Re: [RFC PATCH 0/3] Dealing with NFS re-export and cross mounts

["J. Bruce Fields" <bfields@fieldses.org>]

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.