[patch 074/173] mm: memcontrol: fix get_active_memcg return value

From: Muchun Song <redacted>
Subject: mm: memcontrol: fix get_active_memcg return value

We use a global percpu int_active_memcg variable to store the remote memcg
when we are in the interrupt context.  But get_active_memcg always return
the current->active_memcg or root_mem_cgroup.  The remote memcg (set in
the interrupt context) is ignored.  This is not what we want.  So fix it.

Link: https://lkml.kernel.org/r/20210223091101.42150-1-songmuchun@bytedance.com
Fixes: 37d5985c003d ("mm: kmem: prepare remote memcg charging infra for interrupt contexts")
Signed-off-by: Muchun Song <redacted>
Reviewed-by: Shakeel Butt <redacted>
Reviewed-by: Roman Gushchin <redacted>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <redacted>
Cc: <redacted>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/memcontrol.c |   10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

--- a/mm/memcontrol.c~mm-memcontrol-fix-get_active_memcg-return-value
+++ a/mm/memcontrol.c

@@ -1061,13 +1061,9 @@ static __always_inline struct mem_cgroup
 
 	rcu_read_lock();
 	memcg = active_memcg();
-	if (memcg) {
-		/* current->active_memcg must hold a ref. */
-		if (WARN_ON_ONCE(!css_tryget(&memcg->css)))
-			memcg = root_mem_cgroup;
-		else
-			memcg = current->active_memcg;
-	}
+	/* remote memcg must hold a ref. */
+	if (memcg && WARN_ON_ONCE(!css_tryget(&memcg->css)))
+		memcg = root_mem_cgroup;
 	rcu_read_unlock();
 
 	return memcg;

_