[dm-devel] dm-raid: stack limits instead of overwriting them.

[Mikulas Patocka]

On Thu, 24 Sep 2020, Mike Snitzer wrote:

> On Thu, Sep 24 2020 at  2:12pm -0400,
> Mikulas Patocka <mpatocka at redhat.com> wrote:
> 
> > 
> > 
> > On Thu, 24 Sep 2020, Mikulas Patocka wrote:
> > 
> > > 
> > > 
> > > On Thu, 24 Sep 2020, Mike Snitzer wrote:
> > > 
> > > > WAIT... Could it be that raid_io_hints _really_ meant to special case
> > > > raid0 and raid10 -- due to their striping/splitting requirements!?
> > > > So, not raid1 but raid0?
> > > > 
> > > > E.g.:
> > > > 
> > > > diff --git a/drivers/md/dm-raid.c b/drivers/md/dm-raid.c
> > > > index 56b723d012ac..6dca932d6f1d 100644
> > > > --- a/drivers/md/dm-raid.c
> > > > +++ b/drivers/md/dm-raid.c
> > > > @@ -3730,10 +3730,10 @@ static void raid_io_hints(struct dm_target *ti,
> > > > struct queue_limits *limits)
> > > >         blk_limits_io_opt(limits, chunk_size_bytes *
> > > > 	mddev_data_stripes(rs));
> > > > 
> > > >         /*
> > > > -        * RAID1 and RAID10 personalities require bio splitting,
> > > > -        * RAID0/4/5/6 don't and process large discard bios properly.
> > > > +        * RAID0 and RAID10 personalities require bio splitting,
> > > > +        * RAID1/4/5/6 don't and process large discard bios properly.
> > > >          */
> > > > -       if (rs_is_raid1(rs) || rs_is_raid10(rs)) {
> > > > +       if (rs_is_raid0(rs) || rs_is_raid10(rs)) {
> > > >                 limits->discard_granularity = chunk_size_bytes;
> > > >                 limits->max_discard_sectors = rs->md.chunk_sectors;
> > > >         }
> > > > 
> > > > Mike
> > > 
> > > Yes - that's an interesing point.
> > > 
> > > Mikulas
> > 
> > But raid0_handle_discard handles discards with arbitrary start/end 
> > sectors.
> > 
> > So, we don't need to set discard_granularity for that.
> 
> OK, great, I've staged this:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.10&id=c1fda10e1123a37cf9d22740486cd66f43c47846
> 
> Thanks,
> Mike

What if there are multiple targets for a device with different 
discard_granularity - we would overwrite the settings made by another 
target.

Should there be:

limits->discard_granularity = max(limits->discard_granularity, chunk_size_bytes);

or perhaps:

limits->discard_granularity = lcm_not_zero(limits->discard_granularity, chunk_size_bytes);

Mikulas