linux-kernel - Re: [PATCH v4 4/5] virtiofs: Skip submounts in sget

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Message-ID: <20210521103921.153a243d@bahia.lan>
Date:   Fri, 21 May 2021 10:39:21 +0200
From:   Greg Kurz <groug@...d.org>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     virtualization@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtio-fs-list <virtio-fs@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Max Reitz <mreitz@...hat.com>, Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH v4 4/5] virtiofs: Skip submounts in sget_fc()

On Fri, 21 May 2021 10:26:27 +0200
Miklos Szeredi <miklos@...redi.hu> wrote:

> On Thu, 20 May 2021 at 17:47, Greg Kurz <groug@...d.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

> Thanks,
> Miklos