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

[Mike Snitzer <snitzer@redhat.com>]

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 ;)

> 
> 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.

And blk_stack_limts() does use max() for discard_granularity.

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);
> >         }
> >  }
> >
> >
> > --
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> >
>