How do I get the current subvolid of a mounted btrfs?

Question

How do you get the top level subvolid of a mounted btrfs? I want the actual subvolid that is mounted at this moment and not the default subvolid (which could be different.)

Answer 1

Look at /proc/mounts or the output of mount, the subvolid is always listed in the mount options.

/dev/loop0 /mnt/btrfs btrfs rw,relatime,space_cache,subvolid=5,subvol=/ 0 0
                                                    ^^^^^^^^^^