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

From: Vlastimil Babka
Date: Wed Jan 18 2017 - 04:49:10 EST


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