Re: confusing mountinfo output when bind-mounting files

[Tycho Andersen <tycho.andersen@canonical.com>]

On Mon, Mar 21, 2016 at 11:22:06AM -0600, Chris Murphy wrote:
> On Mon, Mar 21, 2016 at 9:21 AM, Tycho Andersen
> <tycho.andersen@canonical.com> wrote:
> > Hi all,
> >
> > I'm seeing some strange behavior when bind mounting files from a btrfs
> > subvolume. Consider the output below:
> >
> > root@criu2:/tmp# mount -o loop /tmp/tester.btrfs /tmp/dir1
> > root@criu2:/tmp# touch dir1/file
> > root@criu2:/tmp# sudo mount --bind dir1/file dir2/file
> > root@criu2:/tmp# grep "/tmp/dir" /proc/self/mountinfo
> > 128 24 0:45 / /tmp/dir1 rw,relatime shared:107 - btrfs /dev/loop0 rw,space_cache,subvolid=5,subvol=/
> > 129 24 0:45 /file /tmp/dir2/file rw,relatime shared:107 - btrfs /dev/loop0 rw,space_cache,subvolid=5,subvol=/file
> > root@criu2:/tmp# btrfs --version
> > btrfs-progs v4.4
> > root@criu2:/tmp# uname -a
> > Linux criu2 4.4.0-8-generic #23-Ubuntu SMP Wed Feb 24 20:45:30 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
> >
> > The issue here is that the "subvol=" mount option for the target of the bind
> > mount is "/file" when no such subvolume actually exists. Is this
> > intended? It's confusing to say the least, but seems like a bug to me.
> 
> Since btrfs mount subvol=<name> is a bind mount behind the scene, I'm
> not sure the mount info code distinguishes between bind mounts.
> 
> At the moment, this is something of a secret decoder ring where if you
> see subvolid=5 first, then anything after that other than / is just
> not true (can't be). Hence probably why both subvolid and subvol are
> listed for now; you kinda have to parse them both.

Ok, so if I'm trying to compare superblocks from userspace, I should
look for subvolid and see if those match, and if that isn't present
then look for subvol?

Tycho