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From: Miklos Szeredi <miklos@szeredi.hu>
To: Greg Kurz <groug@kaod.org>
Cc: virtualization@lists.linux-foundation.org,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	virtio-fs-list <virtio-fs@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	Max Reitz <mreitz@redhat.com>, Vivek Goyal <vgoyal@redhat.com>
Subject: Re: [PATCH v4 4/5] virtiofs: Skip submounts in sget_fc()
Date: Fri, 21 May 2021 14:37:25 +0200	[thread overview]
Message-ID: <CAJfpegvBB-zRuZAM0m7fxMFCfw=CzN3uT3CqoQrRgizaTH4sOw@mail.gmail.com> (raw)
In-Reply-To: <20210521120616.49d52565@bahia.lan>

On Fri, 21 May 2021 at 12:06, Greg Kurz <groug@kaod.org> wrote:
>
> On Fri, 21 May 2021 10:50:34 +0200
> Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> > On Fri, 21 May 2021 at 10:39, Greg Kurz <groug@kaod.org> wrote:
> > >
> > > On Fri, 21 May 2021 10:26:27 +0200
> > > Miklos Szeredi <miklos@szeredi.hu> wrote:
> > >
> > > > On Thu, 20 May 2021 at 17:47, Greg Kurz <groug@kaod.org> wrote:
> > > > >
> > > > > All submounts share the same virtio-fs device instance as the root
> > > > > mount. If the same virtiofs filesystem is mounted again, sget_fc()
> > > > > is likely to pick up any of these submounts and reuse it instead of
> > > > > the root mount.
> > > > >
> > > > > On the server side:
> > > > >
> > > > > # mkdir ${some_dir}
> > > > > # mkdir ${some_dir}/mnt1
> > > > > # mount -t tmpfs none ${some_dir}/mnt1
> > > > > # touch ${some_dir}/mnt1/THIS_IS_MNT1
> > > > > # mkdir ${some_dir}/mnt2
> > > > > # mount -t tmpfs none ${some_dir}/mnt2
> > > > > # touch ${some_dir}/mnt2/THIS_IS_MNT2
> > > > >
> > > > > On the client side:
> > > > >
> > > > > # mkdir /mnt/virtiofs1
> > > > > # mount -t virtiofs myfs /mnt/virtiofs1
> > > > > # ls /mnt/virtiofs1
> > > > > mnt1 mnt2
> > > > > # grep virtiofs /proc/mounts
> > > > > myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0
> > > > > none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel)
> > > > > none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel)
> > > > >
> > > > > And now remount it again:
> > > > >
> > > > > # mount -t virtiofs myfs /mnt/virtiofs2
> > > > > # grep virtiofs /proc/mounts
> > > > > myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0
> > > > > none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel)
> > > > > none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel)
> > > > > myfs /mnt/virtiofs2 virtiofs rw,seclabel,relatime 0 0
> > > > > # ls /mnt/virtiofs2
> > > > > THIS_IS_MNT2
> > > > >
> > > > > Submount mnt2 was picked-up instead of the root mount.
> > > >
> > >
> > > > Why is this a problem?
> > > >
> > >
> > > It seems very weird to mount the same filesystem again
> > > and to end up in one of its submounts. We should have:
> > >
> > > # ls /mnt/virtiofs2
> > > mnt1 mnt2
> >
> > Okay, sorry, I understand the problem.  The solution is wrong,
> > however: the position of the submount on that list is no indication
> > that it's the right one (it's possible that the root sb will go away
> > and only a sub-sb will remain).
> >
>
> Ah... I had myself convinced this could not happen, i.e. you can't
> unmount a parent sb with a sub-sb still mounted.

No, but it's possible for sub-sb to continue existing after it's no
longer a submount of original mount.
>
> How can this happen ?

E.g. move the submount out of the way, then unmount the parent, or
detach submount (umount -l) while keeping something open in there and
umount the parent.

> > Even just setting a flag in the root, indicating that it's the root
> > isn't fully going to solve the problem.
> >
> > Here's issue in full:
> >
> > case 1:  no connection for "myfs" exists
> >     - need to create fuse_conn, sb
> >
> > case 2: connection for "myfs" exists but only sb for submount
>
> How would we know this sb isn't a root sb ?
>
> >     - only create sb for root, reuse fuse_conn
> >
> > case 3: connection for "myfs" as well as root sb exists
> >    - reuse sb
> >
> > I'll think about how to fix this properly, it's probably going to be
> > rather more involved...
> >
>
> Sure. BTW I'm wondering why we never reuse sbs for submounts ?

Right, same general issue.

An sb can be identified by its root nodeid, so I guess the proper fix
to make the root nodeid be the key for virtio_fs_test_super().

Thanks,
Miklos

  reply	other threads:[~2021-05-21 12:37 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-20 15:46 [PATCH v4 0/5] virtiofs: propagate sync() to file server Greg Kurz
2021-05-20 15:46 ` [PATCH v4 1/5] fuse: Fix leak in fuse_dentry_automount() error path Greg Kurz
2021-05-20 19:45   ` Al Viro
2021-05-21  7:54     ` Miklos Szeredi
2021-05-21  8:15       ` Greg Kurz
2021-05-21  8:23         ` Miklos Szeredi
2021-05-21  8:08     ` Greg Kurz
2021-05-20 15:46 ` [PATCH v4 2/5] fuse: Call vfs_get_tree() for submounts Greg Kurz
2021-05-21  8:19   ` Miklos Szeredi
2021-05-21  8:28     ` Greg Kurz
2021-05-22 17:50   ` kernel test robot
2021-05-22 20:12   ` kernel test robot
2021-05-20 15:46 ` [PATCH v4 3/5] fuse: Make fuse_fill_super_submount() static Greg Kurz
2021-05-20 15:46 ` [PATCH v4 4/5] virtiofs: Skip submounts in sget_fc() Greg Kurz
2021-05-21  8:26   ` Miklos Szeredi
2021-05-21  8:39     ` Greg Kurz
2021-05-21  8:50       ` Miklos Szeredi
2021-05-21 10:06         ` Greg Kurz
2021-05-21 12:37           ` Miklos Szeredi [this message]
2021-05-21 13:36             ` Greg Kurz
2021-05-20 15:46 ` [PATCH v4 5/5] virtiofs: propagate sync() to file server Greg Kurz
2021-05-21 10:08   ` Greg Kurz
2021-05-21 12:51     ` Miklos Szeredi
2021-08-15 14:14   ` Amir Goldstein
2021-08-16 15:29     ` Vivek Goyal
2021-08-16 18:57       ` Amir Goldstein
2021-08-16 19:11         ` Vivek Goyal
2021-08-16 19:46           ` Amir Goldstein
2021-08-28 15:21       ` Miklos Szeredi
2021-08-30 17:01         ` Vivek Goyal
2021-08-30 17:36           ` Miklos Szeredi

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