[dm-devel] [PATCH 07/18] dm: always defer request allocation to the owner of the request_queue

[Mike Snitzer]

On Wed, Jan 25 2017 at 12:25pm -0500,
Christoph Hellwig <hch at lst.de> wrote:

> DM already calls blk_mq_alloc_request on the request_queue of the
> underlying device if it is a blk-mq device.  But now that we allow drivers
> to allocate additional data and initialize it ahead of time we need to do
> the same for all drivers.   Doing so and using the new cmd_size
> infrastructure in the block layer greatly simplifies the dm-rq and mpath
> code, and should also make arbitrary combinations of SQ and MQ devices
> with SQ or MQ device mapper tables easily possible as a further step.
> 
> Signed-off-by: Christoph Hellwig <hch at lst.de>
> Reviewed-by: Hannes Reinecke <hare at suse.com>
> Reviewed-by: Mike Snitzer <snitzer at redhat.com>
...
> diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c
> index 3f12916..8d06834 100644
> --- a/drivers/md/dm-rq.c
> +++ b/drivers/md/dm-rq.c
> @@ -185,7 +163,7 @@ static void end_clone_bio(struct bio *clone)
>  
>  static struct dm_rq_target_io *tio_from_request(struct request *rq)
>  {
> -	return (rq->q->mq_ops ? blk_mq_rq_to_pdu(rq) : rq->special);
> +	return blk_mq_rq_to_pdu(rq);
>  }

Noticed after further review that it seems a bit weird to have the non
blk-mq support in drivers calling blk_mq_rq_to_pdu().  But I'm not sure
a blk_rq_to_pdu() macro to blk_mq_rq_to_pdu() is the right thing.  What
do you guys think?