release/4.8.20/mm-page_alloc-fix-check-for-NULL-preferred_zone.patch

From 710471ac9447ddc698ed9466b7aa7b60ad4805d2 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@suse.cz>
Date: Tue, 24 Jan 2017 15:18:32 -0800
Subject: [PATCH] mm, page_alloc: fix check for NULL preferred_zone

commit ea57485af8f4221312a5a95d63c382b45e7840dc upstream.

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/7c459f26-13a6-a817-e508-b65b903a8378@suse.cz

This patch (of 4):

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.  Also document this in first_zones_zonelist() comment per Michal
Hocko.

Fixes: c33d6c06f60f ("mm, page_alloc: avoid looking up the first zone in a zonelist twice")
Link: http://lkml.kernel.org/r/20170120103843.24587-2-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Ganapatrao Kulkarni <gpkulkarni@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 7f2ae99e5daf..3a73b3b73dcc 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -998,12 +998,16 @@ static __always_inline struct zoneref *next_zones_zonelist(struct zoneref *z,
  * @zonelist - The zonelist to search for a suitable zone
  * @highest_zoneidx - The zone index of the highest zone to return
  * @nodes - An optional nodemask to filter the zonelist with
- * @zone - The first suitable zone found is returned via this parameter
+ * @return - Zoneref pointer for the first suitable zone found (see below)
  *
  * This function returns the first zone at or below a given zone index that is
  * within the allowed nodemask. The zoneref returned is a cursor that can be
  * used to iterate the zonelist with next_zones_zonelist by advancing it by
  * one before calling.
+ *
+ * When no eligible zone is found, zoneref->zone is NULL (zoneref itself is
+ * never NULL). This may happen either genuinely, or due to concurrent nodemask
+ * update due to cpuset modification.
  */
 static inline struct zoneref *first_zones_zonelist(struct zonelist *zonelist,
 					enum zone_type highest_zoneidx,
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 212a017a2f02..6fa120e8e9a1 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3687,7 +3687,7 @@ 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;
 	}
-- 
2.10.1