Re: [RFC 1/4] mm, page_alloc: fix check for NULL preferred_zone

[Vlastimil Babka]

On 01/18/2017 10:31 AM, Michal Hocko wrote:
> On Tue 17-01-17 23:16:07, Vlastimil Babka wrote:
> > Since commit c33d6c06f60f ("mm, page_alloc: avoid looking up the first zone in
> > a zonelist twice") we have a wrong check for NULL preferred_zone, which can
> > theoretically happen due to concurrent cpuset modification. We check the
> > zoneref pointer which is never NULL and we should check the zone pointer.
> >
> > Fixes: c33d6c06f60f ("mm, page_alloc: avoid looking up the first zone in a zonelist twice")
> > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> > ---
> >  mm/page_alloc.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 34ada718ef47..593a11d8bc6b 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3763,7 +3763,7 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
> >  	 */
> >  	ac.preferred_zoneref = first_zones_zonelist(ac.zonelist,
> >  					ac.high_zoneidx, ac.nodemask);
> > -	if (!ac.preferred_zoneref) {
> > +	if (!ac.preferred_zoneref->zone) {
>
> When can the ->zone be NULL?

Either we get a genuinely screwed nodemask, or there's a concurrent cpuset 
update and nodes in zonelist are ordered in such a way that we see all of them 
as not being available to us in the nodemask/current->mems_alowed, when we 
iterate the zonelist, so we reach the end of zonelist. The zonelists are 
terminated with a zoneref with NULL zone pointer.

> >  		page = NULL;
> >  		goto no_zone;
> >  	}
> > --
> > 2.11.0