block: add a blk_account_io_merge_bio helper

Move the non-"new_io" branch of blk_account_io_start() into separate
function.  Fix merge accounting for discards (they were counted as write
merges).

The new blk_account_io_merge_bio() doesn't call update_io_ticks() unlike
blk_account_io_start(), as there is no reason for that.

[hch: rebased]
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -636,6 +636,16 @@ void blk_put_request(struct request *req)
 }
 EXPORT_SYMBOL(blk_put_request);
 
+static void blk_account_io_merge_bio(struct request *req)
+{
+	if (!blk_do_io_stat(req))
+		return;
+
+	part_stat_lock();
+	part_stat_inc(req->part, merges[op_stat_group(req_op(req))]);
+	part_stat_unlock();
+}
+
 bool bio_attempt_back_merge(struct request *req, struct bio *bio,
 		unsigned int nr_segs)
 {
@@ -656,7 +666,7 @@ bool bio_attempt_back_merge(struct request *req, struct bio *bio,
 
 	bio_crypt_free_ctx(bio);
 
-	blk_account_io_start(req, false);
+	blk_account_io_merge_bio(req);
 	return true;
 }
 
@@ -682,7 +692,7 @@ bool bio_attempt_front_merge(struct request *req, struct bio *bio,
 
 	bio_crypt_do_front_merge(req, bio);
 
-	blk_account_io_start(req, false);
+	blk_account_io_merge_bio(req);
 	return true;
 }
 
@@ -704,7 +714,7 @@ bool bio_attempt_discard_merge(struct request_queue *q, struct request *req,
 	req->__data_len += bio->bi_iter.bi_size;
 	req->nr_phys_segments = segments + 1;
 
-	blk_account_io_start(req, false);
+	blk_account_io_merge_bio(req);
 	return true;
 no_merge:
 	req_set_nomerge(q, req);
@@ -1329,7 +1339,7 @@ blk_status_t blk_insert_cloned_request(struct request_queue *q, struct request *
 		return BLK_STS_IOERR;
 
 	if (blk_queue_io_stat(q))
-		blk_account_io_start(rq, true);
+		blk_account_io_start(rq);
 
 	/*
 	 * Since we have a scheduler attached on the top device,
@@ -1433,16 +1443,13 @@ void blk_account_io_done(struct request *req, u64 now)
 	}
 }
 
-void blk_account_io_start(struct request *rq, bool new_io)
+void blk_account_io_start(struct request *rq)
 {
 	if (!blk_do_io_stat(rq))
 		return;
 
 	part_stat_lock();
-	if (!new_io)
-		part_stat_inc(rq->part, merges[rq_data_dir(rq)]);
-	else
-		rq->part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
+	rq->part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
 	update_io_ticks(rq->part, jiffies, false);
 	part_stat_unlock();
 }

block/blk-exec.c

@@ -55,7 +55,7 @@ void blk_execute_rq_nowait(struct request_queue *q, struct gendisk *bd_disk,
 	rq->rq_disk = bd_disk;
 	rq->end_io = done;
 
-	blk_account_io_start(rq, true);
+	blk_account_io_start(rq);
 
 	/*
 	 * don't check dying flag for MQ because the request won't

block/blk-mq.c

@@ -1822,7 +1822,7 @@ static void blk_mq_bio_to_request(struct request *rq, struct bio *bio,
 	blk_rq_bio_prep(rq, bio, nr_segs);
 	blk_crypto_rq_bio_prep(rq, bio, GFP_NOIO);
 
-	blk_account_io_start(rq, true);
+	blk_account_io_start(rq);
 }
 
 static blk_status_t __blk_mq_issue_directly(struct blk_mq_hw_ctx *hctx,

block/blk.h

@@ -185,7 +185,7 @@ bool bio_attempt_discard_merge(struct request_queue *q, struct request *req,
 bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
 		unsigned int nr_segs, struct request **same_queue_rq);
 
-void blk_account_io_start(struct request *req, bool new_io);
+void blk_account_io_start(struct request *req);
 void blk_account_io_done(struct request *req, u64 now);
 
 /*