[PATCH 4.4 16/66] mm: prevent double decrease of nr_reserved_highatomic

[Greg Kroah-Hartman]

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Minchan Kim <minchan@xxxxxxxxxx>

commit 4855e4a7f29d6d10b0b9c84e189c770c9a94e91e upstream.

There is race between page freeing and unreserved highatomic.

 CPU 0				    CPU 1

    free_hot_cold_page
      mt = get_pfnblock_migratetype
      set_pcppage_migratetype(page, mt)
    				    unreserve_highatomic_pageblock
    				    spin_lock_irqsave(&zone->lock)
    				    move_freepages_block
    				    set_pageblock_migratetype(page)
    				    spin_unlock_irqrestore(&zone->lock)
      free_pcppages_bulk
        __free_one_page(mt) <- mt is stale

By above race, a page on CPU 0 could go non-highorderatomic free list
since the pageblock's type is changed.  By that, unreserve logic of
highorderatomic can decrease reserved count on a same pageblock severak
times and then it will make mismatch between nr_reserved_highatomic and
the number of reserved pageblock.

So, this patch verifies whether the pageblock is highatomic or not and
decrease the count only if the pageblock is highatomic.

Link: http://lkml.kernel.org/r/1476259429-18279-3-git-send-email-minchan@xxxxxxxxxx
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Sangseok Lee <sangseok.lee@xxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Miles Chen <miles.chen@xxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 mm/page_alloc.c |   24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1748,13 +1748,25 @@ static void unreserve_highatomic_pageblo
 						struct page, lru);
 
 			/*
-			 * It should never happen but changes to locking could
-			 * inadvertently allow a per-cpu drain to add pages
-			 * to MIGRATE_HIGHATOMIC while unreserving so be safe
-			 * and watch for underflows.
+			 * In page freeing path, migratetype change is racy so
+			 * we can counter several free pages in a pageblock
+			 * in this loop althoug we changed the pageblock type
+			 * from highatomic to ac->migratetype. So we should
+			 * adjust the count once.
 			 */
-			zone->nr_reserved_highatomic -= min(pageblock_nr_pages,
-				zone->nr_reserved_highatomic);
+			if (get_pageblock_migratetype(page) ==
+							MIGRATE_HIGHATOMIC) {
+				/*
+				 * It should never happen but changes to
+				 * locking could inadvertently allow a per-cpu
+				 * drain to add pages to MIGRATE_HIGHATOMIC
+				 * while unreserving so be safe and watch for
+				 * underflows.
+				 */
+				zone->nr_reserved_highatomic -= min(
+						pageblock_nr_pages,
+						zone->nr_reserved_highatomic);
+			}
 
 			/*
 			 * Convert to ac->migratetype and avoid the normal