[PATCH] mm/memcontrol: fix warning in mem_cgroup_page_lruvec()

[Hugh Dickins]

Boot a CONFIG_MEMCG=y kernel with "cgroup_disabled=memory" and you are
met by a series of warnings from the VM_WARN_ON_ONCE_PAGE(!memcg, page)
recently added to the inline mem_cgroup_page_lruvec().

An earlier attempt to place that warning, in mem_cgroup_lruvec(), had
been careful to do so after weeding out the mem_cgroup_disabled() case;
but was itself invalid because of the mem_cgroup_lruvec(NULL, pgdat) in
clear_pgdat_congested() and age_active_anon().

Warning in mem_cgroup_page_lruvec() was once useful in detecting a KSM
charge bug, so may be worth keeping: but skip if mem_cgroup_disabled().

Fixes: 9a1ac2288cf1 ("mm/memcontrol:rewrite mem_cgroup_page_lruvec()")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---

 include/linux/memcontrol.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- 5.11-rc2/include/linux/memcontrol.h	2020-12-27 20:39:36.751923135 -0800
+++ linux/include/linux/memcontrol.h	2021-01-03 19:38:24.822978559 -0800
@@ -665,7 +665,7 @@ static inline struct lruvec *mem_cgroup_
 {
 	struct mem_cgroup *memcg = page_memcg(page);
 
-	VM_WARN_ON_ONCE_PAGE(!memcg, page);
+	VM_WARN_ON_ONCE_PAGE(!memcg && !mem_cgroup_disabled(), page);
 	return mem_cgroup_lruvec(memcg, pgdat);
 }