Re: dm table: simplify discard support processing

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Jul 14 2011 at 11:43am -0400,
Milan Broz <mbroz@redhat.com> wrote:

> On 07/14/2011 04:30 PM, Mike Snitzer wrote:
> > Remove 'discards_supported' from the dm_table structure.  The same
> > information can be easily discovered from the table's target(s) in
> > dm_table_supports_discards().
> > 
> > Also DMWARN if a target sets 'discards_supported' override but forgets
> > to set 'num_discard_requests'.
> 
> Why not BUG_ON? It is bug in code on static attribute, isn't? :-)

Because we don't _need_ to BUG the system over programmer error for
how that target implements discards (given discard support is basically
optional, sometimes nice to have, increasingly nice to have in future).

> > @@ -1426,6 +1422,9 @@ bool dm_table_supports_discards(struct dm_table *t)
> >  	while (i < dm_table_get_num_targets(t)) {
> >  		ti = dm_table_get_target(t, i++);
> >  
> > +		if (!ti->num_discard_requests)
> > +			return 0;
> > +		
> 
> >  		if (ti->discards_supported)
> >  			return 1;
> 
> What if next target has ti->num_discard_requests = 0 here?
> Shouldn't it loop through the all targets always?

Yes it should, but I'm not sure if we are on the same page as to why.

Background:
If a table has a target with discards_supported=1 then the final DM
device's queue must export the ability to handle discards.  But only
targets with num_discard_requests>0 will actually have discards past to
them.

ti->discards_supported is used for targets like thinp.  Even if the the
thinp pool's underlying data device doesn't support discards the 'thin'
and 'thin-pool' targets do.

So back to the original question: yes, we need to look at all targets to
make sure that one of them doesn't have discards_supported set.

I'll fix this up and send v2.

Thanks,
Mike