My system mounts the wrong btrfs partition, from the wrong disk!

["Niccolò Belli" <darkbasic@linuxsystems.it>]

This is something pretty unbelievable, so I had to repeat it several times 
before finding the courage to actually post it to the mailing list :)

After dozens of data loss I don't trust my btrfs partition that much, so I 
make a backup copy with dd weekly. Yesterday I was going to do some 
balancing and deduplication, but since the disk was full I had to remove 
the content of a whole subvolume (16GB) to make some space available to the 
tools (I had the backup made with dd on an external drive).

After the balance and deduplication I attached and mounted the external 
drive with the backup and then I mounted the img file with the copy of the 
partition. To my great wonder the subvolume in the backup which should have 
had the files I deleted was empty! So I rebooted to a live usb and mounted 
the backup again: my files were still there, phew! Then I mounted the 
partition in the laptop's disk and I tried to copy the files from the 
backup and it complained that they already existed! If I unmount both the 
backup and the real disk and then I mount the real disk it's empty again as 
it should.

These are the exact steps to reproduce it from the live usb:

# Opening the encrypted partition from the real disk and then mounting it
cryptsetup luksOpen /dev/sda5 cryptroot
mount -o noatime,compress=lzo,autodefrag /dev/mapper/cryptroot /real_disk

ls /real_disk/@Pictures --> empty (as it should)

# Mounting the external disk with the backup
mount /dev/sdb1 /external_disk

# Mounting the unencrypted backup from the external disk
mount /external_disk/backup.img /backup

ls /backup/@Pictures ---> empty (*it shouldn't!*)

umount /backup
umount /external_disk
cryptsetup luksClose cryptroot

# Mounting the external disk with the backup
mount /dev/sdb1 /external_disk

# Mounting the unencrypted backup from the external disk
mount /external_disk/backup.img /backup

ls /backup/@Pictures ---> 16GB of photos (as it should)

# Opening the encrypted partition from the real disk and then mounting it
cryptsetup luksOpen /dev/sda5 cryptroot
mount -o noatime,compress=lzo,autodefrag /dev/mapper/cryptroot /real_disk

ls /real_disk/@Pictures --> 16GB of photos (it *shouldn't!*)


I really don't know where the bug may lie, probably not even in btrfs but I 
didn't know where to report it. I'm using Archlinux with kernel 4.8.10 and 
the live is an Arch live usb with kernel 4.8 too.

Niccolò Belli