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From:   Alexander Larsson <alexl@redhat.com>
Date:   Fri, 20 Jan 2023 09:43:21 +0100
Message-ID: <CAL7ro1EESqanEQnGnLqd_WvoxL0ybn0XJwtMAFxap=w-a=-Vig@mail.gmail.com>
Subject: Re: [RFC PATCH RESEND 1/1] fs/namespace: defer free_mount from namespace_unlock
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Eric Chanudet <echanude@redhat.com>, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org,
        Andrew Halaney <ahalaney@redhat.com>
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On Thu, Jan 19, 2023 at 11:09 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> On Thu, Jan 19, 2023 at 04:14:55PM -0500, Eric Chanudet wrote:
> > From: Alexander Larsson <alexl@redhat.com>
> >
> > Use call_rcu to defer releasing the umount'ed or detached filesystem
> > when calling namepsace_unlock().
> >
> > Calling synchronize_rcu_expedited() has a significant cost on RT kernel
> > that default to rcupdate.rcu_normal_after_boot=1.
> >
> > For example, on a 6.2-rt1 kernel:
> > perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount mnt
> >            0.07464 +- 0.00396 seconds time elapsed  ( +-  5.31% )
> >
> > With this change applied:
> > perf stat -r 10 --null --pre 'mount -t tmpfs tmpfs mnt' -- umount mnt
> >         0.00162604 +- 0.00000637 seconds time elapsed  ( +-  0.39% )
> >
> > Waiting for the grace period before completing the syscall does not seem
> > mandatory. The struct mount umount'ed are queued up for release in a
> > separate list and no longer accessible to following syscalls.
>
> Again, NAK.  If a filesystem is expected to be shut down by umount(2),
> userland expects it to have been already shut down by the time the
> syscall returns.
>
> It's not just visibility in namespace; it's "can I pull the disk out?".
> Or "can the shutdown get to taking the network down?", for that matter.

In the usecase we're worrying about, all the unmounts are lazy (i.e.
MNT_DETACH). What about delaying the destroy in that case? That seems
in line with the expected behaviour of lazy shutdown. I.e. you can't
rely on it to be settled anyway.


-- 
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 Alexander Larsson                                Red Hat, Inc
       alexl@redhat.com         alexander.larsson@gmail.com