What kinds of things... (really: nfs mounting files)

[Guy Harris]

>I too was surprised to find it work (SunOS 4.0 mounted a DG/UX 4.02 file).
>But if you stop and think that all diskless clients 'mount' a swap _file_
>from the server, it makes some sense that it should work.

The quotes around "mount" are significant.  There's no direct connection
between the fact that mounting a file accessed over NFS atop a directory
works and that attaching a file accessed over NFS as a swap area works. 
"mounting" a swap file means "getting a file handle for it and
associating it with a "swap area" entry; reads from and writes to that
swap area get turned into NFS "read" and "write" calls using that file
handle. 

"mounting" something on top of a directory, if the thing being mounted
is being mounted over NFS, means "getting a file handle for that
something and flagging that something as being 'mounted on' the
directory".  There's no reason why it must *ipso facto* be possible to
mount a non-directory file atop a directory; the implementation happens
to let it work, and this may even be deliberate, but I could imagine an
implementation that supported attaching files accessed over NFS as swap
areas but that got confused by an attempt to mount a file atop a
directory (it might not be a *good* implementation, but then I didn't
say it was...).