The risk of redundant IO accounting was not taken into consideration
when commit 18a25da84354 ("dm: ensure bio submission follows a
depth-first tree walk") introduced IO splitting in terms of recursion
via generic_make_request().
Fix this by subtracting the split bio's payload from the IO stats that
were already accounted for by start_io_acct() upon dm_make_request()
entry. This repeat oscillation of the IO accounting, up then down,
isn't ideal but refactoring DM core's IO splitting to pre-split bios
_before_ they are accounted turned out to be an excessive amount of
change that will need a full development cycle to refine and verify.
Before this fix:
/dev/mapper/stripe_dev is a 4-way stripe using a 32k chunksize, so
bios are split on 32k boundaries.
# fio --name=16M --filename=/dev/mapper/stripe_dev --rw=write --bs=64k
--size=16M \
--iodepth=1 --ioengine=libaio --direct=1 --refill_buffers
with debugging added:
[103898.310264] device-mapper: core: start_io_acct: dm-2 WRITE
bio->bi_iter.bi_sector=0 len=128
[103898.318704] device-mapper: core: __split_and_process_bio: recursing for
following split bio:
[103898.329136] device-mapper: core: start_io_acct: dm-2 WRITE
bio->bi_iter.bi_sector=64 len=64
...
16M written yet 136M (278528 * 512b) accounted:
# cat /sys/block/dm-2/stat | awk '{ print $7 }'
278528
After this fix:
16M written and 16M (32768 * 512b) accounted:
# cat /sys/block/dm-2/stat | awk '{ print $7 }'
32768
Fixes: 18a25da84354 ("dm: ensure bio submission follows a depth-first tree
walk")
Cc: [email protected] # 4.16+
Reported-by: Bryan Gurney <[email protected]>
Signed-off-by: Mike Snitzer <[email protected]>
---
drivers/md/dm.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index fcb97b0a5743..fbadda68e23b 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1584,6 +1584,9 @@ static void init_clone_info(struct clone_info *ci, struct
mapped_device *md,
ci->sector = bio->bi_iter.bi_sector;
}
+#define __dm_part_stat_sub(part, field, subnd) \
+ (part_stat_get(part, field) -= (subnd))
+
/*
* Entry point to split a bio into clones and submit them to the targets.
*/
@@ -1638,6 +1641,19 @@ static blk_qc_t __split_and_process_bio(struct
mapped_device *md,
struct bio *b = bio_split(bio, bio_sectors(bio)
- ci.sector_count,
GFP_NOIO,
&md->queue->bio_split);
ci.io->orig_bio = b;
+
+ /*
+ * Adjust IO stats for each split, otherwise
upon queue
+ * reentry there will be redundant IO
accounting.
+ * NOTE: this is a stop-gap fix, a proper fix
involves
+ * significant refactoring of DM core's bio
splitting
+ * (by eliminating DM's splitting and just
using bio_split)
+ */
+ part_stat_lock();
+ __dm_part_stat_sub(&dm_disk(md)->part0,
+
sectors[op_stat_group(bio_op(bio))], ci.sector_count);
+ part_stat_unlock();
+
bio_chain(b, bio);
ret = generic_make_request(bio);
break;
--
2.15.0
