linux-kernel - Re: [syzbot] [fuse?] WARNING in fuse

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Message-ID: <CAJfpegu=U7sdWvw63ULkr=5T05cqVd3H9ytPOPrkLtwUwsy5Kw@mail.gmail.com>
Date: Fri, 18 Oct 2024 12:54:06 +0200
From: Miklos Szeredi <miklos@...redi.hu>
To: Joanne Koong <joannelkoong@...il.com>
Cc: syzbot <syzbot+217a976dc26ef2fa8711@...kaller.appspotmail.com>, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [fuse?] WARNING in fuse_writepages

On Fri, 4 Oct 2024 at 21:04, Joanne Koong <joannelkoong@...il.com> wrote:

> > The warning is complaining about this WARN_ON here
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/fuse/file.c#n1989.
> > I think this warning can get triggered if there's a race between a
> > write() and a close() where the page is dirty in the cache after the
> > release has happened. Then when writeback (eg fuse_writepages()) is
> > triggered, we hit this warning. (this possibility has always existed,
> > it was surfaced after this refactoring commit 4046d3adcca4: "move fuse
> > file initialization to wpa allocation time" but the actual logic
> > hasn't been changed).
>
> Actually, it's not clear how this WARN_ON is getting triggered.
>
> I will wait for syzbot to surface a repro first before taking further action.

I think the issue is that fuse_writepages() might be called with no
dirty pages after all writable opens were closed.  The exact mechanism
is unclear, but it's pretty likely that this is the case.

Commit 672c3b7457fc ("fuse: move initialization of fuse_file to
fuse_writepages() instead of in callback") broke this case.

Maybe reverting this is the simplest fix?

Thanks,
Miklos