mq-deadline: don't call req_get_ioprio from the I/O completion handler

req_get_ioprio looks at req->bio to find the I/O priority, which is not
set when completing bios that the driver fully iterated through.

Stash away the dd_per_prio in the elevator private data instead of looking
it up again to optimize the code a bit while fixing the regression from
removing the per-request ioprio value.

Fixes: 6975c1a ("block: remove the ioprio field from struct request")
Reported-by: Chris Bainbridge <chris.bainbridge@gmail.com>
Reported-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Chris Bainbridge <chris.bainbridge@gmail.com>
Tested-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20241126102136.619067-1-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Christoph Hellwig

block/mq-deadline.c

@@ -685,10 +685,9 @@ static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
 
 	prio = ioprio_class_to_prio[ioprio_class];
 	per_prio = &dd->per_prio[prio];
-	if (!rq->elv.priv[0]) {
+	if (!rq->elv.priv[0])
 		per_prio->stats.inserted++;
-		rq->elv.priv[0] = (void *)(uintptr_t)1;
-	}
+	rq->elv.priv[0] = per_prio;
 
 	if (blk_mq_sched_try_insert_merge(q, rq, free))
 		return;
@@ -753,18 +752,14 @@ static void dd_prepare_request(struct request *rq)
  */
 static void dd_finish_request(struct request *rq)
 {
-	struct request_queue *q = rq->q;
-	struct deadline_data *dd = q->elevator->elevator_data;
-	const u8 ioprio_class = dd_rq_ioclass(rq);
-	const enum dd_prio prio = ioprio_class_to_prio[ioprio_class];
-	struct dd_per_prio *per_prio = &dd->per_prio[prio];
+	struct dd_per_prio *per_prio = rq->elv.priv[0];
 
 	/*
 	 * The block layer core may call dd_finish_request() without having
 	 * called dd_insert_requests(). Skip requests that bypassed I/O
 	 * scheduling. See also blk_mq_request_bypass_insert().
 	 */
-	if (rq->elv.priv[0])
+	if (per_prio)
 		atomic_inc(&per_prio->stats.completed);
 }