On Mon, Jan 10, 2022 at 07:44:16PM +0100, Richard Weinberger wrote: > Currently when re-exporting a NFS share the NFS cross mount feature does > not work [0]. > This RFC patch series outlines an approach to address the problem. > > Crossing mounts does not work for two reasons: > > 1. As soon the NFS client (on the re-exporting server) sees a different > filesystem id, it installs an automount. That way the other filesystem > will be mounted automatically when someone enters the directory. > But the cross mount logic of KNFS does not know about automount. > The three patches in this series address the problem and teach both KNFSD > and the exportfs logic of NFS to deal with automount. > > 2. When KNFSD detects crossing of a mount point, it asks rpc.mountd to install > a new export for the target mount point. Beside of authentication rpc.mountd > also has to find a filesystem id for the new export. Is the to be exported > filesystem a NFS share, rpc.mountd cannot derive a filesystem id from it and > refuses to export. In the logs you’ll see error such as: > mountd: Cannot export /srv/nfs/vol0, possibly unsupported filesystem or fsid= required > To deal with that I changed rpc.mountd to use an arbitrary fsid. > Since this is a gross hack we need to agree on an approach to derive filesystem > ids for NFS mounts. > > rpc.mountd could: > a) re-use the fsid from the original NFS server. > Beside of requesting this information, the problem with that approach is > that the original fsid might conflict with an existing export. > b) derive the fsid from stat->st_dev. > c) allocate a free fsid. > > One use case to consider is load balancing. When multiple NFS servers re-export > a NFS mount, they need to use the same fsid for crossed mounts. > So I'm a little puzzled which approach is best. What do you think? > > Known issues: > - Only tested with NFSv3 (both server and client) so far. > > [0] https://marc.info/?l=linux-nfs&m=161653016627277&w=2 v4 testing would definitely be good, that's the case we'll care most about. --b.
