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

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Sep 25 2020 at  8:04am -0400,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> 
> 
> On Thu, 24 Sep 2020, Mike Snitzer wrote:
> 
> > On Thu, Sep 24 2020 at  2:12pm -0400,
> > Mikulas Patocka <mpatocka@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);

I'd go with max().  So I can fix up the stable@ patch to actually stack
the limits for raid10.

But it should be noted that there is this patch queued for Jens to pull
in (he actually _did_ pull the MD pull request but then rebased without
it):
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.10&id=828d14fd7a6cf

I haappened to have seized on it and will need to adjust given the
changes have been dropped at this point:
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.10&id=dd29d4b556979dae3cb6460d019c36073af7a3fc

Mike