The only user is cgwb_bdi_init and that one depends on
CONFIG_CGROUP_WRITEBACK which in turn depends on CONFIG_MEMCG
so it doesn't make much sense to definte an empty stub for
!CONFIG_MEMCG. Moreover ERR_PTR(-EINVAL) is ugly and would lead
to runtime crashes if used in unguarded code paths. Better fail
during compilation.

Reviewed-by: Vladimir Davydov <vdavydov@parallels.com>
Signed-off-by: Michal Hocko <mhocko@kernel.org>
---
 include/linux/memcontrol.h | 2 --
 1 file changed, 2 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 42f118ae04cf..292e6701f3fd 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -497,8 +497,6 @@ void mem_cgroup_split_huge_fixup(struct page *head);
 #else /* CONFIG_MEMCG */
 struct mem_cgroup;
 
-#define mem_cgroup_root_css ((struct cgroup_subsys_state *)ERR_PTR(-EINVAL))
-
 static inline void mem_cgroup_events(struct mem_cgroup *memcg,
 				     enum mem_cgroup_events_index idx,
 				     unsigned int nr)
-- 
2.1.4