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

[Mike Snitzer]

On Wed, Apr 13 2022 at 11:57P -0400,
Ming Lei <ming.lei at redhat.com> wrote:

> 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 at 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.

Recall that ->orig_bio is updated after the bio_split() at the bottom of
dm_split_and_process_bio().

That bio_split() is based on ci->sector_count, which is reduced as a
side-effect of dm_accept_partial_bio() reducing tio->len_ptr.  It is
pretty circuitous so I can absolutely understand why you didn't
immediately appreciate the interface.  The block comment above
dm_accept_partial_bio() does a pretty comprehensive job of explaining.

But basically dm_accept_partial_bio() provides DM targets access to
control DM core's splitting if they find that they cannot accommodate
the entirety of the clone bio that is sent to their ->map.
dm_accept_partial_bio() may only ever be called from a target's ->map

> 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?

https://gitlab.com/cryptsetup/cryptsetup.git

> 
> > 
> > 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.

Yeah, something like that would be good.

Mike