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
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