On Mon, Jan 28 2019 at 7:50am -0500,
gregkh@xxxxxxxxxxxxxxxxxxx <gregkh@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>.
>
> thanks,
>
> greg k-h
>
> ------------------ original commit in Linus's tree ------------------
>
> From a1e1cb72d96491277ede8d257ce6b48a381dd336 Mon Sep 17 00:00:00 2001
> From: Mike Snitzer <snitzer@xxxxxxxxxx>
> 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@xxxxxxxxxxxxxxx # 4.16+
> Reported-by: Bryan Gurney <bgurney@xxxxxxxxxx>
> Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>
> Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
>
> 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(+)
