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From:   "NeilBrown" <neilb@suse.de>
To:     "Patrick Goetz" <pgoetz@math.utexas.edu>
Cc:     "J. Bruce Fields" <bfields@fieldses.org>,
        "Wang Yugui" <wangyugui@e16-tech.com>, linux-nfs@vger.kernel.org
Subject: Re: any idea about auto export multiple btrfs snapshots?
In-reply-to: <3ac9c4f3-6bc2-9753-6f0f-937aa4f13efa@math.utexas.edu>
References: <162432531379.17441.15110145423567943074@noble.neil.brown.name>,
 <20210622112253.DAEE.409509F4@e16-tech.com>,
 <20210622151407.C002.409509F4@e16-tech.com>,
 <162440994038.28671.7338874000115610814@noble.neil.brown.name>,
 <20210623153548.GF20232@fieldses.org>,
 <162448589701.28671.8402117125966499268@noble.neil.brown.name>,
 <3ac9c4f3-6bc2-9753-6f0f-937aa4f13efa@math.utexas.edu>
Date:   Fri, 25 Jun 2021 09:27:38 +1000
Message-id: <162457725899.28671.14092003979067994699@noble.neil.brown.name>
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On Fri, 25 Jun 2021, Patrick Goetz wrote:
> 
> On 6/23/21 5:04 PM, NeilBrown wrote:
> > 
> > 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.
> 
> I'm very confused about what you're talking about.  Is this documented 
> somewhere? I mean, I do use autofs, but see that as a separate software 
> system working with NFS.
> 

autofs (together with the user-space automountd) is a special filesystem
that provides automount functionality to the sysadmin.
It makes use of some core automount functionality in the Linux VFS.
This functionality is referred to as "managed" dentries.
See "Revalidation and automounts" in https://lwn.net/Articles/649115/.

autofs makes use of this functionality to provide automounts.  NFS makes
use of this same functionality to provide the same mount-point structure
on the client that it finds on the server.

I don't think there is any documentation specifically about NFS using
this infrastructure.  It should be largely transparent to users.

Suppose that on the server "/export/foo" is a mount of some
filesystem, and you nfs4 mount "server:/export" to "/import" on the
client.
Then you will at first see only "/import" in /proc/mounts on client.
If you "ls -ld /import/foo" you will still only see /import.
But if you "ls -l /import/foo" so it lists the contents of that other
filesytem, then check /proc/mounts, you will now see "/import" and
"/import/foo".

After a while (between 500 and 1000 seconds I think) of not accessing
/import/foo, that entry will disappear from /proc/mounts.

I'm sure you will recognise this as very similar to autofs behaviour.
It uses the same core functionality.  The timeout for inactive NFS
sub-filesystems to be unmounted can be controlled via
/proc/sys/fs/nfs/nfs_mountpoint_timeout and, since Linux 5.7, via the
nfs_mountpoint_expiry_timeout module parameter.
These aren't documented.

Note that I'm no longer sure that btrfs using automount like this would
actually make things easier for nfsd.  But in some ways I think it would
be the "right" thing to do.

NeilBrown