On Wed, 28 Jul 2021, Al Viro wrote:

On Wed, Jul 28, 2021 at 08:37:45AM +1000, NeilBrown wrote:

quoted

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.

NeilBrown