Re: [PATCH v4] fs/namespace: defer RCU sync for MNT_DETACH umount

Eric Chanudet
2025-04-09T16:09:01Z

> > > On Wed, Apr 09, 2025 at 12:37:06PM +0200, Christian Brauner wrote:
> > > > On Tue, Apr 08, 2025 at 04:58:34PM -0400, Eric Chanudet wrote:
> > > > > Attempt to re-spin this series based on the feedback received in v3 that
> > > > > pointed out the need to wait the grace-period in namespace_unlock()
> > > > > before calling the deferred mntput().
> > > >
> > > > I still hate this with a passion because it adds another special-sauce
> > > > path into the unlock path. I've folded the following diff into it so it
> > > > at least doesn't start passing that pointless boolean and doesn't
> > > > introduce __namespace_unlock(). Just use a global variable and pick the
> > > > value off of it just as we do with the lists. Testing this now:

My apologies, I went with the feedback from v3[1] and failed to parse
the context surrounding it.

[1] https://lore.kernel.org/all/Znx-WGU5Wx6RaJyD@casper.infradead.org/

> > > > @@ -2094,7 +2088,7 @@ static int do_umount(struct mount *mnt, int flags)
> > > > }
> > > > out:
> > > > unlock_mount_hash();
> > > > - __namespace_unlock(flags & MNT_DETACH);
> > > > + namespace_unlock();
> > > > return retval;
> > > > }
> > > >
> > > >

I believe you skipped setting unmounted_lazily in this hunk?

With this, I have applied your patch for the following discussion and
down thread. Happy to send a v5, should this patch be deemed worth
pursuing.

On Wed, Apr 09, 2025 at 04:25:10PM +0200, Sebastian Andrzej Siewior wrote:
> On 2025-04-09 16:02:29 [+0200], Mateusz Guzik wrote:
> > On Wed, Apr 09, 2025 at 03:14:44PM +0200, Sebastian Andrzej Siewior wrote:
> > > One question: Do we need this lazy/ MNT_DETACH case? Couldn't we handle
> > > them all via queue_rcu_work()?
> > > If so, couldn't we have make deferred_free_mounts global and have two
> > > release_list, say release_list and release_list_next_gp? The first one
> > > will be used if queue_rcu_work() returns true, otherwise the second.
> > > Then once defer_free_mounts() is done and release_list_next_gp not
> > > empty, it would move release_list_next_gp -> release_list and invoke
> > > queue_rcu_work().
> > > This would avoid the kmalloc, synchronize_rcu_expedited() and the
> > > special-sauce.
> > >
> >
> > To my understanding it was preferred for non-lazy unmount consumers to
> > wait until the mntput before returning.

Unless I misunderstand the statement, and from the previous thread[2],
this is a requirement of the user API.

[2] https://lore.kernel.org/all/Y8m+M%2FffIEEWbfmv@ZenIV/

> > On Wed, Apr 09, 2025 at 03:14:44PM +0200, Sebastian Andrzej Siewior wrote:
> > > On 2025-04-09 12:37:06 [+0200], Christian Brauner wrote:
> > > > diff --git a/fs/namespace.c b/fs/namespace.c
> > > > index e5b0b920dd97..25599428706c 100644
> > > > --- a/fs/namespace.c
> > > > +++ b/fs/namespace.c
> > > > @@ -1840,29 +1842,21 @@ static void __namespace_unlock(bool lazy)
> > > …
> > > > + d = kmalloc(sizeof(struct deferred_free_mounts), GFP_KERNEL);
> > > > + if (d) {
> > > > + hlist_move_list(&head, &d->release_list);
> > > > + INIT_RCU_WORK(&d->rwork, defer_free_mounts);
> > > > + queue_rcu_work(system_wq, &d->rwork);
> > >
> > > Couldn't we do system_unbound_wq?

I think we can, afaict we don't need locality? I'll run some tests with
system_unbound_wq.

Thanks,

--
Eric Chanudet