Horton

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

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <dgrowleyml@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Julien Rouhaud <rjuju123@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Yura Sokolov <y.sokolov@postgrespro.ru>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2022-10-06T20:05:13Z
Lists: pgsql-hackers

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Here's a v2 incorporating discussed changes.

In reordering enum MemoryContextMethodID, I arranged to avoid using
000 and 111 as valid IDs, since those bit patterns will appear in
zeroed and wipe_mem'd memory respectively.  Those should probably be
more-or-less-permanent exclusions, so I added comments about it.

I also avoided using 001: based on my work with converting guc.c to use
palloc [1], it seems that pfree'ing a malloc-provided pointer is likely
to see 001 a lot, at least on 64-bit glibc platforms.  I've not stuck
my nose into the glibc sources to see how consistent that might be,
but it definitely recurred several times while I was chasing down
places needing adjustment in that patch.

I'm not sure if there are any platform-dependent reasons to avoid
other bit-patterns, but we do still have a little bit of flexibility
left here if anyone has candidates.

			regards, tom lane

[1] https://www.postgresql.org/message-id/2982579.1662416866%40sss.pgh.pa.us

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