blk-mq: bypass IO scheduler's limit_depth for passthrough request

Commit 01e99ae ("blk-mq: insert passthrough request into
hctx->dispatch directly") gives high priority to passthrough requests and
bypass underlying IO scheduler. But as we allocate tag for such request it
still runs io-scheduler's callback limit_depth, while we really want is to
give full sbitmap-depth capabity to such request for acquiring available
tag.
blktrace shows PC requests(dmraid -s -c -i) hit bfq's limit_depth:
  8,0    2        0     0.000000000 39952 1,0  m   N bfq [bfq_limit_depth] wr_busy 0 sync 0 depth 8
  8,0    2        1     0.000008134 39952  D   R 4 [dmraid]
  8,0    2        2     0.000021538    24  C   R [0]
  8,0    2        0     0.000035442 39952 1,0  m   N bfq [bfq_limit_depth] wr_busy 0 sync 0 depth 8
  8,0    2        3     0.000038813 39952  D   R 24 [dmraid]
  8,0    2        4     0.000044356    24  C   R [0]

This patch introduce a new wrapper to make code not that ugly.

Signed-off-by: Lin Feng <linf@wangsu.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20210415033920.213963-1-linf@wangsu.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -361,11 +361,12 @@ static struct request *__blk_mq_alloc_request(struct blk_mq_alloc_data *data)
 
 	if (e) {
 		/*
-		 * Flush requests are special and go directly to the
+		 * Flush/passthrough requests are special and go directly to the
 		 * dispatch list. Don't include reserved tags in the
 		 * limiting, as it isn't useful.
 		 */
 		if (!op_is_flush(data->cmd_flags) &&
+		    !blk_op_is_passthrough(data->cmd_flags) &&
 		    e->type->ops.limit_depth &&
 		    !(data->flags & BLK_MQ_REQ_RESERVED))
 			e->type->ops.limit_depth(data->cmd_flags, data);

include/linux/blkdev.h

@@ -274,6 +274,12 @@ static inline bool bio_is_passthrough(struct bio *bio)
 	return blk_op_is_scsi(op) || blk_op_is_private(op);
 }
 
+static inline bool blk_op_is_passthrough(unsigned int op)
+{
+	return (blk_op_is_scsi(op & REQ_OP_MASK) ||
+			blk_op_is_private(op & REQ_OP_MASK));
+}
+
 static inline unsigned short req_get_ioprio(struct request *req)
 {
 	return req->ioprio;