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: David Rowley <dgrowleyml@gmail.com>, Julien Rouhaud <rjuju123@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Yura Sokolov <y.sokolov@postgrespro.ru>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2022-09-08T22:15:23Z
Lists: pgsql-hackers 
Hi,

On 2022-09-08 14:10:36 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > If there is a finalfunc, they're typically going to return data from within
> > the current memory context, but could legitimately also return part of the
> > data from curaggcontext. Perhaps we could forbid that?
> 
> No, I don't think we can get away with that.  See int8inc() for a
> counterexample.

What I was suggesting a bit below the bit quoted above, was that we'd copy
whenever there's no finalfunc or if the finalfunc doesn't take an internal
parameter. And that finalfuncs returning byref with an internal parameter can
be expected to return memory allocated in the right context (which we of
course could check with an assert).  It's not super satisfying - but I don't
think it'd have the problem you describe above.

Alternatively we could add a column to pg_aggregate denoting this. That'd only
be permissible to set for a superuser presumably.


This business with interpreting random memory as a palloc'd chunk seems like a
fundamentally wrong approach worth incurring some pain to fix.

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