1 files changed, 106 insertions, 48 deletions

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 323c9cb28066..a12b1763508d 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -641,6 +641,14 @@ bool blk_mq_complete_request(struct request *rq)
 }
 EXPORT_SYMBOL(blk_mq_complete_request);
 
+/**
+ * blk_mq_start_request - Start processing a request
+ * @rq: Pointer to request to be started
+ *
+ * Function used by device drivers to notify the block layer that a request
+ * is going to be processed now, so blk layer can do proper initializations
+ * such as starting the timeout timer.
+ */
 void blk_mq_start_request(struct request *rq)
 {
 	struct request_queue *q = rq->q;
@@ -1327,6 +1335,12 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
 	return (queued + errors) != 0;
 }
 
+/**
+ * __blk_mq_run_hw_queue - Run a hardware queue.
+ * @hctx: Pointer to the hardware queue to run.
+ *
+ * Send pending requests to the hardware.
+ */
 static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
 {
 	int srcu_idx;
@@ -1424,6 +1438,15 @@ select_cpu:
 	return next_cpu;
 }
 
+/**
+ * __blk_mq_delay_run_hw_queue - Run (or schedule to run) a hardware queue.
+ * @hctx: Pointer to the hardware queue to run.
+ * @async: If we want to run the queue asynchronously.
+ * @msecs: Microseconds of delay to wait before running the queue.
+ *
+ * If !@async, try to run the queue now. Else, run the queue asynchronously and
+ * with a delay of @msecs.
+ */
 static void __blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async,
 					unsigned long msecs)
 {
@@ -1445,12 +1468,28 @@ static void __blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async,
 				    msecs_to_jiffies(msecs));
 }
 
+/**
+ * blk_mq_delay_run_hw_queue - Run a hardware queue asynchronously.
+ * @hctx: Pointer to the hardware queue to run.
+ * @msecs: Microseconds of delay to wait before running the queue.
+ *
+ * Run a hardware queue asynchronously with a delay of @msecs.
+ */
 void blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, unsigned long msecs)
 {
 	__blk_mq_delay_run_hw_queue(hctx, true, msecs);
 }
 EXPORT_SYMBOL(blk_mq_delay_run_hw_queue);
 
+/**
+ * blk_mq_run_hw_queue - Start to run a hardware queue.
+ * @hctx: Pointer to the hardware queue to run.
+ * @async: If we want to run the queue asynchronously.
+ *
+ * Check if the request queue is not in a quiesced state and if there are
+ * pending requests to be sent. If this is true, run the queue to send requests
+ * to hardware.
+ */
 void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
 {
 	int srcu_idx;
@@ -1474,6 +1513,11 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
 }
 EXPORT_SYMBOL(blk_mq_run_hw_queue);
 
+/**
+ * blk_mq_run_hw_queue - Run all hardware queues in a request queue.
+ * @q: Pointer to the request queue to run.
+ * @async: If we want to run the queue asynchronously.
+ */
 void blk_mq_run_hw_queues(struct request_queue *q, bool async)
 {
 	struct blk_mq_hw_ctx *hctx;
@@ -1625,7 +1669,11 @@ void __blk_mq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
 	blk_mq_hctx_mark_pending(hctx, ctx);
 }
 
-/*
+/**
+ * blk_mq_request_bypass_insert - Insert a request at dispatch list.
+ * @rq: Pointer to request to be inserted.
+ * @run_queue: If we should run the hardware queue after inserting the request.
+ *
  * Should only be used carefully, when the caller knows we want to
  * bypass a potential IO scheduler on the target device.
  */
@@ -1668,28 +1716,20 @@ static int plug_rq_cmp(void *priv, struct list_head *a, struct list_head *b)
 	struct request *rqa = container_of(a, struct request, queuelist);
 	struct request *rqb = container_of(b, struct request, queuelist);
 
-	if (rqa->mq_ctx < rqb->mq_ctx)
-		return -1;
-	else if (rqa->mq_ctx > rqb->mq_ctx)
-		return 1;
-	else if (rqa->mq_hctx < rqb->mq_hctx)
-		return -1;
-	else if (rqa->mq_hctx > rqb->mq_hctx)
-		return 1;
+	if (rqa->mq_ctx != rqb->mq_ctx)
+		return rqa->mq_ctx > rqb->mq_ctx;
+	if (rqa->mq_hctx != rqb->mq_hctx)
+		return rqa->mq_hctx > rqb->mq_hctx;
 
 	return blk_rq_pos(rqa) > blk_rq_pos(rqb);
 }
 
 void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 {
-	struct blk_mq_hw_ctx *this_hctx;
-	struct blk_mq_ctx *this_ctx;
-	struct request_queue *this_q;
-	struct request *rq;
 	LIST_HEAD(list);
-	LIST_HEAD(rq_list);
-	unsigned int depth;
 
+	if (list_empty(&plug->mq_list))
+		return;
 	list_splice_init(&plug->mq_list, &list);
 
 	if (plug->rq_count > 2 && plug->multiple_queues)
@@ -1697,42 +1737,27 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 
 	plug->rq_count = 0;
 
-	this_q = NULL;
-	this_hctx = NULL;
-	this_ctx = NULL;
-	depth = 0;
-
-	while (!list_empty(&list)) {
-		rq = list_entry_rq(list.next);
-		list_del_init(&rq->queuelist);
-		BUG_ON(!rq->q);
-		if (rq->mq_hctx != this_hctx || rq->mq_ctx != this_ctx) {
-			if (this_hctx) {
-				trace_block_unplug(this_q, depth, !from_schedule);
-				blk_mq_sched_insert_requests(this_hctx, this_ctx,
-								&rq_list,
-								from_schedule);
-			}
-
-			this_q = rq->q;
-			this_ctx = rq->mq_ctx;
-			this_hctx = rq->mq_hctx;
-			depth = 0;
+	do {
+		struct list_head rq_list;
+		struct request *rq, *head_rq = list_entry_rq(list.next);
+		struct list_head *pos = &head_rq->queuelist; /* skip first */
+		struct blk_mq_hw_ctx *this_hctx = head_rq->mq_hctx;
+		struct blk_mq_ctx *this_ctx = head_rq->mq_ctx;
+		unsigned int depth = 1;
+
+		list_for_each_continue(pos, &list) {
+			rq = list_entry_rq(pos);
+			BUG_ON(!rq->q);
+			if (rq->mq_hctx != this_hctx || rq->mq_ctx != this_ctx)
+				break;
+			depth++;
 		}
 
-		depth++;
-		list_add_tail(&rq->queuelist, &rq_list);
-	}
-
-	/*
-	 * If 'this_hctx' is set, we know we have entries to complete
-	 * on 'rq_list'. Do those.
-	 */
-	if (this_hctx) {
-		trace_block_unplug(this_q, depth, !from_schedule);
+		list_cut_before(&rq_list, &list, pos);
+		trace_block_unplug(head_rq->q, depth, !from_schedule);
 		blk_mq_sched_insert_requests(this_hctx, this_ctx, &rq_list,
 						from_schedule);
-	}
+	} while(!list_empty(&list));
 }
 
 static void blk_mq_bio_to_request(struct request *rq, struct bio *bio,
@@ -1828,6 +1853,17 @@ insert:
 	return BLK_STS_OK;
 }
 
+/**
+ * blk_mq_try_issue_directly - Try to send a request directly to device driver.
+ * @hctx: Pointer of the associated hardware queue.
+ * @rq: Pointer to request to be sent.
+ * @cookie: Request queue cookie.
+ *
+ * If the device has enough resources to accept a new request now, send the
+ * request directly to device driver. Else, insert at hctx->dispatch queue, so
+ * we can try send it another time in the future. Requests inserted at this
+ * queue have higher priority.
+ */
 static void blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 		struct request *rq, blk_qc_t *cookie)
 {
@@ -1905,6 +1941,22 @@ static void blk_add_rq_to_plug(struct blk_plug *plug, struct request *rq)
 	}
 }
 
+/**
+ * blk_mq_make_request - Create and send a request to block device.
+ * @q: Request queue pointer.
+ * @bio: Bio pointer.
+ *
+ * Builds up a request structure from @q and @bio and send to the device. The
+ * request may not be queued directly to hardware if:
+ * * This request can be merged with another one
+ * * We want to place request at plug queue for possible future merging
+ * * There is an IO scheduler active at this queue
+ *
+ * It will not queue the request if there is an error with the bio, or at the
+ * request creation.
+ *
+ * Returns: Request queue cookie.
+ */
 static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 {
 	const int is_sync = op_is_sync(bio->bi_opf);
@@ -1950,7 +2002,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 
 	plug = blk_mq_plug(q, bio);
 	if (unlikely(is_flush_fua)) {
-		/* bypass scheduler for flush rq */
+		/* Bypass scheduler for flush requests */
 		blk_insert_flush(rq);
 		blk_mq_run_hw_queue(data.hctx, true);
 	} else if (plug && (q->nr_hw_queues == 1 || q->mq_ops->commit_rqs ||
@@ -1978,6 +2030,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 
 		blk_add_rq_to_plug(plug, rq);
 	} else if (q->elevator) {
+		/* Insert the request at the IO scheduler queue */
 		blk_mq_sched_insert_request(rq, false, true, true);
 	} else if (plug && !blk_queue_nomerges(q)) {
 		/*
@@ -2004,8 +2057,13 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 		}
 	} else if ((q->nr_hw_queues > 1 && is_sync) ||
 			!data.hctx->dispatch_busy) {
+		/*
+		 * There is no scheduler and we can try to send directly
+		 * to the hardware.
+		 */
 		blk_mq_try_issue_directly(data.hctx, rq, &cookie);
 	} else {
+		/* Default case. */
 		blk_mq_sched_insert_request(rq, false, true, true);
 	}