Re: Hot standby queries see transient all-zeros pages

Hi,

On 2024-12-13 16:38:05 -0800, Noah Misch wrote:
> On Fri, Dec 13, 2024 at 05:41:15PM -0500, Andres Freund wrote:
> > Afaics we didn't do anything about this issue?
>
> postgr.es/c/e656657 fixed this. I thought this was longstanding, but it
> turned out to have started on 2024-04-02.

Ah, that makes sense. I was a bit surprised to find this thread without any
replies...

> > Hm. Leaving RBM_ZERO_AND_LOCK aside, is it actually always safe to do
> > RestoreBlockImage() into a buffer that currently is pinned? Not sure if
> > there's actually all that much guarantee what transient state one can read
> > when reading a page concurrently to a memcpy(). I suspect it's practically
> > rare to see a problem, but one could imagine an memcpy implementation that
> > uses non-temporal writes, which afaict would leave you open to seeing quite
> > random states when reading concurrently, as the cache coherence protocol
> > doesn't protect anymore.
>
> I wondered about that, too. I didn't dig too deep.
> https://pubs.opengroup.org/onlinepubs/9799919799/functions/memcpy.html and
> https://en.cppreference.com/w/cpp/string/byte/memcpy were both silent about
> the topic.

Hm. Perhaps it'd be worth having a small stress test in the tests that'd make
problems like this more apparent. Even if it's not a problem current libc's,
who knows what happens down the line.

> > On 2024-05-12 10:16:58 -0700, Noah Misch wrote:
> > > I suspect the fix is to add a ReadBufferMode specified as, "If the block is
> > > already in shared_buffers, do RBM_NORMAL and exclusive-lock the buffer.
> > > Otherwise, do RBM_ZERO_AND_LOCK."
> >
> > I think that should work. At least in the current code it looks near trivial
> > to implement, although the branch differences are going to be annoying.
> >
> > As usual the hardest part would probably be the naming. Maybe
> > RBM_ZERO_ON_MISS_LOCK? RBM_LOCK_ZERO_ON_MISS? RBM_DWIM?
>
> It turned out RBM_ZERO_AND_LOCK long worked that way, and postgr.es/c/e656657
> just had to restore that longstanding behavior. The existing comment "Don't
> read from disk, caller will initialize." does allude to this (but I didn't
> originally catch the subtle point).
>
> If RBM_ZERO_AND_LOCK hadn't existed so long, I'd rename it. Perhaps it
> deserves a rename anyway? Of those, I'd pick RBM_ZERO_ON_MISS_LOCK. I also
> considered RBM_RECENT_OR_ZERO, borrowing a term from ReadRecentBuffer().

At least we could make the documentation for the mode in the enum clearer...

Greetings,

Andres Freund