Re: BUG: scheduling while atomic: cron/668/0x10c9a0c0 (was: Re: mm, page_alloc: avoid looking up the first zone in a zonelist twice)

[Mel Gorman <mgorman@techsingularity.net>]

On Tue, May 31, 2016 at 11:44:24PM +0200, Vlastimil Babka wrote:
> On 05/30/2016 05:56 PM, Mel Gorman wrote:
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index dba8cfd0b2d6..f2c1e47adc11 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3232,6 +3232,9 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >  		 * allocations are system rather than user orientated
> >  		 */
> >  		ac->zonelist = node_zonelist(numa_node_id(), gfp_mask);
> > +		ac->preferred_zoneref = first_zones_zonelist(ac->zonelist,
> > +					ac->high_zoneidx, ac->nodemask);
> > +		ac->classzone_idx = zonelist_zone_idx(ac->preferred_zoneref);
> >  		page = get_page_from_freelist(gfp_mask, order,
> >  						ALLOC_NO_WATERMARKS, ac);
> >  		if (page)
> > 
> 
> Even if that didn't help for this report, I think it's needed too
> (except the classzone_idx which doesn't exist anymore?).
> 
> And I think the following as well. (the changed comment could be also
> just deleted).
> 

Why?

The comment is fine but I do not see why the recalculation would occur.

In the original code, the preferred_zoneref for statistics is calculated
based on either the supplied nodemask or cpuset_current_mems_allowed during
the initial attempt. It then relies on the cpuset checks in the slowpath
to encorce mems_allowed but the preferred zone doesn't change.

With your proposed change, it's possible that the
preferred_zoneref recalculation points to a zoneref disallowed by
cpuset_current_mems_sllowed. While it'll be skipped during allocation,
the statistics will still be against a zone that is potentially outside
what is allowed.

-- 
Mel Gorman
SUSE Labs