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Date:   Thu, 15 Jul 2021 11:45:34 -0400
From:   "J. Bruce Fields" <bfields@fieldses.org>
To:     NeilBrown <neilb@suse.de>
Cc:     Chuck Lever <chuck.lever@oracle.com>, Chris Mason <clm@fb.com>,
        Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>, linux-nfs@vger.kernel.org,
        Wang Yugui <wangyugui@e16-tech.com>,
        Ulli Horlacher <framstag@rus.uni-stuttgart.de>,
        linux-btrfs@vger.kernel.org
Subject: Re: [PATCH/RFC] NFSD: handle BTRFS subvolumes better.
Message-ID: <20210715154534.GA24492@fieldses.org>
References: <20210613115313.BC59.409509F4@e16-tech.com>
 <20210310074620.GA2158@tik.uni-stuttgart.de>
 <162632387205.13764.6196748476850020429@noble.neil.brown.name>
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On Thu, Jul 15, 2021 at 02:37:52PM +1000, NeilBrown wrote:
> To fix this, we need to report a different fsid for each subvolume, but
> need to use the same fsid that we currently use for the top-level
> volume.  Changing this (by rebooting a server to new code), might
> confuse the client.  I don't think it would be a major problem (stale
> filehandles shouldn't happen), but it is best avoided.
...
> Again, we really want an API to get this from the filesystem.  Changing
> it later has no cost, so we don't need any commitment from the btrfs team
> that this is what they will provide if/when we do get such an API.

"No cost" makes me a little nervous, are we sure nobody will notice the
mountd-on-fileid changing?

Fileid and fsid changes I'd worry about more, though I wouldn't rule it
out if that'd stand in the way of a bug fix.

Thanks for looking into this.

--b.