3.5.0-rc6: btrfs and LVM snapshots -> wrong devicename in /proc/mounts

[Arnd Hannemann <arnd@arndnet.de>]

Hi,

using btrfs with LVM snapshots seems to be confusing /proc/mounts
After mounting a snapshot of an original filesystem, the devicename of the
original filesystem is overwritten with that of the snapshot in /proc/mounts.

Steps to reproduce:

arnd@kallisto:/mnt$ sudo mount /dev/vg0/original /mnt/original
[  107.041432] device fsid 5c3e8ca2-da56-4ade-9fef-103a6a8a70c2 devid 1 transid 4 /dev/mapper/vg0-original
[  107.044691] btrfs: no dev_stats entry found for device /dev/mapper/vg0-original (devid 1) (OK on first mount after mkfs)

arnd@kallisto:/mnt$ cat /proc/mounts  | grep /mnt
/dev/mapper/vg0-original /mnt/original btrfs rw,relatime,ssd,space_cache 0 0

arnd@kallisto:/mnt$ sudo lvcreate -L 10M --snap --name snapshot /dev/vg0/original
arnd@kallisto:/mnt$ sudo mount /dev/vg0/testsnap /mnt/testsnap
[  215.432281] device fsid 5c3e8ca2-da56-4ade-9fef-103a6a8a70c2 devid 1 transid 4 /dev/mapper/vg0-testsnap

arnd@kallisto:/mnt$ cat /proc/mounts  | grep /mnt
/dev/mapper/vg0-testsnap /mnt/original btrfs rw,relatime,ssd,space_cache 0 0
/dev/mapper/vg0-testsnap /mnt/testsnap btrfs rw,relatime,ssd,space_cache 0 0

arnd@kallisto:/mnt$ sudo umount /dev/vg0/testsnap
arnd@kallisto:/mnt$ cat /proc/mounts | grep /mnt
/dev/mapper/vg0-testsnap /mnt/original btrfs rw,relatime,ssd,space_cache 0 0


Best regards,
Arnd