[dm-devel] [PATCH 14/15] block/bsg: move queue creation into bsg_setup_queue

[Christoph Hellwig]

On Wed, Jan 11, 2017 at 05:01:22PM -0500, Mike Snitzer wrote:
> But I've seen you reference the need to stop multipath from allocating
> its own requests.  Are you referring to old request_fn request-based
> multipath's clone_old_rq:alloc_old_clone_request?

Yes, that one is the issue.  It allocates a struct request "blind",
that is without known what queue it goes to.  With this queue (or blk-mq
for that matter) we need to know the queue, because the request structures
might have additional data behind it and require additional initialization
for drivers that require per-request data.  We make use of the per-request
data for SCSI passthrough in this patch.

> Or how blk-mq request-based multipath gets a request from the blk-mq tag
> space (via blk_mq_alloc_request)?

That's fine because it works on the queue of the device that I/O is
submitted to.