Hey Greg,

Turns out the fixes I marked for stable@ aren't actually fixing anything (after applying the changes: code is cleaner but no meaningful difference in IO stats).

That is because DM was already patching up the 'sectors' portion of the stats after a split. The other fields I documented in the following header aren't tied to payload (see __part_start_io_acct).

So no _real_ need to backport to 5.4 or 5.10. Sorry for the noise.

Mike

On Sat, Jan 29 2022 at 8:31P -0500, gregkh@linuxfoundation.org gregkh@linuxfoundation.org wrote:

The patch below does not apply to the 5.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From f524d9c95fab54783d0038f7a3e8c014d5b56857 Mon Sep 17 00:00:00 2001 From: Mike Snitzer snitzer@redhat.com Date: Fri, 28 Jan 2022 10:58:40 -0500 Subject: [PATCH] dm: revert partial fix for redundant bio-based IO accounting

Reverts a1e1cb72d9649 ("dm: fix redundant IO accounting for bios that need splitting") because it was too narrow in scope (only addressed redundant 'sectors[]' accounting and not ios, nsecs[], etc).

Cc: stable@vger.kernel.org Signed-off-by: Mike Snitzer snitzer@redhat.com Link: https://lore.kernel.org/r/20220128155841.39644-3-snitzer@redhat.com Signed-off-by: Jens Axboe axboe@kernel.dk

diff --git a/drivers/md/dm.c b/drivers/md/dm.c index c0ae8087c602..9849114b3c08 100644 --- a/drivers/md/dm.c +++ b/drivers/md/dm.c @@ -1442,9 +1442,6 @@ 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.

*/ @@ -1480,18 +1477,6 @@ static void __split_and_process_bio(struct mapped_device *md, 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);
  trace_block_split(b, bio->bi_iter.bi_sector);
  submit_bio_noacct(bio);