[PATCH 2/4] dm: fix redundant IO accounting for bios that need splitting

[Mike Snitzer <snitzer@redhat.com>]

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: stable@vger.kernel.org # 4.16+
Reported-by: Bryan Gurney <bgurney@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 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