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Subject: Re: Bug in MultiXact replay compat logic for older minor version
 after crash-recovery
From: Andrey Borodin <x4mmm@yandex-team.ru>
In-Reply-To: <18a40051-abc2-4fc2-8dcc-4dc39aa3e79e@iki.fi>
Date: Fri, 20 Mar 2026 14:55:52 +0300
Cc: =?utf-8?B?IuauteWdpOS7gSjliLvpn6cpIg==?= <duankunren.dkr@alibaba-inc.com>,
 pgsql-hackers <pgsql-hackers@postgresql.org>
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References: 
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 <18a40051-abc2-4fc2-8dcc-4dc39aa3e79e@iki.fi>
To: Heikki Linnakangas <hlinnaka@iki.fi>
Archived-At: 
 <https://www.postgresql.org/message-id/F513DFBB-DCFE-4B99-B01E-DDB04414857C%40yandex-team.ru>
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> On 19 Mar 2026, at 23:11, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>=20
> :-(. This is a gift that keeps giving.

Well, maybe, we could leaving that deadlock in place for some time...

> Idea 3:
>=20
> I think a better fix is to accept that our tracking is a little =
imprecise and use SimpleLruDoesPhysicalPageExist() to check if the page =
exists. I suspect that's too expensive to do on every =
RecordNewMultiXact() call that crosses a page, but perhaps we could do =
it once at StartupMultiXact().
>=20
> Or perhaps track last-zeroed page separately from latest_page_number, =
and if we haven't seen any XLOG_MULTIXACT_ZERO_OFF_PAGE records yet =
after startup, call SimpleLruDoesPhysicalPageExist() to determine if =
initialization is needed. Attached patch does that.

SimpleLruDoesPhysicalPageExist() does not detect recently zeroed pages =
via buffers, because it goes directly to FS.
I tried this approach when implementing deadlock fix, it did not work =
for me.


Best regards, Andrey Borodin.=