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

We can enhance nfsd to understand that some automounts can be managed.
"internal mounts" where a filesystem provides an automount point and
mounts its own directories, can be handled differently by nfsd.

This series addresses all these issues.  After a few enhancements to the
VFS to provide needed support, they enhance exportfs and nfsd to cope
with the concept of internal mounts, and then enhance btrfs to provide
them.

I'm half asleep right now; will review and reply in detail tomorrow.
The first impression is that it's going to be brittle; properties of
mount really should not depend upon the nature of mountpoint - too many
ways for that to go wrong.

Anyway, tomorrow...