[PATCH 04/48] memcg: add mem_cgroup_root_css

[Tejun Heo <tj@kernel.org>]

Add global mem_cgroup_root_css which points to the root memcg css.
This will be used by cgroup writeback support.  If memcg is disabled,
it's defined as ERR_PTR(-EINVAL).

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
aCc: Michal Hocko <mhocko@suse.cz>
---
 include/linux/memcontrol.h | 4 ++++
 mm/memcontrol.c            | 2 ++
 2 files changed, 6 insertions(+)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 5fe6411..294498f 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -68,6 +68,8 @@ enum mem_cgroup_events_index {
 };
 
 #ifdef CONFIG_MEMCG
+extern struct cgroup_subsys_state *mem_cgroup_root_css;
+
 void mem_cgroup_events(struct mem_cgroup *memcg,
 		       enum mem_cgroup_events_index idx,
 		       unsigned int nr);
@@ -196,6 +198,8 @@ 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)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index cf25d1a..fda7025 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -71,6 +71,7 @@ EXPORT_SYMBOL(memory_cgrp_subsys);
 
 #define MEM_CGROUP_RECLAIM_RETRIES	5
 static struct mem_cgroup *root_mem_cgroup __read_mostly;
+struct cgroup_subsys_state *mem_cgroup_root_css __read_mostly;
 
 /* Whether the swap controller is active */
 #ifdef CONFIG_MEMCG_SWAP
@@ -4551,6 +4552,7 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 	/* root ? */
 	if (parent_css == NULL) {
 		root_mem_cgroup = memcg;
+		mem_cgroup_root_css = &memcg->css;
 		page_counter_init(&memcg->memory, NULL);
 		memcg->high = PAGE_COUNTER_MAX;
 		memcg->soft_limit = PAGE_COUNTER_MAX;
-- 
2.1.0