mm: memcg/slab: fix root memcg vmstats

If we reparent the slab objects to the root memcg, when we free the slab
object, we need to update the per-memcg vmstats to keep it correct for
the root memcg.  Now this at least affects the vmstat of
NR_KERNEL_STACK_KB for !CONFIG_VMAP_STACK when the thread stack size is
smaller than the PAGE_SIZE.

David said:
 "I assume that without this fix that the root memcg's vmstat would
  always be inflated if we reparented"

Fixes: ec9f02384f60 ("mm: workingset: fix vmstat counters for shadow nodes")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Christopher Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Roman Gushchin <guro@fb.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yafang Shao <laoar.shao@gmail.com>
Cc: Chris Down <chris@chrisdown.name>
Cc: <stable@vger.kernel.org>	[5.3+]
Link: https://lkml.kernel.org/r/20201110031015.15715-1-songmuchun@bytedance.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 3dcbf24d2227a3350e855f5fd3015a0cbc20e070..29459a6ce1c7ad06f00d9cc3873a4126fd1914cb 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -867,8 +867,13 @@ void __mod_lruvec_slab_state(void *p, enum node_stat_item idx, int val)
        rcu_read_lock();
        memcg = mem_cgroup_from_obj(p);
 
-       /* Untracked pages have no memcg, no lruvec. Update only the node */
-       if (!memcg || memcg == root_mem_cgroup) {
+       /*
+        * Untracked pages have no memcg, no lruvec. Update only the
+        * node. If we reparent the slab objects to the root memcg,
+        * when we free the slab object, we need to update the per-memcg
+        * vmstats to keep it correct for the root memcg.
+        */
+       if (!memcg) {
                __mod_node_page_state(pgdat, idx, val);
        } else {
                lruvec = mem_cgroup_lruvec(memcg, pgdat);