From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
cc: Matthias van de Meent <boekewurm+postgres@gmail.com>,
        David Rowley <dgrowleyml@gmail.com>,
        PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Add bump memory context type and use it for tuplesorts
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Comments: In-reply-to Tomas Vondra <tomas.vondra@enterprisedb.com>
	message dated "Fri, 16 Feb 2024 23:46:03 +0100"
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Date: Fri, 16 Feb 2024 18:14:44 -0500
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Tomas Vondra <tomas.vondra@enterprisedb.com> writes:
> Maybe I'm completely misunderstanding the implication of those limits,
> but doesn't this mean the claim that we can support 8 memory context
> types is not quite true, and the limit is 4, because the 4 IDs are
> already used for malloc stuff?

Well, correct code would still work, but we will take a hit in our
ability to detect bogus chunk pointers if we convert any of the four
remaining bit-patterns to valid IDs.  That has costs for debugging.
The particular bit patterns we left unused were calculated to make it
likely that we could detect a malloced-instead-of-palloced chunk (at
least with glibc); but in general, reducing the number of invalid
patterns makes it more likely that a just-plain-bad pointer would
escape detection.

I am less concerned about that than I was in 2022, because people
have already had some time to flush out bugs associated with the
GUC malloc->palloc conversion.  Still, maybe we should think harder
about whether we can free up another ID bit before we go eating
more ID types.  It's not apparent to me that the "bump context"
idea is valuable enough to foreclose ever adding more context types,
yet it will be pretty close to doing that if we commit it as-is.

If we do kick this can down the road, then I concur with eating 010
next, as it seems the least likely to occur in glibc-malloced
chunks.

> One thing that confuses me a bit is that the comments introduced by
> 80ef92675823 talk about glibc, but the related discussion in [1] talks a
> lot about FreeBSD, NetBSD, ... which don't actually use glibc (AFAIK).

The conclusion was that the specific invalid values didn't matter as
much on the other platforms as they do with glibc.  But right now you
have a fifty-fifty chance that a pointer to garbage will look valid.
Do we want to increase those odds?

			regards, tom lane