On Tue, Dec 26, 2017 at 10:22:53PM -0500, Mike Snitzer wrote:

All requests allocated from a request_queue with this callback set can
failover their requests during completion.

This callback is expected to use the blk_steal_bios() interface to
transfer a request's bios back to an upper-layer bio-based
request_queue.

This will be used by both NVMe multipath and DM multipath.  Without it
DM multipath cannot get access to NVMe-specific error handling that NVMe
core provides in nvme_complete_rq().

And the whole point is that it should not get any such access.

The reason why we did nvme multipathing differently is because the
design of dm-multipath inflicts so much pain on users that we absolutely
want to avoid it this time around.