Re: VM corruption on standby

Hi,

On 2025-08-19 15:56:05 +0300, Yura Sokolov wrote:
> 09.08.2025 22:54, Kirill Reshke пишет:
> > On Thu, 7 Aug 2025 at 21:36, Aleksander Alekseev
> > <aleksander(at)tigerdata(dot)com> wrote:
> >
> >> Perhaps there was a good
> >> reason to update the VM *before* creating WAL records I'm unaware of.
> >
> > Looks like 503c730 intentionally does it this way; however, I have not
> > yet fully understood the reasoning behind it.
>
> I repeat: there was no intention. Neither in commit message, nor in
> discussion about.
>
> There was intention to move visibilitymap_clear under heap page lock and
> into critical section, but there were no any word about logging.
>
> I believe, it was just an unfortunate oversight that the change is made
> before logging.

The normal pattern *is* to modify the buffer while holding an exclusive lock,
in a critical section, before WAL logging. Look at
src/backend/access/transam/README:

> The general schema for executing a WAL-logged action is
>
> 1. Pin and exclusive-lock the shared buffer(s) containing the data page(s)
> to be modified.
>
> 2. START_CRIT_SECTION() (Any error during the next three steps must cause a
> PANIC because the shared buffers will contain unlogged changes, which we
> have to ensure don't get to disk. Obviously, you should check conditions
> such as whether there's enough free space on the page before you start the
> critical section.)
>
> 3. Apply the required changes to the shared buffer(s).
>
> 4. Mark the shared buffer(s) as dirty with MarkBufferDirty(). (This must
> happen before the WAL record is inserted; see notes in SyncOneBuffer().)
> Note that marking a buffer dirty with MarkBufferDirty() should only
> happen iff you write a WAL record; see Writing Hints below.
>
> 5. If the relation requires WAL-logging, build a WAL record using
> XLogBeginInsert and XLogRegister* functions, and insert it. (See
> "Constructing a WAL record" below). Then update the page's LSN using the
> returned XLOG location. For instance,
>
> XLogBeginInsert();
> XLogRegisterBuffer(...)
> XLogRegisterData(...)
> recptr = XLogInsert(rmgr_id, info);
>
> PageSetLSN(dp, recptr);
>
> 6. END_CRIT_SECTION()
>
> 7. Unlock and unpin the buffer(s).

Greetings,

Andres Freund