Horton

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

David Rowley <dgrowleyml@gmail.com>

View thread View on postgr.es
From: David Rowley <dgrowleyml@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
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: 2022-09-08T23:33:37Z
Lists: pgsql-hackers 
On Fri, 9 Sept 2022 at 10:53, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I hate to give up MemoryContextContains altogether.  The assertions
> that David nuked in b76fb6c2a had some value I think,

Those can be put back if we decide to keep MemoryContextContains.
Those newly added Asserts just temporarily had to go due to b76fb6c2a
making MemoryContextContains temporarily always return false.

> The implementation I suggested upthread would reliably distinguish
> malloc from palloc, and while it is potentially a tad expensive
> I don't think it's too much so for Assert checks.  I don't have an
> objection to trying to get to a place where we only use it in
> Assert, though.

I really think the Assert only form of MemoryContextContains() is the
best move, and if it's doing Assert only, then we can do the
loop-over-the-blocks idea as you described and I drafted in [1].

If the need comes up that we're certain we always have a pointer to
some allocated chunk, but need to know if it's in some memory context,
then the proper form of expressing that, I think, should be:

if (GetMemoryChunkContext(pointer) == somecontext)

If we're worried about getting that wrong, we can beef up the
MemoryChunk struct with a magic_number field in
MEMORY_CONTEXT_CHECKING builds to ensure we catch any code which
passes invalid pointers.

David

[1] https://www.postgresql.org/message-id/CAApHDvoKjOmPQeokicwDuO-_Edh=tKp23-=jskYcyKfw5QuDhA@mail.gmail.com

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