Re: [PATCH 5/8] dm: always setup ->orig_bio in alloc_io

[Ming Lei <ming.lei@redhat.com>]

On Wed, Apr 13, 2022 at 10:25:45PM -0400, Mike Snitzer wrote:
> On Wed, Apr 13 2022 at  8:36P -0400,
> Ming Lei <ming.lei@redhat.com> wrote:
> 
> > On Wed, Apr 13, 2022 at 01:58:54PM -0400, Mike Snitzer wrote:
> > > 
> > > The bigger issue with this patch is that you've caused
> > > dm_submit_bio_remap() to go back to accounting the entire original bio
> > > before any split occurs.  That is a problem because you'll end up
> > > accounting that bio for every split, so in split heavy workloads the
> > > IO accounting won't reflect when the IO is actually issued and we'll
> > > regress back to having very inaccurate and incorrect IO accounting for
> > > dm_submit_bio_remap() heavy targets (e.g. dm-crypt).
> > 
> > Good catch, but we know the length of mapped part in original bio before
> > calling __map_bio(), so io->sectors/io->offset_sector can be setup here,
> > something like the following delta change should address it:
> > 
> > diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> > index db23efd6bbf6..06b554f3104b 100644
> > --- a/drivers/md/dm.c
> > +++ b/drivers/md/dm.c
> > @@ -1558,6 +1558,13 @@ static int __split_and_process_bio(struct clone_info *ci)
> >  
> >  	len = min_t(sector_t, max_io_len(ti, ci->sector), ci->sector_count);
> >  	clone = alloc_tio(ci, ti, 0, &len, GFP_NOIO);
> > +
> > +	if (ci->sector_count > len) {
> > +		/* setup the mapped part for accounting */
> > +		dm_io_set_flag(ci->io, DM_IO_SPLITTED);
> > +		ci->io->sectors = len;
> > +		ci->io->sector_offset = bio_end_sector(ci->bio) - ci->sector;
> > +	}
> >  	__map_bio(clone);
> >  
> >  	ci->sector += len;
> > @@ -1603,11 +1610,6 @@ static void dm_split_and_process_bio(struct mapped_device *md,
> >  	if (error || !ci.sector_count)
> >  		goto out;
> >  
> > -	/* setup the mapped part for accounting */
> > -	dm_io_set_flag(ci.io, DM_IO_SPLITTED);
> > -	ci.io->sectors = bio_sectors(bio) - ci.sector_count;
> > -	ci.io->sector_offset = bio_end_sector(bio) - bio->bi_iter.bi_sector;
> > -
> >  	bio_trim(bio, ci.io->sectors, ci.sector_count);
> >  	trace_block_split(bio, bio->bi_iter.bi_sector);
> >  	bio_inc_remaining(bio);
> > 
> > -- 
> > Ming
> > 
> 
> Unfortunately we do need splitting after __map_bio() because a dm
> target's ->map can use dm_accept_partial_bio() to further reduce a
> bio's mapped part.
> 
> But I think dm_accept_partial_bio() could be trained to update
> tio->io->sectors?

->orig_bio is just for serving io accounting, but ->orig_bio isn't
passed to dm_accept_partial_bio(), and not gets updated after
dm_accept_partial_bio() is called.

If that is one issue, it must be one existed issue in dm io accounting
since ->orig_bio isn't updated when dm_accept_partial_bio() is called.

So do we have to update it?

> 
> dm_accept_partial_bio() has been around for a long time, it keeps
> growing BUG_ONs that are actually helpful to narrow its use to "normal
> IO", so it should be OK.
> 
> Running 'make check' in a built cryptsetup source tree should be a
> good test for DM target interface functionality.

Care to share the test tree?

> 
> But there aren't automated tests for IO accounting correctness yet.

I did verify io accounting by running dm-thin with blk-throttle, and the
observed throughput is same with expected setting. Running both small bs
and large bs, so non-split and split code path are covered.

Maybe you can add this kind of test into dm io accounting automated test.


Thanks,
Ming