How to unmount device from a path hidden by another mounted device?

Question

I have olddevice mounted to /mnt with:

$ mount olddevice /mnt

I would like to change the device at /mnt mount point. For this purpose, I simply mount newdevice on top of it:

$ mount newdevice /mnt

With that, processes that still have file descriptors on olddevice can keeps working on old device, but new processes using /mnt will use newdevice.

I can detect when olddevice is not used anymore and decide to unmount it. My problem is how to unmount it:

$ umount olddevice
umount: olddevice: umount failed: Invalid argument.

Is it possible to directly unmount it? Or is it mandatory to unmount newdevice first (I do not want that) ?

Answer 1

If you are not already running with mount propagation enabled e.g. as per systemd defaults, run this first:

mount --make-rshared /

Then:

mkdir /root.orig
mount --rbind / /root.orig
mount --make-rprivate /root.orig/mnt

mount newdevice /mnt
...
umount -R /root.orig/mnt    # instead of umount olddevice

Then safely disassemble the magic - taking care not to unmount your entire system:

mount --make-rprivate /root.orig
umount -l /root.orig
rmdir /root.orig

---

Usually, I like to use the recursive variants of mount / umount commands. You say you started with only one filesystem mounted underneath /mnt. In the above sequence, I used umount -R /root.orig/mnt. If there was also a filesystem mounted on a subdirectory of /mnt, this umount -R command might fail half-way through. I.e. because there are no open files on the submount, but there are still some open files on the main mount. IMO this feels similar to how umount -l works. umount -l /path disassembles and detaches a mount tree, and each independent filesystem is shut down as soon as it has no open files.