[PATCH] blk: fix a wrong accounting of hd_struct->in_flight

[Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>]

From: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>

/proc/diskstats would display a strange output as follows.

$ cat /proc/diskstats |grep sda
   8       0 sda 90524 7579 102154 20464 0 0 0 0 0 14096 20089
   8       1 sda1 19085 1352 21841 4209 0 0 0 0 4294967064 15689 4293424691
                                                ~~~~~~~~~~
   8       2 sda2 71252 3624 74891 15950 0 0 0 0 232 23995 1562390
   8       3 sda3 54 487 2188 92 0 0 0 0 0 88 92
   8       4 sda4 4 0 8 0 0 0 0 0 0 0 0
   8       5 sda5 81 2027 2130 138 0 0 0 0 0 87 137

Its reason is the wrong way of accounting hd_struct->in_flight. When a bio is
merged into a request belongs to different partition by ELEVATOR_FRONT_MERGE.

The detailed root cause is as follows.

Assuming that there are two partition, sda1 and sda2.

1. A request for sda2 is in request_queue. Hence sda1's hd_struct->in_flight
   is 0 and sda2's one is 1.

        | hd_struct->in_flight
   ---------------------------
   sda1 |          0
   sda2 |          1
   ---------------------------

2. A bio belongs to sda1 is issued and is merged into the request mentioned on
   step1 by ELEVATOR_BACK_MERGE. The first sector of the request is changed
   from sda2 region to sda1 region. However the two partition's
   hd_struct->in_flight are not changed.

        | hd_struct->in_flight
   ---------------------------
   sda1 |          0
   sda2 |          1
   ---------------------------

3. The request is finished and blk_account_io_done() is called. In this case,
   sda2's hd_struct->in_flight, not a sda1's one, is decremented.

        | hd_struct->in_flight
   ---------------------------
   sda1 |         -1
   sda2 |          1
   ---------------------------

The patch fixes the problem.

Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
---
 block/blk-core.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

Index: linux-2.6.36-rc7/block/blk-core.c
===================================================================
--- linux-2.6.36-rc7.orig/block/blk-core.c	2010-10-07 05:39:52.000000000 +0900
+++ linux-2.6.36-rc7/block/blk-core.c	2010-10-09 05:53:51.000000000 +0900
@@ -1202,6 +1202,8 @@ static int __make_request(struct request
 	const bool unplug = !!(bio->bi_rw & REQ_UNPLUG);
 	const unsigned long ff = bio->bi_rw & REQ_FAILFAST_MASK;
 	int rw_flags;
+	struct hd_struct *src_part;
+	struct hd_struct *dst_part;

 	if ((bio->bi_rw & REQ_HARDBARRIER) &&
 	    (q->next_ordered == QUEUE_ORDERED_NONE)) {
@@ -1268,7 +1270,17 @@ static int __make_request(struct request
 		 * not touch req->buffer either...
 		 */
 		req->buffer = bio_data(bio);
+		src_part = disk_map_sector_rcu(req->rq_disk, blk_rq_pos(req));
 		req->__sector = bio->bi_sector;
+		dst_part = disk_map_sector_rcu(req->rq_disk, blk_rq_pos(req));
+		if (unlikely(src_part != dst_part)) {
+			int rw = rq_data_dir(req);
+
+			part_stat_lock();
+			part_dec_in_flight(src_part, rw);
+			part_inc_in_flight(dst_part, rw);
+			part_stat_unlock();
+		}
 		req->__data_len += bytes;
 		req->ioprio = ioprio_best(req->ioprio, prio);
 		if (!blk_rq_cpu_valid(req))