On Mon, Mar 21, 2016 at 9:21 AM, Tycho Andersen <tycho.ander...@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. -- Chris Murphy
