linux-kernel - Re: [PATCH/RFC] make btrfs subvol mounts appear in /proc/mounts

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Message-ID: <87ziyvj20o.fsf@notabene.neil.brown.name>
Date:	Tue, 03 Nov 2015 12:38:15 +1100
From:	Neil Brown <neil@...wn.name>
To:	Chris Mason <clm@...com>, "J. Bruce Fields" <bfields@...ldses.org>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Linux btrfs Developers List <linux-btrfs@...r.kernel.org>,
	Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH/RFC] make btrfs subvol mounts appear in /proc/mounts

On Tue, Nov 03 2015, Chris Mason wrote:

> On Mon, Nov 02, 2015 at 03:50:12PM -0500, J. Bruce Fields wrote:
>> On Wed, Oct 28, 2015 at 07:25:10AM +0900, Neil Brown wrote:
>> > 
>> > If you create a subvolume in btrfs and access it (by name) without
>> > mounting it, then the subvolume looks like a separate mount to some
>> > extent, returning a different st_dev to stat(), but it doesn't look like
>> > a separate mount in that it isn't listed in /proc/mounts. This
>> > inconsistency can confuse tools.
>> > 
>> > This patch causes these subvolumes to become separate mounts by using
>> > the VFS' automount functionality, much like NFS uses automount when it
>> > discovered mountpoints on the server.
>> > 
>> > The VFS currently makes it impossible to auto-mount a directory on to itself
>> > (i.e. a bind mount).  For NFS this isn't a problem as a new superblock
>> > is created for the child filesystem so there are two separate dentries
>> > (and inodes) for the one directory: one in the parent filesystem, one in
>> > the child (note that the two superblocks share a common connection to
>> > the server so there is still a lot of commonality).
>> > 
>> > BTRFS has chosen instead to use a single superblock for all subvolumes.
>> 
>> Naive question: was there a reason for that choice?
>
> They are really all part of the same FS, the single super better fits.
> Or said another way, it felt like there would be dramatically more duct
> tape around supers-per-subvolume than there was abusing st_dev.
>
> Neil's patch came up after I told him a few of us had tried to do the
> same thing and failed to find clean vfs changes to make it possible...he
> took it as a challenge.  Now I have to remember what it was about our
> past attempts that I didn't like.
>
> I'll test this and queue for 4.5 if it all works out, thanks Neil!

I'd rather resend with proper documentation updates and s-o-b before it
gets queued if that is OK.  So once you are happy, please let me know
and I'll do it "properly".

Thanks,
NeilBrown

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