From mboxrd@z Thu Jan  1 00:00:00 1970
From: Phillip Susi <psusi@cfl.rr.com>
Subject: Re: Snapshots
Date: Thu, 20 Dec 2007 14:45:54 -0500
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Jun'ichi Nomura wrote:
> So, you have to put the linear mapping *before* that.
> Then, the top-level block device will be locked but you can
> still change the underlying mappings.

So using a disk device as a dm target locks it.  DM won't use a disk
device as a target if it is locked.  Are you saying that dm can use
another dm device as a target, even if it is already locked by being
used as another dm target?