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From:   "NeilBrown" <neilb@suse.de>
To:     "Wang Yugui" <wangyugui@e16-tech.com>
Cc:     linux-nfs@vger.kernel.org
Subject: Re: any idea about auto export multiple btrfs snapshots?
In-reply-to: <20210617122852.BE6A.409509F4@e16-tech.com>
References: <20210615231318.F40F.409509F4@e16-tech.com>,
 <162389895501.29912.12470238090250719500@noble.neil.brown.name>
Date:   Fri, 18 Jun 2021 10:32:56 +1000
Message-id: <162397637680.29912.2268876490205517592@noble.neil.brown.name>
Precedence: bulk

On Thu, 17 Jun 2021, Wang Yugui wrote:
> > Can we go back to the beginning.  What, exactly, is the problem you are
> > trying to solve?  How can you demonstrate the problem?
> >=20
> > NeilBrown
>=20
> I nfs/exported a btrfs with 2 subvols and 2 snapshot(subvol).
>=20
> # btrfs subvolume list /mnt/test
> ID 256 gen 53 top level 5 path sub1
> ID 260 gen 56 top level 5 path sub2
> ID 261 gen 57 top level 5 path .snapshot/sub1-s1
> ID 262 gen 57 top level 5 path .snapshot/sub2-s1
>=20
> and then mount.nfs4 it to /nfs/test.
>=20
> # /bin/find /nfs/test/
> /nfs/test/
> find: File system loop detected; '/nfs/test/sub1' is part of the same file =
system loop as '/nfs/test/'.
> /nfs/test/.snapshot
> find: File system loop detected; '/nfs/test/.snapshot/sub1-s1' is part of t=
he same file system loop as '/nfs/test/'.
> find: File system loop detected; '/nfs/test/.snapshot/sub2-s1' is part of t=
he same file system loop as '/nfs/test/'.
> /nfs/test/dir1
> /nfs/test/dir1/a.txt
> find: File system loop detected; '/nfs/test/sub2' is part of the same file =
system loop as '/nfs/test/'
>=20
> /bin/find report 'File system loop detected'. so I though there is
> something wrong.

Certainly something is wrong.  The error message implies that some
directory is reporting the same dev an ino as an ancestor directory.
Presumably /nfs/test and /nfs/test/sub1.
Can you confirm that please. e.g. run the command

   stat /nfs/test /nfs/test/sub1

and examine the output.

As sub1 is considered a different file system, it should have a
different dev number.  NFS will assign a different device number only
when the server reports a different fsid.  The Linux NFS server will
report a different fsid if d_mountpoint() is 'true' for the dentry, and
follow_down() results in no change the the vfsmnt,dentry in a 'struct
path'.

You have already said that d_mountpoint doesn't work for btrfs, so that
is part of the problem.  NFSD doesn't trust d_mountpoint completely as
it only reports that the dentry is a mountpoint in some namespace, not
necessarily in this namespace.  So you really need to fix
nfsd_mountpoint.

I suggest you try adding your "dirty fix" to nfsd_mountpoint() so that
it reports the root of a btrfs subvol as a mountpoint, and see if that
fixes the problem.  It should change the problem at least.  You would
need to get nfsd_mountpoint() to return '1' in this case, not '2'.

NeilBrown