Date: Mon, 24 May 2010 20:08:28 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Trond Myklebust <trond.myklebust@fys.uio.no>
Cc: Neil Brown <neilb@suse.de>, "Dr. J. Bruce Fields" <bfields@fieldses.org>,
        Chuck Lever <chuck.lever@oracle.com>, linux-nfs@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] VFS: fix recent breakage of FS_REVAL_DOT
Message-ID: <20100524190828.GS31073@ZenIV.linux.org.uk>
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On Mon, May 24, 2010 at 01:06:31PM -0400, Trond Myklebust wrote:

> I believe that the answer is that most filehandle types include an
> encoding of the inode number of the export directory. In other words, as
> long as '/a' and '/b' are different directories, then they will result
> in the generation of different filehandles for /a/x and /b/x.
> 
> It seems that is not always the case, though. According to the
> definition of mk_fsid(), it looks as if the 'FSID_UUID8' and
> 'FSID_UUID16' filehandle types only encode the uuid of the filesystem,
> and have no inode information. They will therefore not be able to
> distinguish between an export through '/a' or '/b'.
> 
> Neil, Bruce am I right?

Er?  On server:

mount -t ext2 /dev/sdb1 /srv/nfs4
mount -t ext2 /dev/sda1 /srv/nfs4/a
mount -t ext2 /dev/sda1 /srv/nfs4/b

after that /srv/nfs4/a and /srv/nfs4/b will have the *same* inode, nevermind
the inode number.  I really mean the same filesystem mounted twice; if you
want to include inumber of mountpoint into fsid, fine, turn the above into

mount -t ext2 /dev/sdb1 /srv/nfs4
mount -t ext2 /dev/sda1 /srv/nfs4/a
mount -t ext2 /dev/sda1 /srv/nfs4/b
mount -t ext2 /dev/sda3 /srv/nfs4/a/z
mount -t ext2 /dev/sda3 /srv/nfs4/b/z

At that point you have the same fs (ext2 from sda3) mounted on /srv/nfs4/a/z
and /srv/nfs4/b/z, with the same directory inode overmounted by it in both
mountpoints.  Suppose your referral point is on /a/z/x and /b/z/x resp. and
see the question upthread...