[for-4.16 PATCH v2 1/5] block: establish request failover callback

[Christoph Hellwig]

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.