bcache: fix bio_{start,end}_io_acct with proper device

Commit 85750ae ("bcache: use bio_{start,end}_io_acct") moves the
io account code to the location after bio_set_dev(bio, dc->bdev) in
cached_dev_make_request(). Then the account is performed incorrectly on
backing device, indeed the I/O should be counted to bcache device like
/dev/bcache0.

With the mistaken I/O account, iostat does not display I/O counts for
bcache device and all the numbers go to backing device. In writeback
mode, the hard drive may have 340K+ IOPS which is impossible and wrong
for spinning disk.

This patch introduces bch_bio_start_io_acct() and bch_bio_end_io_acct(),
which switches bio->bi_disk to bcache device before calling
bio_start_io_acct() or bio_end_io_acct(). Now the I/Os are counted to
bcache device, and bcache device, cache device and backing device have
their correct I/O count information back.

Fixes: 85750ae ("bcache: use bio_{start,end}_io_acct")
Signed-off-by: Coly Li <colyli@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Coly Li

drivers/md/bcache/request.c

@@ -617,6 +617,28 @@ static void cache_lookup(struct closure *cl)
 
 /* Common code for the make_request functions */
 
+static inline void bch_bio_start_io_acct(struct gendisk *acct_bi_disk,
+					 struct bio *bio,
+					 unsigned long *start_time)
+{
+	struct gendisk *saved_bi_disk = bio->bi_disk;
+
+	bio->bi_disk = acct_bi_disk;
+	*start_time = bio_start_io_acct(bio);
+	bio->bi_disk = saved_bi_disk;
+}
+
+static inline void bch_bio_end_io_acct(struct gendisk *acct_bi_disk,
+				       struct bio *bio,
+				       unsigned long start_time)
+{
+	struct gendisk *saved_bi_disk = bio->bi_disk;
+
+	bio->bi_disk = acct_bi_disk;
+	bio_end_io_acct(bio, start_time);
+	bio->bi_disk = saved_bi_disk;
+}
+
 static void request_endio(struct bio *bio)
 {
 	struct closure *cl = bio->bi_private;
@@ -668,7 +690,7 @@ static void backing_request_endio(struct bio *bio)
 static void bio_complete(struct search *s)
 {
 	if (s->orig_bio) {
-		bio_end_io_acct(s->orig_bio, s->start_time);
+		bch_bio_end_io_acct(s->d->disk, s->orig_bio, s->start_time);
 		trace_bcache_request_end(s->d, s->orig_bio);
 		s->orig_bio->bi_status = s->iop.status;
 		bio_endio(s->orig_bio);
@@ -728,7 +750,7 @@ static inline struct search *search_alloc(struct bio *bio,
 	s->recoverable		= 1;
 	s->write		= op_is_write(bio_op(bio));
 	s->read_dirty_data	= 0;
-	s->start_time		= bio_start_io_acct(bio);
+	bch_bio_start_io_acct(d->disk, bio, &s->start_time);
 
 	s->iop.c		= d->c;
 	s->iop.bio		= NULL;
@@ -1080,7 +1102,7 @@ static void detached_dev_end_io(struct bio *bio)
 	bio->bi_end_io = ddip->bi_end_io;
 	bio->bi_private = ddip->bi_private;
 
-	bio_end_io_acct(bio, ddip->start_time);
+	bch_bio_end_io_acct(ddip->d->disk, bio, ddip->start_time);
 
 	if (bio->bi_status) {
 		struct cached_dev *dc = container_of(ddip->d,
@@ -1105,7 +1127,8 @@ static void detached_dev_do_request(struct bcache_device *d, struct bio *bio)
 	 */
 	ddip = kzalloc(sizeof(struct detached_dev_io_private), GFP_NOIO);
 	ddip->d = d;
-	ddip->start_time = bio_start_io_acct(bio);
+	bch_bio_start_io_acct(d->disk, bio, &ddip->start_time);
+
 	ddip->bi_end_io = bio->bi_end_io;
 	ddip->bi_private = bio->bi_private;
 	bio->bi_end_io = detached_dev_end_io;