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


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