NFS proxy-only mode

From Linux NFS

It could be useful to have a mode where an NFS server is dedicated to reexporting all the exports from *one* other NFS server. It would have no other exports whatsoever.

This would allow the re-export server to support crossmount-like behavior, skip adding its own filesystem identifier to each filehandle (fixing problems with filehandle length limits), and avoid the need for manual assignment of filesystem identifiers with the fsid= option.

Possible implementation (needs more details):

- Create a new /proc/fs/nfsd/proxy_only file. Before starting the server, mount the original nfs server, then write the path to the mount to /proc/fs/nfsd/proxy_only. This interface is per-container. It also works for v3, which wouldn't currently possible with in-kernel mounting, though this feature is not terribly useful in that case as nested v3 mounts are rarer.

- the NFS mount can't allow redirection to other servers, unless those servers observe all the same filehandles.

- add a new export operation which accepts a filehandle and returns a superblock.

- In theory, this could work with a filesystem other than NFS, if there was a filesystem or group of filesystems that coordinated their filehandles.