Re: [RFC PATCH RESEND 1/1] fs/namespace: defer free_mount from namespace_unlock

[Alexander Larsson]

On Thu, Jan 19, 2023 at 11:09 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Jan 19, 2023 at 04:14:55PM -0500, Eric Chanudet wrote:
> > From: Alexander Larsson <alexl@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx         alexander.larsson@xxxxxxxxx