I experienced an error where mkfs.btrfs failed when an unrelated cryptoloop device was mounted. $ sudo /tmp/btrfs-progs/mkfs.btrfs /dev/mapper/mydisk WARNING! - Btrfs v0.20-rc1-37-g91d9eec-dirty IS EXPERIMENTAL WARNING! - see http://btrfs.wiki.kernel.org before using error checking /dev/mapper/mydisk mount status $ sudo losetup -a /dev/loop0: [0014]:71314 (/home/michael/loopfile), encryption aes (type 18) I found that resolve_loop_device() method in utils.c was returning "aes" as the associated file, instead of /home/michael/loopfile. This eventually causes the realpath() call in is_same_blk_file() to fail. The attached patch fixes this problem. Regards, Michael McMaster