From: Ian Kent <raven@themaw.net>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Mateusz Guzik <mguzik@redhat.com>, NeilBrown <neilb@suse.com>,
Andrew Morton <akpm@linux-foundation.org>,
autofs mailing list <autofs@vger.kernel.org>,
Kernel Mailing List <linux-kernel@vger.kernel.org>,
Al Viro <viro@ZenIV.linux.org.uk>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Omar Sandoval <osandov@osandov.com>
Subject: Re: [PATCH 3/4] autofs - make mountpoint checks namespace aware
Date: Wed, 21 Sep 2016 06:44:22 +0800 [thread overview]
Message-ID: <1474411462.22440.2.camel@themaw.net> (raw)
In-Reply-To: <87oa3iikgf.fsf@x220.int.ebiederm.org>
On Tue, 2016-09-20 at 11:50 -0500, Eric W. Biederman wrote:
> Ian Kent <raven@themaw.net> writes:
>
> > On Sat, 2016-09-17 at 22:10 +0200, Mateusz Guzik wrote:
> > > On Wed, Sep 14, 2016 at 02:14:45PM +0800, Ian Kent wrote:
> > > > If an automount mount is clone(2)ed into a file system that is
> > > > propagation private, when it later expires in the originating
> > > > namespace subsequent calls to autofs ->d_automount() for that
> > > > dentry in the original namespace will return ELOOP until the
> > > > mount is manually umounted in the cloned namespace.
> > > >
> > > > In the same way, if an autofs mount is triggered by automount(8)
> > > > running within a container the dentry will be seen as mounted in
> > > > the root init namespace and calls to ->d_automount() in that namespace
> > > > will return ELOOP until the mount is umounted within the container.
> > > >
> > > > Also, have_submounts() can return an incorect result when a mount
> > > > exists in a namespace other than the one being checked.
> > > >
> > > > @@ -460,7 +460,7 @@ static int autofs4_d_manage(struct dentry *dentry,
> > > > bool
> > > > rcu_walk)
> > > >
> > > > if (ino->flags & AUTOFS_INF_WANT_EXPIRE)
> > > > return 0;
> > > > - if (d_mountpoint(dentry))
> > > > + if (is_local_mountpoint(dentry))
> > > > return 0;
> > > > inode = d_inode_rcu(dentry);
> > > > if (inode && S_ISLNK(inode->i_mode))
> > >
> > > This change is within RCU lookup.
> > >
> > > is_local_mountpoint may end up calling __is_local_mountpoint, which will
> > > optionally take the namespace_sem lock, resulting in a splat:
> >
> > Yes, that's a serious problem I missed.
> >
> > snip ...
> >
> > > I don't know this code. Perhaps it will be perfectly fine performance wise
> > > to
> > > just drop out of RCU lookup in this case.
> >
> > It's a bit worse than that.
> >
> > I think being able to continue the rcu-walk for an already mounted dentry
> > that
> > is not being expired is an important part of the performance improvement
> > given
> > by the series that added this.
> >
> > Can you confirm that Neil?
> >
> > But for the case here the existing test can allow rcu-walk to continue for a
> > dentry that would attempt to trigger an automount so it's also a bug in the
> > existing code.
>
> I don't think the existing code is buggy. As I read __follow_mount_rcu
> if DCACHE_NEED_AUTOMOUNT is set on the dentry after return from d_manage
> the code will break out of the rcu walk.
That's right.
I'm working on follow up patches now.
>
> > Any thoughts on how we can handle this Neil, I'm having a bit of trouble
> > working
> > out how to resolve this one.
>
> I believe in this case d_mountpoint is enough for the rcu walk case. If
> the mountpoint turns out not to be local __follow_mount_rcu will kick
> out of the rcu walk as it will return false. Because:
> return !mounted && !(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT);
>
> I am not quite certain about the non-rcu case. That can't be
> is_local_mountpoint as that is inside a spinlock and is_local_mountpoint
> can sleep. Perhaps d_mountpoint is simply correct for autofs4_d_manage.
That's right, I think I have other mistakes there too.
I'm checking those too.
>
> Eric
>
next prev parent reply other threads:[~2016-09-20 22:44 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-09-14 6:14 [PATCH 1/4] fs - make is_local_mountpoint() usable by others Ian Kent
2016-09-14 6:14 ` [PATCH 2/4] fs - add have_local_submounts() Ian Kent
2016-09-14 6:14 ` [PATCH 3/4] autofs - make mountpoint checks namespace aware Ian Kent
2016-09-14 17:28 ` Eric W. Biederman
2016-09-15 0:09 ` Ian Kent
2016-09-15 0:32 ` Omar Sandoval
2016-09-15 2:08 ` Eric W. Biederman
2016-09-15 4:12 ` Ian Kent
2016-09-15 8:19 ` Ian Kent
2016-09-16 0:47 ` Eric W. Biederman
2016-09-16 2:58 ` Ian Kent
2016-09-19 0:58 ` Ian Kent
2016-09-20 16:09 ` Eric W. Biederman
2016-09-15 1:03 ` Ian Kent
2016-09-16 21:14 ` Omar Sandoval
2016-09-17 20:10 ` Mateusz Guzik
2016-09-19 1:36 ` Ian Kent
2016-09-20 16:50 ` Eric W. Biederman
2016-09-20 22:44 ` Ian Kent [this message]
2016-09-20 23:00 ` Ian Kent
2016-09-22 1:33 ` Ian Kent
2016-09-22 15:43 ` Eric W. Biederman
2016-09-23 0:55 ` Ian Kent
2016-09-23 1:37 ` Eric W. Biederman
2016-09-23 4:26 ` Ian Kent
2016-09-23 12:00 ` Ian Kent
2016-09-28 10:18 ` Ian Kent
2016-09-23 19:15 ` Eric W. Biederman
2016-09-24 0:11 ` Ian Kent
2016-09-26 16:05 ` Eric W. Biederman
2016-09-27 1:52 ` Ian Kent
2016-09-27 13:14 ` Eric W. Biederman
2016-09-28 0:19 ` Ian Kent
2016-09-14 6:14 ` [PATCH 4/4] fs - remove unused have_submounts() function Ian Kent
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