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

Ming Lei
2022-04-15T00:15:11Z

On Thu, Apr 14, 2022 at 01:45:33PM -0400, Mike Snitzer wrote:
> On Wed, Apr 13 2022 at 11:57P -0400,
> Ming Lei <ming.lei@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@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.

Go it now, thanks for the explanation.

As you mentioned, it can be addressed in dm_accept_partial_bio()
by updating ti->io->sectors.

Thanks,
Ming