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Subject[PATCH 1/2] Don't merge different partition's IOs
From: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>

PROBLEM:

/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
---------------------------
HOW TO FIX:

The patch fixes the problem by changing a merging rule.

The problem is caused by merging different partition's I/Os. So the patch
check whether a merging bio or request is a same partition as a request or not
by using a partition's start sector and size.

Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>

---
block/blk-core.c | 2 ++
block/blk-merge.c | 11 +++++++++++
include/linux/blkdev.h | 14 ++++++++++++++
3 files changed, 27 insertions(+)
Index: linux-2.6.37-rc3/block/blk-core.c
===================================================================
--- linux-2.6.37-rc3.orig/block/blk-core.c 2010-11-30 13:33:02.000000000 +0900
+++ linux-2.6.37-rc3/block/blk-core.c 2010-12-03 17:15:50.000000000 +0900
@@ -71,6 +71,8 @@ static void drive_stat_acct(struct reque
else {
part_round_stats(cpu, part);
part_inc_in_flight(part, rw);
+ rq->__part_start_sect = part->start_sect;
+ rq->__part_size = part->nr_sects;
}

part_stat_unlock();
Index: linux-2.6.37-rc3/block/blk-merge.c
===================================================================
--- linux-2.6.37-rc3.orig/block/blk-merge.c 2010-11-30 13:33:02.000000000 +0900
+++ linux-2.6.37-rc3/block/blk-merge.c 2010-12-03 17:15:50.000000000 +0900
@@ -235,6 +235,9 @@ int ll_back_merge_fn(struct request_queu
else
max_sectors = queue_max_sectors(q);

+ if (blk_rq_part_sector(req) + blk_rq_part_size(req) <= bio->bi_sector)
+ return 0;
+
if (blk_rq_sectors(req) + bio_sectors(bio) > max_sectors) {
req->cmd_flags |= REQ_NOMERGE;
if (req == q->last_merge)
@@ -259,6 +262,8 @@ int ll_front_merge_fn(struct request_que
else
max_sectors = queue_max_sectors(q);

+ if (bio->bi_sector < blk_rq_part_sector(req))
+ return 0;

if (blk_rq_sectors(req) + bio_sectors(bio) > max_sectors) {
req->cmd_flags |= REQ_NOMERGE;
@@ -382,6 +387,12 @@ static int attempt_merge(struct request_
return 0;

/*
+ * Don't merge different partition's request
+ */
+ if (blk_rq_part_sector(req) != blk_rq_part_sector(next))
+ return 0;
+
+ /*
* not contiguous
*/
if (blk_rq_pos(req) + blk_rq_sectors(req) != blk_rq_pos(next))
Index: linux-2.6.37-rc3/include/linux/blkdev.h
===================================================================
--- linux-2.6.37-rc3.orig/include/linux/blkdev.h 2010-11-30 13:33:02.000000000 +0900
+++ linux-2.6.37-rc3/include/linux/blkdev.h 2010-11-30 16:46:19.000000000 +0900
@@ -91,6 +91,8 @@ struct request {
/* the following two fields are internal, NEVER access directly */
unsigned int __data_len; /* total data len */
sector_t __sector; /* sector cursor */
+ sector_t __part_start_sect; /* partition's start sector*/
+ sector_t __part_size; /* partition's size*/

struct bio *bio;
struct bio *biotail;
@@ -724,6 +726,8 @@ static inline struct request_queue *bdev
* blk_rq_err_bytes() : bytes left till the next error boundary
* blk_rq_sectors() : sectors left in the entire request
* blk_rq_cur_sectors() : sectors left in the current segment
+ * blk_rq_part_sector() : partition's start sector
+ * blk_rq_part_size() : partition's size
*/
static inline sector_t blk_rq_pos(const struct request *rq)
{
@@ -752,6 +756,16 @@ static inline unsigned int blk_rq_cur_se
return blk_rq_cur_bytes(rq) >> 9;
}

+static inline sector_t blk_rq_part_sector(const struct request *rq)
+{
+ return rq->__part_start_sect;
+}
+
+static inline sector_t blk_rq_part_size(const struct request *rq)
+{
+ return rq->__part_size;
+}
+
/*
* Request issue related functions.
*/


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