NFS has nothing do with it. Right now, you can mount over any
directory you want!
The comment for do_mount in fs/super.c says:
* [21-Mar-97] T.Schoebel-Theuer: Now this can be overridden when
* supplying a leading "!" before the dir_name, allowing "stacks" of
* mounted filesystems. The stacking will only influence any pathname lookups
* _after_ the mount, but open file descriptors or working directories that
* are now covered remain valid. For example, when you overmount /home, any
* process with old cwd /home/joe will continue to use the old versions,
* as long as relative paths are used, but absolute paths like /home/joe/xxx
* will go to the new "top of stack" version. In general, crossing a
* mount point will always go to the top of stack element.
* Anyone using this new feature must know what he/she is doing.
I can't see any check for a `!' anywhere in the code; perhaps this
`feature' is now default behaviour?
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