> This is a 4.4 feature -- stackable mounts. There was a paper about it
> in the Winter 1995 Usenix.
> 
> It's annoying, though, when I edit fstab and do mount -a, in which
> case it remounts multiple instances of some of the file systems. It
> perhaps should avoid mounting the _same_ file system twice on the same
> mount point.

I agree with this.

The mount itself is not a problem - you are permitted to mount something on
top of the root of a filesystem.

However, the exact trace of what happens worries me:

mount /dev/sd1a /foo
mount /dev/sd0a /foo
umount /dev/sd1a

I see the above as mounting sd0a on the root of the sd1a filesystem.  As
such, shouldn't sd1a become "busy", and therefore unmountable?

What happens if:

mount /dev/sd1a /foo
mount /dev/sd0a /foo/bar
umount /dev/sd1a

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David Jones, M.A.Sc student, Electronics Group (VLSI), University of Toronto
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