[PATCH] mm/memcg: Fix incorrect flushing of lruvec data in obj_stock

[Waiman Long]

When mod_objcg_state() is called with a pgdat that is different from
that in the obj_stock, the old lruvec data cached in obj_stock are
flushed out. Unfortunately, they were flushed to the new pgdat and
hence the wrong node, not the one cached in obj_stock.

Fix that by flushing the data to the cached pgdat instead.

Fixes: 68ac5b3c8db2 ("mm/memcg: cache vmstat data in percpu memcg_stock_pcp")
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 mm/memcontrol.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ae1f5d0cb581..881ec4ddddcd 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3106,17 +3106,19 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
 		stock->cached_pgdat = pgdat;
 	} else if (stock->cached_pgdat != pgdat) {
 		/* Flush the existing cached vmstat data */
+		struct pglist_data *oldpg = stock->cached_pgdat;
+
+		stock->cached_pgdat = pgdat;
 		if (stock->nr_slab_reclaimable_b) {
-			mod_objcg_mlstate(objcg, pgdat, NR_SLAB_RECLAIMABLE_B,
+			mod_objcg_mlstate(objcg, oldpg, NR_SLAB_RECLAIMABLE_B,
 					  stock->nr_slab_reclaimable_b);
 			stock->nr_slab_reclaimable_b = 0;
 		}
 		if (stock->nr_slab_unreclaimable_b) {
-			mod_objcg_mlstate(objcg, pgdat, NR_SLAB_UNRECLAIMABLE_B,
+			mod_objcg_mlstate(objcg, oldpg, NR_SLAB_UNRECLAIMABLE_B,
 					  stock->nr_slab_unreclaimable_b);
 			stock->nr_slab_unreclaimable_b = 0;
 		}
-		stock->cached_pgdat = pgdat;
 	}
 
 	bytes = (idx == NR_SLAB_RECLAIMABLE_B) ? &stock->nr_slab_reclaimable_b
-- 
2.18.1