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