confusing mountinfo output when bind-mounting files

From: Tycho Andersen
Date: Mon Mar 21 2016 - 11:21:53 EST

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.