[PATCH v2] md: raid0: account for split bio in iostat accounting

[David Jeffery <djeffery@redhat.com>]

When a bio is split by md raid0, the newly created bio will not be tracked
by md for I/O accounting. Only the portion of I/O still assigned to the
original bio which was reduced by the split will be accounted for. This
results in md iostat data sometimes showing I/O values far below the actual
amount of data being sent through md.

md_account_bio() needs to be called for all bio generated by the bio split.

A simple example of the issue was generated using a raid0 device on partitions
to the same device. Since all raid0 I/O then goes to one device, it makes it
easy to see a gap between the md device and its sd storage. Reading an lvm
device on top of the md device, the iostat output (some 0 columns and extra
devices removed to make the data more compact) was:

Device             tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read
md2               0.00         0.00         0.00         0.00          0
sde               0.00         0.00         0.00         0.00          0
md2            1364.00    411496.00         0.00         0.00     411496
sde            1734.00    646144.00         0.00         0.00     646144
md2            1699.00    510680.00         0.00         0.00     510680
sde            2155.00    802784.00         0.00         0.00     802784
md2             803.00    241480.00         0.00         0.00     241480
sde            1016.00    377888.00         0.00         0.00     377888
md2               0.00         0.00         0.00         0.00          0
sde               0.00         0.00         0.00         0.00          0

I/O was generated doing large direct I/O reads (12M) with dd to a linear
lvm volume on top of the 4 leg raid0 device.

The md2 reads were showing as roughly 2/3 of the reads to the sde device
containing all of md2's raid partitions. The sum of reads to sde was
1826816 kB, which was the expected amount as it was the amount read by
dd. With the patch, the total reads from md will match the reads from
sde and be consistent with the amount of I/O generated.

Fixes: 10764815ff47 ("md: add io accounting for raid0 and raid5") 
Signed-off-by: David Jeffery <djeffery@redhat.com>
Tested-by: Laurence Oberman <loberman@redhat.com>
Reviewed-by: Laurence Oberman <loberman@redhat.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
---

Patch v2: ported to apply on top of md-next

 drivers/md/raid0.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/md/raid0.c b/drivers/md/raid0.c
index abbd77977f98..c50a7abda744 100644
--- a/drivers/md/raid0.c
+++ b/drivers/md/raid0.c
@@ -553,8 +553,7 @@ static void raid0_map_submit_bio(struct mddev *mddev, struct bio *bio)
 	sector_t bio_sector = bio->bi_iter.bi_sector;
 	sector_t sector = bio_sector;
 
-	if (bio->bi_pool != &mddev->bio_set)
-		md_account_bio(mddev, &bio);
+	md_account_bio(mddev, &bio);
 
 	zone = find_zone(mddev->private, &sector);
 	switch (conf->layout) {
-- 
2.41.0