Patch "mm, page_alloc: fix fast-path race with cpuset update or removal" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

mm, page_alloc: fix fast-path race with cpuset update or removal

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-page_alloc-fix-fast-path-race-with-cpuset-update-or-removal.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Vlastimil Babka <[email protected]>
Date: Tue, 24 Jan 2017 15:18:35 -0800
Subject: mm, page_alloc: fix fast-path race with cpuset update or removal

From: Vlastimil Babka <[email protected]>

commit 16096c25bf0ca5d87e4fa6ec6108ba53feead212 upstream.

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.

One possible cause is that in the fast path we find the preferred zoneref according to current mems_allowed, so that it points to the middle of the zonelist, skipping e.g. zones of node 1 completely. If the mems_allowed is updated to contain only node 1, we never reach it in the zonelist, and trigger OOM before checking the cpuset_mems_cookie.

This patch fixes the particular case by redoing the preferred zoneref search if we switch back to the original nodemask. The condition is also slightly changed so that when the last non-root cpuset is removed, we don't miss it.

Note that this is not a full fix, and more patches will follow.

---
 mm/page_alloc.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

@@ -3783,9 +3783,17 @@ retry_cpuset:
 	/*
 	 * 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 (cpusets_enabled())
+	if (unlikely(ac.nodemask != nodemask)) {
 		ac.nodemask = nodemask;
+		ac.preferred_zoneref = first_zones_zonelist(ac.zonelist,
+						ac.high_zoneidx, ac.nodemask);
+		if (!ac.preferred_zoneref->zone)
+			goto no_zone;
+	}
+
 	page = __alloc_pages_slowpath(alloc_mask, order, &ac);
 
 no_zone:

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/mm-page_alloc-fix-fast-path-race-with-cpuset-update-or-removal.patch queue-4.9/mm-page_alloc-fix-check-for-null-preferred_zone.patch queue-4.9/mm-mempolicy.c-do-not-put-mempolicy-before-using-its-nodemask.patch queue-4.9/mm-page_alloc-fix-premature-oom-when-racing-with-cpuset-mems-update.patch queue-4.9/mm-page_alloc-move-cpuset-seqcount-checking-to-slowpath.patch