Feedback-ID: id4a34324:Fastmail
Date: Tue, 23 May 2023 19:28:45 -0700
From: Andres Freund <andres@anarazel.de>
To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: thomas.munro@gmail.com, myon@debian.org,
	pgsql-hackers@lists.postgresql.org
Subject: Re: could not extend file "base/5/3501" with FileFallocate():
 Interrupted system call
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Hi,

On 2023-05-24 10:56:28 +0900, Kyotaro Horiguchi wrote:
> At Wed, 26 Apr 2023 11:37:55 +1200, Thomas Munro <thomas.munro@gmail.com> wrote in 
> > On Tue, Apr 25, 2023 at 12:16 PM Andres Freund <andres@anarazel.de> wrote:
> > > On 2023-04-24 15:32:25 -0700, Andres Freund wrote:
> > > > We obviously can add a retry loop to FileFallocate(), similar to what's
> > > > already present e.g. in FileRead(). But I wonder if we shouldn't go a bit
> > > > further, and do it for all the fd.c routines where it's remotely plausible
> > > > EINTR could be returned? It's a bit silly to add EINTR retries one-by-one to
> > > > the functions.
> > > >
> > > >
> > > > The following are documented to potentially return EINTR, without fd.c having
> > > > code to retry:
> > > >
> > > > - FileWriteback() / pg_flush_data()
> > > > - FileSync() / pg_fsync()
> > > > - FileFallocate()
> > > > - FileTruncate()
> > > >
> > > > With the first two there's the added complication that it's not entirely
> > > > obvious whether it'd be better to handle this in File* or pg_*. I'd argue the
> > > > latter is a bit more sensible?
> > >
> > > A prototype of that approach is attached. I pushed the retry handling into the
> > > pg_* routines where applicable.  I guess we could add pg_* routines for
> > > FileFallocate(), FilePrewarm() etc as well, but I didn't do that here.
> > 
> > One problem we ran into with the the shm_open() case (which is nearly
> > identical under the covers, since shm_open() just opens a file in a
> > tmpfs on Linux) was that a simple retry loop like this could never
> > terminate if the process was receiving a lot of signals from the
> > recovery process, which is why we went with the idea of masking
> > signals instead.  Eventually we should probably grow the file in
> > smaller chunks with a CFI in between so that we both guarantee that we
> > make progress (by masking for smaller size increases) and service
> > interrupts in a timely fashion (by unmasking between loops).  I don't
> > think that applies here because we're not trying to fallocate
> > humongous size increases in one go, but I just want to note that we're
> > making a different choice.  I think this looks reasonable for the use
> > cases we actually have here.
> 
> FWIW I share the same feeling about looping by EINTR without signals
> being blocked. If we just retry the same operation without processing
> signals after getting EINTR, I think blocking signals is better. We
> could block signals more gracefully, but I'm not sure it's worth the
> complexity.

I seriously doubt it's a good path to go down in this case. As Thomas
mentioned, this case isn't really comparable to the shm_open() one, due to the
bounded vs unbounded amount of memory we're dealing with.

What would be the benefit?

Greetings,

Andres Freund