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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Fri, 21 May 2021 10:26:27 +0200
Message-ID: <CAJfpegugQM-ChaGiLyfPkbFr9c=_BiOBQkJTeEz5yN0ujO_O4A@mail.gmail.com>
Subject: Re: [PATCH v4 4/5] virtiofs: Skip submounts in sget_fc()
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>
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Precedence: bulk

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?

Thanks,
Miklos