Ext4 data corruption trouble [Updated]

(But, still, you do want to remount the loopback-mounted filesystem first, or that umount won't be able to do e.g. journal flushes...)

And the answer is no: it ends up calling do_emergency_remount(), which does a straight iteration over all super_blocks: there is no dependency analysis of any kind: I'd expect (given the way super_blocks is built) to unmount the backing store fs *before* the loopback-mounted fs.

(Perhaps do_emergency_remount() should iterate over super_blocks in reverse order?)