memcg: add mem_cgroup_root_css

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>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 5fe6411..294498f 100644 (file)
--- 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 c23c1a3..b22a92b 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -77,6 +77,7 @@ 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
@@ -4441,6 +4442,7 @@ 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;