cgroup, memcg: allocate cgroup ID from 1

Currently, cgroup->id is allocated from 0, which is always assigned to
the root cgroup; unfortunately, memcg wants to use ID 0 to indicate
invalid IDs and ends up incrementing all IDs by one.

It's reasonable to reserve 0 for special purposes.  This patch updates
cgroup core so that ID 0 is not used and the root cgroups get ID 1.
The ID incrementing is removed form memcg.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Li Zefan <lizefan@huawei.com>

1 files changed, 2 insertions, 6 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 29501f040568..1d0b29715b73 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -527,18 +527,14 @@ static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
 
 static inline unsigned short mem_cgroup_id(struct mem_cgroup *memcg)
 {
-        /*
-         * The ID of the root cgroup is 0, but memcg treat 0 as an
-         * invalid ID, so we return (cgroup_id + 1).
-         */
-        return memcg->css.cgroup->id + 1;
+        return memcg->css.cgroup->id;
 }
 
 static inline struct mem_cgroup *mem_cgroup_from_id(unsigned short id)
 {
         struct cgroup_subsys_state *css;
 
-        css = css_from_id(id - 1, &memory_cgrp_subsys);
+        css = css_from_id(id, &memory_cgrp_subsys);
         return mem_cgroup_from_css(css);
 }