On Wed, Jul 28, 2021 at 1:44 AM NeilBrown [off-list ref] wrote:

When a filesystem has internal mounts, it controls the filehandles
across all those mounts (subvols) in the filesystem.  So it is useful to
be able to look up a filehandle again one mount, and get a result which
is in a different mount (part of the same overall file system).

This patch makes that possible by changing export_decode_fh() and
export_decode_fh_raw() to take a vfsmount pointer by reference, and
possibly change the vfsmount pointed to before returning.

The core of the change is in reconnect_path() which now not only checks
that the dentry is fully connected, but also that the vfsmnt reported
has the same 'dev' (reported by vfs_getattr) as the dentry.
If it doesn't, we walk up the dparent() chain to find the highest place
where the dev changes without there being a mount point, and trigger an
automount there.

As no filesystems yet provide local-mounts, this does not yet change any
behaviour.

In exportfs_decode_fh_raw() we previously tested for DCACHE_DISCONNECT
before calling reconnect_path().  That test is dropped.  It was only a
minor optimisation and is now inconvenient.

The change in overlayfs needs more careful thought than I have yet given
it.

Just note that overlayfs does not support following auto mounts in layers.
See ovl_dentry_weird(). ovl_lookup() fails if it finds such a dentry.
So I think you need to make sure that the vfsmount was not crossed
when decoding an overlayfs real fh.

Apart from that, I think that your new feature should be opt-in w.r.t
the exportfs_decode_fh() vfs api and that overlayfs should not opt-in
for the cross mount decode.

Thanks,
Amir.