Re: dm-raid: stack limits instead of overwriting them.

[John Dorminy <jdorminy@redhat.com>]

I am impressed at how much I read wrong...

On Thu, Sep 24, 2020 at 1:00 PM Mike Snitzer <snitzer@redhat.com> wrote:
>
> On Thu, Sep 24 2020 at 12:45pm -0400,
> John Dorminy <jdorminy@redhat.com> wrote:
>
> > I don't understand how this works...
> >
> > Can chunk_size_bytes be 0? If not, how is discard_granularity being set to 0?
>
> Yeah, I had same question.. see the reply I just sent in this thread:
> https://www.redhat.com/archives/dm-devel/2020-September/msg00568.html
>
> > I think also limits is local to the ti in question here, initialized
> > by blk_set_stacking_limits() via dm-table.c, and therefore has only
> > default values and not anything to do with the underlying queue. So
> > setting discard_granularity=max(discard_granularity, chunk_size_bytes)
> > doesn't seem like it should be working, unless I'm not understanding
> > what it's there for...
>
> You're reading the dm-table.c limits stacking wrong.  Of course DM stack
> up the underlying device(s) limits ;)

Yep, I failed to read iterate_devices...

>
> >
> > And shouldn't melding in the target's desired io_hints into the
> > existing queue limits be happening in blk_stack_limits() instead?
> > (Also, it does lcm_not_zero() for stacking granularity, instead of
> > max()...)
> >
>
> DM core does do that, the .io_hints hook in the DM target is reserved
> for when the target has additional constraints that blk_stack_limits()
> didn't/couldn't factor in.
Yes, I had erroneously thought the limit-stacking was after getting
the targets' individual limits, not before.

>
> And blk_stack_limts() does use max() for discard_granularity.
... I'm just terrible at reading this morning.

Thanks for pointing out all the things I misread!

>
> Mike
>
> >
> > On Thu, Sep 24, 2020 at 12:29 PM Mikulas Patocka <mpatocka@redhat.com> wrote:
> > >
> > > This patch fixes a warning WARN_ON_ONCE(!q->limits.discard_granularity).
> > > The reason is that the function raid_io_hints overwrote
> > > limits->discard_granularity with zero. We need to properly stack the
> > > limits instead of overwriting them.
> > >
> > > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> > > Cc: stable@vger.kernel.org
> > >
> > > ---
> > >  drivers/md/dm-raid.c |    4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > Index: linux-2.6/drivers/md/dm-raid.c
> > > ===================================================================
> > > --- linux-2.6.orig/drivers/md/dm-raid.c 2020-09-24 18:16:45.000000000 +0200
> > > +++ linux-2.6/drivers/md/dm-raid.c      2020-09-24 18:16:45.000000000 +0200
> > > @@ -3734,8 +3734,8 @@ static void raid_io_hints(struct dm_targ
> > >          * RAID0/4/5/6 don't and process large discard bios properly.
> > >          */
> > >         if (rs_is_raid1(rs) || rs_is_raid10(rs)) {
> > > -               limits->discard_granularity = chunk_size_bytes;
> > > -               limits->max_discard_sectors = rs->md.chunk_sectors;
> > > +               limits->discard_granularity = max(limits->discard_granularity, chunk_size_bytes);
> > > +               limits->max_discard_sectors = min_not_zero(limits->max_discard_sectors, (unsigned)rs->md.chunk_sectors);
> > >         }
> > >  }
> > >
> > >
> > > --
> > > dm-devel mailing list
> > > dm-devel@redhat.com
> > > https://www.redhat.com/mailman/listinfo/dm-devel
> > >
> >
>