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From:   "NeilBrown" <neilb@suse.de>
To:     "Frank Filz" <ffilzlnx@mindspring.com>
Cc:     "'J. Bruce Fields'" <bfields@fieldses.org>,
        "'Chuck Lever'" <chuck.lever@oracle.com>,
        linux-nfs@vger.kernel.org, "'Josef Bacik'" <josef@toxicpanda.com>
Subject: RE: [PATCH v2] BTRFS/NFSD: provide more unique inode number for btrfs export
In-reply-to: <005801d79b9d$b8437b80$28ca7280$@mindspring.com>
References: <162995209561.7591.4202079352301963089@noble.neil.brown.name>,
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 <20210826201916.GB10730@fieldses.org>,
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Date:   Sat, 28 Aug 2021 09:55:01 +1000
Message-id: <163010850148.7591.14454473128413118273@noble.neil.brown.name>
Precedence: bulk

On Sat, 28 Aug 2021, Frank Filz wrote:

> > On Sat, 28 Aug 2021, Frank Filz wrote:
> > >
> > > Changing the fsid for sub-volumes is Ganesha's solution (before adding
> > > that, we couldn't even export the sub-volumes at all).
> >=20
> > What does Ganesha use for the mounted-on-fileid? There doesn't seem to be=
 an
> > "obvious" answer so I wonder what was chosen.
>=20
> We only make mounted_on_fileid different from fileid on our export
> boundaries, and even then, it's not a terribly correct thing for
> FSAL_VFS (our module for interfacing with kernel filesystems) since
> user space to my knowledge has no way to get any information on an
> inode that serves as a mount point.

It is possible to see the mounted-on inode number by doing a readdir()
of the parent directory and looking at d_ino.  It is a bit round-about.

>=20
> What clients actually do anything with mounted_on_fileid, and what
> sorts of things do they do with it? I know the AIX client was
> interested in it (from having worked on security negotiation back in
> 2006), but I have never been able to test Ganesha with an AIX client.
> For normal Linux client operations, what Ganesha does seems to work
> OK.=20

On the Linux client, if you stat() a directory that is a mountpoint on
the server, you will see a directory with the inode number being the
mounted-on-fileid.  That directory is an automount-point, and when you
access anything in it, the 'real' directory gets mounted and the
mounted-on-fileid disappears.
So if you reported a mounted-on-fileid the same as the fileid, which
would be 256 on btrfs, du and find can get confused.  To be safe, the
mounted-of-fileid needs to be different to all ancestors.

NeilBrown