[PATCH 02/12] block: Send flush requests to the I/O scheduler

[Bart Van Assche <bvanassche@xxxxxxx>]

Prevent that zoned writes with the FUA flag set are reordered against each
other or against other zoned writes. Separate the I/O scheduler members
from the flush members in struct request since with this patch applied a
request may pass through both an I/O scheduler and the flush machinery.

Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Damien Le Moal <damien.lemoal@xxxxxxxxxxxxxxxxxx>
Cc: Ming Lei <ming.lei@xxxxxxxxxx>
Cc: Mike Snitzer <snitzer@xxxxxxxxxx>
Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
---
 block/blk-flush.c      |  3 ++-
 block/blk-mq.c         | 11 ++++-------
 block/mq-deadline.c    |  2 +-
 include/linux/blk-mq.h | 27 +++++++++++----------------
 4 files changed, 18 insertions(+), 25 deletions(-)

diff --git a/block/blk-flush.c b/block/blk-flush.c
index 53202eff545e..e0cf153388d8 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -432,7 +432,8 @@ void blk_insert_flush(struct request *rq)
 	 */
 	if ((policy & REQ_FSEQ_DATA) &&
 	    !(policy & (REQ_FSEQ_PREFLUSH | REQ_FSEQ_POSTFLUSH))) {
-		blk_mq_request_bypass_insert(rq, false, true);
+		blk_mq_sched_insert_request(rq, /*at_head=*/false,
+					    /*run_queue=*/true, /*async=*/true);
 		return;
 	}
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index fefc9a728e0e..250556546bbf 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -390,8 +390,7 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
 		INIT_HLIST_NODE(&rq->hash);
 		RB_CLEAR_NODE(&rq->rb_node);
 
-		if (!op_is_flush(data->cmd_flags) &&
-		    e->type->ops.prepare_request) {
+		if (e->type->ops.prepare_request) {
 			e->type->ops.prepare_request(rq);
 			rq->rq_flags |= RQF_ELVPRIV;
 		}
@@ -452,13 +451,11 @@ static struct request *__blk_mq_alloc_requests(struct blk_mq_alloc_data *data)
 		data->rq_flags |= RQF_ELV;
 
 		/*
-		 * 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.
+		 * Do not limit the depth for passthrough requests nor for
+		 * requests with a reserved tag.
 		 */
-		if (!op_is_flush(data->cmd_flags) &&
+		if (e->type->ops.limit_depth &&
 		    !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);
 	}
diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index f10c2a0d18d4..d885ccf49170 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -789,7 +789,7 @@ 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] && !(rq->rq_flags & RQF_FLUSH_SEQ)) {
 		per_prio->stats.inserted++;
 		rq->elv.priv[0] = (void *)(uintptr_t)1;
 	}
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 06caacd77ed6..5e6c79ad83d2 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -169,25 +169,20 @@ struct request {
 		void *completion_data;
 	};
 
-
 	/*
 	 * Three pointers are available for the IO schedulers, if they need
-	 * more they have to dynamically allocate it.  Flush requests are
-	 * never put on the IO scheduler. So let the flush fields share
-	 * space with the elevator data.
+	 * more they have to dynamically allocate it.
 	 */
-	union {
-		struct {
-			struct io_cq		*icq;
-			void			*priv[2];
-		} elv;
-
-		struct {
-			unsigned int		seq;
-			struct list_head	list;
-			rq_end_io_fn		*saved_end_io;
-		} flush;
-	};
+	struct {
+		struct io_cq		*icq;
+		void			*priv[2];
+	} elv;
+
+	struct {
+		unsigned int		seq;
+		struct list_head	list;
+		rq_end_io_fn		*saved_end_io;
+	} flush;
 
 	union {
 		struct __call_single_data csd;