| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Petr Jelinek <petr(dot)jelinek(at)2ndquadrant(dot)com>, Petr Jelinek <petr(at)2ndquadrant(dot)com>, John Gorman <johngorman2(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: PATCH: two slab-like memory allocators |
| Date: | 2017-03-06 17:44:06 |
| Message-ID: | 20170306174406.pfs3as44gc7a5ps2@alap3.anarazel.de |
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| Lists: | pgsql-hackers |
On 2017-03-06 12:40:18 -0500, Robert Haas wrote:
> On Wed, Mar 1, 2017 at 5:55 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > The issue was that on 32bit platforms the Datum returned by some
> > functions (int2int4_sum in this case) isn't actually a separately
> > allocated Datum, but rather just something embedded in a larger
> > struct. That, combined with the following code:
> > if (!peraggstate->resulttypeByVal && !*isnull &&
> > !MemoryContextContains(CurrentMemoryContext,
> > DatumGetPointer(*result)))
> > seems somewhat problematic to me. MemoryContextContains() can give
> > false positives when used on memory that's not a distinctly allocated
> > chunk, and if so, we violate memory lifetime rules. It's quite
> > unlikely, given the required bit patterns, but nonetheless it's making
> > me somewhat uncomfortable.
> >
> > Do others think this isn't an issue and we can just live with it?
>
> I think it's 100% broken to call MemoryContextContains() on something
> that's not guaranteed to be a palloc'd chunk.
I agree, but to me it seems the only fix would be to just yank out the
whole optimization?
- Andres
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