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: 2022-10-06T21:57:01Z
Lists: pgsql-hackers 
I wrote:
> 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 poked at this some more by creating a function that intentionally
does pfree(malloc(N)) for various values of N.

RHEL8, x86_64: the low-order nibble of the header is consistently 0001.

macOS 12.6, arm64: the low-order nibble is consistently 0000.

FreeBSD 13.0, arm64: Usually the low-order nibble is 0000 or 1111,
but for some smaller values of N it sometimes comes up as 0010.

NetBSD 9.2, amd64: results similar to FreeBSD.

I still haven't looked into anybody's source code, but based on these
results I'm inclined to leave both 001 and 010 IDs unused for now.
That'll help the GUC malloc -> palloc transition tremendously, because
people will get fairly clear errors rather than weird assertions
and/or memory corruption.  That'll leave us in a situation where only
one more context ID can be assigned without risk of reducing our error
detection ability, but I'm content with that for now.

			regards, tom lane

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