Re: PATCH: two slab-like memory allocators

On Mon, Mar 6, 2017 at 12:44 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> 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?

Dunno, haven't looked into it.

--
Robert Haas
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