[PATCH v4 3/7] block: Send requeued requests to the I/O scheduler

[Bart Van Assche <bvanassche@acm.org>]

Send requeued requests to the I/O scheduler if the dispatch order
matters such that the I/O scheduler can control the order in which
requests are dispatched.

This patch reworks commit aef1897cd36d ("blk-mq: insert rq with DONTPREP
to hctx dispatch list when requeue"). Instead of sending DONTPREP
requests to the dispatch list, send these to the I/O scheduler and
prevent that the I/O scheduler merges these requests by adding
RQF_DONTPREP to the list of flags that prevent merging
(RQF_NOMERGE_FLAGS).

Cc: Christoph Hellwig <hch@lst.de>
Cc: Damien Le Moal <dlemoal@kernel.org>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 block/blk-mq.c         | 10 +++++-----
 include/linux/blk-mq.h |  4 ++--
 2 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index f440e4aaaae3..453a90767f7a 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1453,13 +1453,13 @@ static void blk_mq_requeue_work(struct work_struct *work)
 	while (!list_empty(&requeue_list)) {
 		rq = list_entry(requeue_list.next, struct request, queuelist);
 		/*
-		 * If RQF_DONTPREP ist set, the request has been started by the
-		 * driver already and might have driver-specific data allocated
-		 * already.  Insert it into the hctx dispatch list to avoid
-		 * block layer merges for the request.
+		 * Only send those RQF_DONTPREP requests to the dispatch list
+		 * that may be reordered freely. If the request order matters,
+		 * send the request to the I/O scheduler.
 		 */
 		list_del_init(&rq->queuelist);
-		if (rq->rq_flags & RQF_DONTPREP)
+		if (rq->rq_flags & RQF_DONTPREP &&
+		    !op_needs_zoned_write_locking(req_op(rq)))
 			blk_mq_request_bypass_insert(rq, 0);
 		else
 			blk_mq_insert_request(rq, BLK_MQ_INSERT_AT_HEAD);
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index f401067ac03a..2610b299ec77 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -62,8 +62,8 @@ typedef __u32 __bitwise req_flags_t;
 #define RQF_RESV		((__force req_flags_t)(1 << 23))
 
 /* flags that prevent us from merging requests: */
-#define RQF_NOMERGE_FLAGS \
-	(RQF_STARTED | RQF_FLUSH_SEQ | RQF_SPECIAL_PAYLOAD)
+#define RQF_NOMERGE_FLAGS                                               \
+	(RQF_STARTED | RQF_FLUSH_SEQ | RQF_DONTPREP | RQF_SPECIAL_PAYLOAD)
 
 enum mq_rq_state {
 	MQ_RQ_IDLE		= 0,