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From:   Amir Goldstein <amir73il@gmail.com>
Date:   Wed, 28 Jul 2021 13:13:10 +0300
Message-ID: <CAOQ4uxjXcVE=4K+3uSYXLsvGgi0o7Nav=DsV=0qG_DanjXB18Q@mail.gmail.com>
Subject: Re: [PATCH 07/11] exportfs: Allow filehandle lookup to cross internal
 mount points.
To:     NeilBrown <neilb@suse.de>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Josef Bacik <josef@toxicpanda.com>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Chuck Lever <chuck.lever@oracle.com>, Chris Mason <clm@fb.com>,
        David Sterba <dsterba@suse.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        Linux Btrfs <linux-btrfs@vger.kernel.org>,
        Miklos Szeredi <miklos@szeredi.hu>
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On Wed, Jul 28, 2021 at 1:44 AM NeilBrown <neilb@suse.de> wrote:
>
> When a filesystem has internal mounts, it controls the filehandles
> across all those mounts (subvols) in the filesystem.  So it is useful to
> be able to look up a filehandle again one mount, and get a result which
> is in a different mount (part of the same overall file system).
>
> This patch makes that possible by changing export_decode_fh() and
> export_decode_fh_raw() to take a vfsmount pointer by reference, and
> possibly change the vfsmount pointed to before returning.
>
> The core of the change is in reconnect_path() which now not only checks
> that the dentry is fully connected, but also that the vfsmnt reported
> has the same 'dev' (reported by vfs_getattr) as the dentry.
> If it doesn't, we walk up the dparent() chain to find the highest place
> where the dev changes without there being a mount point, and trigger an
> automount there.
>
> As no filesystems yet provide local-mounts, this does not yet change any
> behaviour.
>
> In exportfs_decode_fh_raw() we previously tested for DCACHE_DISCONNECT
> before calling reconnect_path().  That test is dropped.  It was only a
> minor optimisation and is now inconvenient.
>
> The change in overlayfs needs more careful thought than I have yet given
> it.

Just note that overlayfs does not support following auto mounts in layers.
See ovl_dentry_weird(). ovl_lookup() fails if it finds such a dentry.
So I think you need to make sure that the vfsmount was not crossed
when decoding an overlayfs real fh.

Apart from that, I think that your new feature should be opt-in w.r.t
the exportfs_decode_fh() vfs api and that overlayfs should not opt-in
for the cross mount decode.

Thanks,
Amir.