Re: FAILED: patch "[PATCH] dm: fix redundant IO accounting for bios that need splitting" failed to apply to 4.20-stable tree

On Mon, Jan 28, 2019 at 10:31:41AM -0500, Mike Snitzer wrote:

On Mon, Jan 28 2019 at 7:50am -0500, gregkh@linuxfoundation.org gregkh@linuxfoundation.org wrote:

...

The patch below does not apply to the 4.20-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 a1e1cb72d96491277ede8d257ce6b48a381dd336 Mon Sep 17 00:00:00 2001 From: Mike Snitzer snitzer@redhat.com Date: Thu, 17 Jan 2019 10:48:01 -0500 Subject: [PATCH] dm: fix redundant IO accounting for bios that need splitting

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

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;
  
  

Seems to apply fine.. not sure what the problem is on your end:

$ git checkout stable/linux-4.20.y Previous HEAD position was 8fe28cb58bcb... Linux 4.20 HEAD is now at 9f1a389a0b5b... Linux 4.20.5

$ git show a1e1cb72d96491277ede8d257ce6b48a381dd336 | patch -p1 --dry patching file drivers/md/dm.c Hunk #1 succeeded at 1578 (offset -6 lines). Hunk #2 succeeded at 1626 (offset -15 lines).

$ git cherry-pick a1e1cb72d96491277ede8d257ce6b48a381dd336 [detached HEAD 3d6015ea633a] dm: fix redundant IO accounting for bios that need splitting Date: Thu Jan 17 10:48:01 2019 -0500 1 file changed, 16 insertions(+)

Try building it, it blows up into tiny pieces :)

I guess I need a different script that says, "the patch applied, but broke the build", but it is so rare it's almost not worth it...

thanks,

greg k-h