linux-kernel - Re: [PATCH v2] fuse: In fuse_flush only wait if someone wants the return code

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Date:   Fri, 30 Sep 2022 16:41:37 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Tycho Andersen <tycho@...ho.pizza>
Cc:     Eric Biederman <ebiederm@...ssion.com>,
        "Serge E. Hallyn" <serge@...lyn.com>, linux-kernel@...r.kernel.org,
        fuse-devel@...ts.sourceforge.net
Subject: Re: [PATCH v2] fuse: In fuse_flush only wait if someone wants the
 return code

On Fri, 30 Sept 2022 at 16:01, Tycho Andersen <tycho@...ho.pizza> wrote:
>
> On Fri, Sep 30, 2022 at 03:35:16PM +0200, Miklos Szeredi wrote:
> > On Thu, 29 Sept 2022 at 18:40, Tycho Andersen <tycho@...ho.pizza> wrote:
> > >
> > > If a fuse filesystem is mounted inside a container, there is a problem
> > > during pid namespace destruction. The scenario is:
> > >
> > > 1. task (a thread in the fuse server, with a fuse file open) starts
> > >    exiting, does exit_signals(), goes into fuse_flush() -> wait
> >
> > Can't the same happen through
> >
> >   fuse_flush -> fuse_sync_writes -> fuse_set_nowrite -> wait
> >
> > ?
>
> Looks like yes, though I haven't seen this in the wild, I guess
> because there aren't multiple writers most of the time the user code
> that causes this.
>
> I'm not exactly sure how to fix this. Reading through 3be5a52b30aa
> ("fuse: support writable mmap"), we don't want to allow multiple
> writes since that may do allocations, which could cause deadlocks. But
> in this case we have no reliable way to wait (besides a busy loop, I
> suppose).
>
> Maybe just a check for PF_EXITING and a pr_warn() with "echo 1 >
> /sys/fs/fuse/connections/$N/abort" or something?

AFAICS it should be perfectly normal (and trivial to trigger) for an
exiting process to have its dirty pages flushed through fuse_flush().

We could do that asynchronously as well, generally there are no
promises about dirty pages being synced as part of the process exiting
.  But ordering between dirty page flushing and sending the FUSE_FLUSH
request should be kept.  Which needs more complexity, unfortunately.

Thanks,
Miklos