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

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Date:   Tue, 20 Sep 2022 13:02:31 -0500
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Tycho Andersen <tycho@...ho.pizza>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        Eric Biederman <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org, fuse-devel@...ts.sourceforge.net,
        "Serge E. Hallyn" <serge@...lyn.com>
Subject: Re: [PATCH] fuse: In fuse_flush only wait if someone wants the
 return code

On Mon, Sep 19, 2022 at 09:03:41AM -0600, Tycho Andersen wrote:
> Hi Miklos,
> 
> On Thu, Sep 01, 2022 at 08:06:47AM -0600, Tycho Andersen wrote:
> > From: "Eric W. Biederman" <ebiederm@...ssion.com>
> > 
> > In my very light testing this resolves a hang where a thread of the
> > fuse server was accessing the fuse filesystem (the fuse server is
> > serving up), when the fuse server is killed.
> > 
> > The practical problem is that the fuse server file descriptor was
> > being closed after the file descriptor into the fuse filesystem so
> > that the fuse filesystem operations were being blocked for instead of
> > being aborted.  Simply skipping the unnecessary wait resolves this
> > issue.
> > 
> > This is just a proof of concept and someone should look to see if the
> > fuse max_background limit could cause a problem with this approach.

I tried to track this down last week, but it looks to me like since
the max_background is per-connection, this should work as expected
and not affect any other connections.

> > Additionally testing PF_EXITING is a very crude way to tell if someone
> > wants the return code from the vfs flush operation.  As such in the
> > long run it probably makes sense to get some direct vfs support for
> > knowing if flush needs to block until all of the flushing is complete
> > and a status/return code can be returned.
> > 
> > Unless I have missed something this is a generic optimization that can
> > apply to many network filesystems.
> > 
> > Al, vfs folks? (igrab/iput sorted so as not to be distractions).
> > 
> > Perhaps a .flush_async method without a return code and a
> > filp_close_async function without a return code to take advantage of
> > this in the general sense.
> > 
> > Waiting potentially indefinitely for user space in do_exit seems like a
> > bad idea.  Especially when all that the wait is for is to get a return
> > code that will never be examined.
> > 
> > Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> > [tycho: small fixups for releasing fuse file + nocred flag]
> > Signed-off-by: Tycho Andersen <tycho@...ho.pizza>
> > Reported-by: Tycho Andersen <tycho@...ho.pizza>
> > Tested-by: "Serge E. Hallyn" <serge@...lyn.com>
> 
> Any chance you're willing to take this patch? We're still seeing this
> a lot and it would be great to get it fixed.
> 
> Thanks.
> 
> Tycho