Re: [PATCH 0/3 v2] NFS: NFSD: Allow crossing mounts when re-exporting

[Jeff Layton]

On Wed, 2022-12-07 at 09:43 +0100, Richard Weinberger wrote:
> Currently when re-exporting a NFS share the NFS cross mount feature does
> not work [0].
> This 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.
> This patch series addresses 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 errors such as:
> 
> mountd: Cannot export /srv/nfs/vol0, possibly unsupported filesystem or fsid= required
> 
> To deal with that I've changed rpc.mountd to use generate and store fsids [1].
> Since the kernel side of my changes did change for a long time I decided to
> try upstreaming it first.
> A 3rd iteration of my rpc.mountd will happen soon.
> 
> [0] https://marc.info/?l=linux-nfs&m=161653016627277&w=2
> [1] https://lore.kernel.org/linux-nfs/20220217131531.2890-1-richard@xxxxxx/
> 
> Changes since v1:
> https://lore.kernel.org/linux-nfs/20221117191151.14262-1-richard@xxxxxx/
> 
> - Use LOOKUP_AUTOMOUNT only when NFSEXP_CROSSMOUNT is set (Jeff Layton)
> 
> Richard Weinberger (3):
>   NFSD: Teach nfsd_mountpoint() auto mounts
>   fs: namei: Allow follow_down() to uncover auto mounts
>   NFS: nfs_encode_fh: Remove S_AUTOMOUNT check
> 
>  fs/namei.c            | 6 +++---
>  fs/nfs/export.c       | 2 +-
>  fs/nfsd/vfs.c         | 8 ++++++--
>  include/linux/namei.h | 2 +-
>  4 files changed, 11 insertions(+), 7 deletions(-)
> 

This set looks reasonable to me.

Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>