On Fri, Mar 12, 2021 at 06:44:55PM +0300, Sergei Shtepa wrote:

DM_INTERPOSED_FLAG allow to create DM targets on "the fly".
Underlying block device opens without a flag FMODE_EXCL.
DM target receives bio from the original device via
bdev_interposer.

This is more of a philopical comment, but the idea of just letting the
interposed reopen the device by itself seems like a bad idea.  I think
that is probably better hidden in the block layer interposer attachment
function, which could do the extra blkdev_get_by_dev for the caller.