mm, page_alloc: fix check for NULL preferred_zone

Patch series "fix premature OOM regression in 4.7+ due to cpuset races".

This is v2 of my attempt to fix the recent report based on LTP cpuset
stress test [1].  The intention is to go to stable 4.9 LTSS with this,
as triggering repeated OOMs is not nice.  That's why the patches try to
be not too intrusive.

Unfortunately why investigating I found that modifying the testcase to
use per-VMA policies instead of per-task policies will bring the OOM's
back, but that seems to be much older and harder to fix problem.  I have
posted a RFC [2] but I believe that fixing the recent regressions has a
higher priority.

Longer-term we might try to think how to fix the cpuset mess in a better
and less error prone way.  I was for example very surprised to learn,
that cpuset updates change not only task->mems_allowed, but also
nodemask of mempolicies.  Until now I expected the parameter to
alloc_pages_nodemask() to be stable.  I wonder why do we then treat
cpusets specially in get_page_from_freelist() and distinguish HARDWALL
etc, when there's unconditional intersection between mempolicy and
cpuset.  I would expect the nodemask adjustment for saving overhead in
g_p_f(), but that clearly doesn't happen in the current form.  So we
have both crazy complexity and overhead, AFAICS.

[1] https://lkml.kernel.org/r/CAFpQJXUq-JuEP=QPidy4p_=FN0rkH5Z-kfB4qBvsf6jMS87Edg@mail.gmail.com
[2] https://lkml.kernel.org/r/[email protected]

This patch (of 4):

Since commit c33d6c06f6 ("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.  Also document this in first_zones_zonelist() comment per Michal
Hocko.

Fixes: c33d6c06f6 ("mm, page_alloc: avoid looking up the first zone in a zonelist twice")
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Vlastimil Babka <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Acked-by: Hillf Danton <[email protected]>
Cc: Ganapatrao Kulkarni <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/page_alloc.c

@@ -3784,7 +3784,7 @@ retry_cpuset:
 	 */
 	ac.preferred_zoneref = first_zones_zonelist(ac.zonelist,
 					ac.high_zoneidx, ac.nodemask);
-	if (!ac.preferred_zoneref) {
+	if (!ac.preferred_zoneref->zone) {
 		page = NULL;
 		goto no_zone;
 	}