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Date:   Fri, 21 May 2021 15:36:14 +0200
From:   Greg Kurz <groug@kaod.org>
To:     Miklos Szeredi <miklos@szeredi.hu>
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()
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On Fri, 21 May 2021 14:37:25 +0200
Miklos Szeredi <miklos@szeredi.hu> wrote:

> 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.
> 

Ok, I get it now. Thanks for the clarification.

> > > 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().
> 

Cool, I was thinking about doing this exactly. :)

> Thanks,
> Miklos