I'm worried that the 'dm-' prefix is really indicating that devicemapper is being used, but the rest of the logic is assuming this is a multipath device.  What about other dm stuff like dmcrypt?  A more detailed description of what is going on in the commit message or a comment would help here.

BTW, the commit also needs a Signed-off-by line before we can merge it... see teh SubmittingPatches file in teh root dir.

Thanks!