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Date:   Fri, 30 Jul 2021 00:34:32 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     NeilBrown <neilb@suse.de>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Josef Bacik <josef@toxicpanda.com>,
        "J. Bruce Fields" <bfields@fieldses.org>,
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        David Sterba <dsterba@suse.com>, linux-fsdevel@vger.kernel.org,
        linux-nfs@vger.kernel.org, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 05/11] VFS: new function: mount_is_internal()
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On Wed, Jul 28, 2021 at 01:32:45PM +1000, NeilBrown wrote:
> On Wed, 28 Jul 2021, Al Viro wrote:
> > On Wed, Jul 28, 2021 at 08:37:45AM +1000, NeilBrown wrote:
> > > This patch introduces the concept of an "internal" mount which is a
> > > mount where a filesystem has create the mount itself.
> > > 
> > > Both the mounted-on-dentry and the mount's root dentry must refer to the
> > > same superblock (they may be the same dentry), and the mounted-on dentry
> > > must be an automount.
> > 
> > And what happens if you mount --move it?
> > 
> > 
> If you move the mount, then the mounted-on dentry would not longer be an
> automount (....  I assume???...) so it would not longer be
> mount_is_internal().
> 
> I think that is reasonable.  Whoever moved the mount has now taken over
> responsibility for it - it no longer is controlled by the filesystem.
> The moving will have removed the mount from the list of auto-expire
> mounts, and the mount-trap will now be exposed and can be mounted-on
> again.
> 
> It would be just like unmounting the automount, and bind-mounting the
> same dentry elsewhere.

Once more, with feeling: what happens to your function if it gets called during
mount --move?

What locking environment is going to be provided for it?  And what is going
to provide said environment?