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

[Michal Hocko]

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?

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

-- 
Michal Hocko
SUSE Labs