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

[Ian Kent]

On 13/12/22 01:06, Jeff Layton wrote:
> 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>

Right, looks ok to me too, at least from the POV of that follow_down()

change.


Reviewed-by: Ian Kent <raven@xxxxxxxxxx>


Ian