mm, page_alloc: fix premature OOM when racing with cpuset mems update

Ganapatrao Kulkarni reported that the LTP test cpuset01 in stress mode
triggers OOM killer in few seconds, despite lots of free memory.  The
test attempts to repeatedly fault in memory in one process in a cpuset,
while changing allowed nodes of the cpuset between 0 and 1 in another
process.

The problem comes from insufficient protection against cpuset changes,
which can cause get_page_from_freelist() to consider all zones as
non-eligible due to nodemask and/or current->mems_allowed.  This was
masked in the past by sufficient retries, but since commit 682a338
("mm, page_alloc: inline the fast path of the zonelist iterator") we fix
the preferred_zoneref once, and don't iterate over the whole zonelist in
further attempts, thus the only eligible zones might be placed in the
zonelist before our starting point and we always miss them.

A previous patch fixed this problem for current->mems_allowed.  However,
cpuset changes also update the task's mempolicy nodemask.  The fix has
two parts.  We have to repeat the preferred_zoneref search when we
detect cpuset update by way of seqcount, and we have to check the
seqcount before considering OOM.

[[email protected]: fix typo in comment]
Link: http://lkml.kernel.org/r/[email protected]
Fixes: c33d6c0 ("mm, page_alloc: avoid looking up the first zone in a zonelist twice")
Signed-off-by: Vlastimil Babka <[email protected]>
Reported-by: Ganapatrao Kulkarni <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Acked-by: Hillf Danton <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: tehcaster

mm/page_alloc.c

@@ -3555,6 +3555,17 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	no_progress_loops = 0;
 	compact_priority = DEF_COMPACT_PRIORITY;
 	cpuset_mems_cookie = read_mems_allowed_begin();
+	/*
+	 * We need to recalculate the starting point for the zonelist iterator
+	 * because we might have used different nodemask in the fast path, or
+	 * there was a cpuset modification and we are retrying - otherwise we
+	 * could end up iterating over non-eligible zones endlessly.
+	 */
+	ac->preferred_zoneref = first_zones_zonelist(ac->zonelist,
+					ac->high_zoneidx, ac->nodemask);
+	if (!ac->preferred_zoneref->zone)
+		goto nopage;
+
 
 	/*
 	 * The fast path uses conservative alloc_flags to succeed only until
@@ -3715,6 +3726,13 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 				&compaction_retries))
 		goto retry;
 
+	/*
+	 * It's possible we raced with cpuset update so the OOM would be
+	 * premature (see below the nopage: label for full explanation).
+	 */
+	if (read_mems_allowed_retry(cpuset_mems_cookie))
+		goto retry_cpuset;
+
 	/* Reclaim has failed us, start killing things */
 	page = __alloc_pages_may_oom(gfp_mask, order, ac, &did_some_progress);
 	if (page)
@@ -3728,10 +3746,11 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 
 nopage:
 	/*
-	 * When updating a task's mems_allowed, it is possible to race with
-	 * parallel threads in such a way that an allocation can fail while
-	 * the mask is being updated. If a page allocation is about to fail,
-	 * check if the cpuset changed during allocation and if so, retry.
+	 * When updating a task's mems_allowed or mempolicy nodemask, it is
+	 * possible to race with parallel threads in such a way that our
+	 * allocation can fail while the mask is being updated. If we are about
+	 * to fail, check if the cpuset changed during allocation and if so,
+	 * retry.
 	 */
 	if (read_mems_allowed_retry(cpuset_mems_cookie))
 		goto retry_cpuset;
@@ -3822,15 +3841,9 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
 	/*
 	 * Restore the original nodemask if it was potentially replaced with
 	 * &cpuset_current_mems_allowed to optimize the fast-path attempt.
-	 * Also recalculate the starting point for the zonelist iterator or
-	 * we could end up iterating over non-eligible zones endlessly.
 	 */
-	if (unlikely(ac.nodemask != nodemask)) {
+	if (unlikely(ac.nodemask != nodemask))
 		ac.nodemask = nodemask;
-		ac.preferred_zoneref = first_zones_zonelist(ac.zonelist,
-						ac.high_zoneidx, ac.nodemask);
-		/* If we have NULL preferred zone, slowpath wll handle that */
-	}
 
 	page = __alloc_pages_slowpath(alloc_mask, order, &ac);