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Date:   Wed, 23 Jun 2021 18:25:59 -0400
From:   "J. Bruce Fields" <bfields@fieldses.org>
To:     NeilBrown <neilb@suse.de>
Cc:     Wang Yugui <wangyugui@e16-tech.com>, linux-nfs@vger.kernel.org
Subject: Re: any idea about auto export multiple btrfs snapshots?
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On Thu, Jun 24, 2021 at 08:04:57AM +1000, NeilBrown wrote:
> On Thu, 24 Jun 2021, J. Bruce Fields wrote:
> > Is there any hope of solving this problem within btrfs?
> > 
> > It doesn't seem like it should have been that difficult for it to give
> > subvolumes separate superblocks and vfsmounts.
> > 
> > But this has come up before, and I think the answer may have been that
> > it's just too late to fix.
> 
> It is never too late to do the right thing!
> 
> Probably the best approach to fixing this completely on the btrfs side
> would be to copy the auto-mount approach used in NFS.  NFS sees multiple
> different volumes on the server and transparently creates new vfsmounts,
> using the automount infrastructure to mount and unmount them.  BTRFS
> effective sees multiple volumes on the block device and could do the
> same thing.

Yes, that makes sense to me.

> I can only think of one change to the user-space API (other than
> /proc/mounts contents) that this would cause and I suspect it could be
> resolved if needed.
> 
> Currently when you 'stat' the mountpoint of a btrfs subvol you see the
> root of that subvol.  However when you 'stat' the mountpoint of an NFS
> sub-filesystem (before any access below there) you see the mountpoint
> (s_dev matches the parent).  This is how automounts are expected to work
> and if btrfs were switched to use automounts for subvols, stating the
> mountpoint would initially show the mountpoint, not the subvol root.
> 
> If this were seen to be a problem I doubt it would be hard to add
> optional functionality to automount so that 'stat' triggers the mount.

One other thing I'm not sure about: how do cold cache lookups of
filehandles for (possibly not-yet-mounted) subvolumes work?

> All we really need is:
> 1/ someone to write the code
> 2/ someone to review the code
> 3/ someone to accept the code

Hah.  Still, the special exceptions for btrfs seem to be accumulating.
I wonder if that's happening outside nfs as well.

--b.