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

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Message-ID: <1AFA78FF-3F09-47E3-BE13-F5BB7F9C779C@oracle.com>
Date:   Sat, 10 Dec 2022 16:09:05 +0000
From:   Chuck Lever III <chuck.lever@...cle.com>
To:     Richard Weinberger <richard@....at>
CC:     Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Jeff Layton <jlayton@...nel.org>,
        Anna Schumaker <anna@...nel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Al Viro <viro@...iv.linux.org.uk>, Ian Kent <raven@...maw.net>,
        "chris.chilvers@...sbroker.com" <chris.chilvers@...sbroker.com>,
        "david.young@...sbroker.com" <david.young@...sbroker.com>,
        "luis.turcitu@...sbroker.com" <luis.turcitu@...sbroker.com>,
        "david@...ma-star.at" <david@...ma-star.at>,
        "benmaynard@...gle.com" <benmaynard@...gle.com>
Subject: Re: [PATCH 0/3 v2] NFS: NFSD: Allow crossing mounts when re-exporting



> On Dec 7, 2022, at 3:43 AM, Richard Weinberger <richard@....at> 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@nod.at/
> 
> Changes since v1:
> https://lore.kernel.org/linux-nfs/20221117191151.14262-1-richard@nod.at/
> 
> - 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(-)
> 
> -- 
> 2.26.2
> 

This series is a bit late for inclusion in v6.2. The next opportunity
will be v6.3 in a couple of months. I prefer to have a "final" version
of patches by -rc5.

I'm waiting for review comments on v2 of this series.


--
Chuck Lever