Ext4 data corruption trouble [Updated]

It might be helpful to have a way to put all filesystems in a read only state, for the benefit of shutdown code that only requires (file level) read access, such as code to shutdown RAID devices.

A more general problem is that you might have loopback mounts and nested block devices, so what you really need is a combined operation that does the topological sort and quiesces filesystems and block devices in reverse stacking order.