[PATCH] zsmalloc: fix zs_can_compact() integer overflow

[Sergey Senozhatsky]

zs_can_compact() has two race conditions in its core calculation:

unsigned long obj_wasted = zs_stat_get(class, OBJ_ALLOCATED) -
				zs_stat_get(class, OBJ_USED);

1) classes are not locked, so the numbers of allocated and used
   objects can change by the concurrent ops happening on other CPUs
2) shrinker invokes it from preemptible context

Depending on the circumstances, OBJ_ALLOCATED can become less
than OBJ_USED, which can result in either very high or negative
`total_scan' value calculated in do_shrink_slab().

Take a pool stat snapshot and use it instead of racy zs_stat_get()
calls.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>        [4.3+]
---
 mm/zsmalloc.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 107ec06..1bc2a98 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -2262,10 +2262,13 @@ static void SetZsPageMovable(struct zs_pool *pool, struct zspage *zspage)
 static unsigned long zs_can_compact(struct size_class *class)
 {
 	unsigned long obj_wasted;
+	unsigned long obj_allocated = zs_stat_get(class, OBJ_ALLOCATED);
+	unsigned long obj_used = zs_stat_get(class, OBJ_USED);
 
-	obj_wasted = zs_stat_get(class, OBJ_ALLOCATED) -
-		zs_stat_get(class, OBJ_USED);
+	if (obj_allocated <= obj_used)
+		return 0;
 
+	obj_wasted = obj_allocated - obj_used;
 	obj_wasted /= get_maxobj_per_zspage(class->size,
 			class->pages_per_zspage);
 
-- 
2.8.2