Re: freespace buffer locking

On Tue, 2 Dec 2025 at 03:41, Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> Hi,

Hi!

> I.e. we modify the buffer without even holding a share lock on the page. That
seems ... not ok.

I was recently confused by these lines too doing some related hacking
on my work.

>
> Now, this normally won't happen, was the FSM isn't WAL
> logged, but still. And I think there may be special circumstances where the
> page is included in a WAL record, e.g. as part of an CREATE DATABASE. And
> there's FreeSpaceMapPrepareTruncateRel() - which hopefully can't run
> concurrently with fsm_vacuum_page(), but would seem to court WAL corruption,
> if it ever did.

Yep, also extension may want to run log_newpage_range for FSM fork for
whatever reason.
Extension then should rely on that when running log_newpage_range,
other activity will not
cause any hazards or corruptions. Is it?

> Besides modifying the page while not even share locked, there are a few other
> oddities:
>
>
> There seem to be some other oddities:
> - GetRecordedFreeSpace() does fsm_get_avail() without locking
> - fsm_vacuum_page() does fsm_get_avail(), fsm_get_max_avail() without locking

IIRC, Given these all are single-byte reads will there be no actual
troubles? Like, we already can
read corrupted info from FSM fork, because this is just a "hint", and
fsm callers are ready to not-trust the results. Just my 2c

>
> ISTM we clearly should take a lock in fsm_vacuum_page() to reset fp_next_slot,
> that just seems like a nasty hard to find bug waiting to happen. Changing it
> to not look at the page without a lock seems a bit more challenging.
>

I agree on the grounds of enforcing good examples of buf lock/pin
management along the internals.
Like, maybe all of this actually works on HEAD as expected, but I'm
not sure it is worth its additional complexity.

--
Best regards,
Kirill Reshke