[PATCH] mm/memcg: warn on missing memcg on mem_cgroup_page_lruvec()

[Lorenzo Stoakes <lstoakes@xxxxxxxxx>]

Move memcg check to mem_cgroup_page_lruvec() as there are callers which
may invoke this with !memcg in mem_cgroup_lruvec(), whereas they should
not in mem_cgroup_page_lruvec().

We expect that we have always charged a page to the memcg before
mem_cgroup_page_lruvec() is invoked, so add a warning to assert that this
is the case.

Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
Reported-by: syzbot+ce635500093181f39c1c@xxxxxxxxxxxxxxxxxxxxxxxxx
---
 include/linux/memcontrol.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 87ed56dc75f9..3e6a1df3bdb9 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -618,7 +618,6 @@ static inline struct lruvec *mem_cgroup_lruvec(struct mem_cgroup *memcg,
 		goto out;
 	}
 
-	VM_WARN_ON_ONCE(!memcg);
 	if (!memcg)
 		memcg = root_mem_cgroup;
 
@@ -645,7 +644,10 @@ static inline struct lruvec *mem_cgroup_lruvec(struct mem_cgroup *memcg,
 static inline struct lruvec *mem_cgroup_page_lruvec(struct page *page,
 						struct pglist_data *pgdat)
 {
-	return mem_cgroup_lruvec(page_memcg(page), pgdat);
+	struct mem_cgroup *memcg = page_memcg(page);
+
+	VM_WARN_ON_ONCE_PAGE(!memcg, page);
+	return mem_cgroup_lruvec(memcg, pgdat);
 }
 
 static inline bool lruvec_holds_page_lru_lock(struct page *page,
-- 
2.29.2