Using an upper layer path and/or a workdir path that are already used by
another overlay mount is not allowed and may fail with EBUSY. Using
-partially overlapping paths is not allowed but will not fail with EBUSY.
+partially overlapping paths is not allowed and may fail with EBUSY.
If files are accessed from two overlayfs mounts which share or overlap the
upper layer and/or workdir path the behavior of the overlay is undefined,
though it will not result in a crash or deadlock.