On Fri, Jul 14 2017 at  3:12am -0400,
Christoph Hellwig [off-list ref] wrote:

Btw, we might want to expedite this to 4.13, a 4.13 now defaults
to blk-mq for scsi, and this patch would make sure that dm keeps
on just working with that switch.

Don't think we need to rush anything in response to that change in
SCSI's default.  old .request_fn DM multipath works happily ontop of
blk-mq devices (so long as all paths are blk-mq).

Similarly, blk-mq DM multipath works ontop of all blk-mq devices.

Lastly, old .request_fn DM multipath ontop of old .request_fn paths
works fine.

It is just blk-mq DM multipath ontop of old .request_fn paths that is
disallowed in current upstream code.

But again, I really don't see why we should even want/need to support
that mode... hence my question raised in this RFC.