From 7db0d1b29a5b7d86442ff2f9d283fb37c56741fc Mon Sep 17 00:00:00 2001
From: Mike Snitzer <snitzer@redhat.com>
Date: Mon, 10 May 2021 14:11:36 +0200
Subject: [PATCH 1/1] dm: fix redundant IO accounting for bios that need
 splitting

Patch-mainline: v5.0-rc4
Git-commit: a1e1cb72d96491277ede8d257ce6b48a381dd336
References: bsc#1183738

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>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Firo Yang <firo.yang@suse.com>
---
 drivers/md/dm.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 3f46b5368f4f..4ba93ca00bd5 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1599,7 +1599,7 @@ static blk_qc_t __split_and_process_bio(struct mapped_device *md,
 {
 	struct clone_info ci;
 	blk_qc_t ret = BLK_QC_T_NONE;
-	int error = 0;
+	int error = 0, cpu;
 
 	if (unlikely(!map)) {
 		bio_io_error(bio);
@@ -1639,6 +1639,19 @@ 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)
+				 */
+				cpu = part_stat_lock();
+				__part_stat_add(cpu, &dm_disk(md)->part0, sectors[bio_data_dir(bio)],
+							-(unsigned long)ci.sector_count);
+				part_stat_unlock();
+
 				bio_chain(b, bio);
 				ret = generic_make_request(bio);
 				break;
-- 
2.26.2