dm: fix redundant IO accounting for bios that need splitting

commit a1e1cb72d96491277ede8d257ce6b48a381dd336 upstream.

[Joseph: cherry-pick part_stat_get() from commit 1226b8dd0e91 ("block:
switch to per-cpu in-flight counters") since we don't want the whole
patch series get involved.]

The risk of redundant IO accounting was not taken into consideration
when commit 18a25da8 ("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: 18a25da8 ("dm: ensure bio submission follows a depth-first tree walk")
Cc: stable@vger.kernel.org # 4.16+
Reported-by: NBryan Gurney <bgurney@redhat.com>
Reviewed-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Signed-off-by: NShile Zhang <shile.zhang@linux.alibaba.com>
Acked-by: NCaspar Zhang <caspar@linux.alibaba.com>

drivers/md/dm.c

@@ -1576,6 +1576,9 @@ 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.
  */
@@ -1624,6 +1627,19 @@ 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;

include/linux/genhd.h

@@ -298,6 +298,9 @@ extern struct hd_struct *disk_map_sector_rcu(struct gendisk *disk,
 #define __part_stat_add(cpu, part, field, addnd)			\
 	(per_cpu_ptr((part)->dkstats, (cpu))->field += (addnd))
 
+#define part_stat_get(part, field)					\
+	(per_cpu_ptr((part)->dkstats, smp_processor_id())->field)
+
 #define part_stat_read(part, field)					\
 ({									\
 	typeof((part)->dkstats->field) res = 0;				\
@@ -336,6 +339,7 @@ static inline void free_part_stats(struct hd_struct *part)
 #define __part_stat_add(cpu, part, field, addnd)				\
 	((part)->dkstats.field += addnd)
 
+#define part_stat_get(part, field)	((part)->dkstats.field)
 #define part_stat_read(part, field)	((part)->dkstats.field)
 
 static inline void part_stat_set_all(struct hd_struct *part, int value)