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>

3 files changed, 6 insertions, 10 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index c6c703f2486b..793f70a48820 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -144,8 +144,8 @@ struct cgroup {
         /*
          * idr allocated in-hierarchy ID.
          *
-         * The ID of the root cgroup is always 0, and a new cgroup
-         * will be assigned with a smallest available ID.
+         * ID 0 is not used, the ID of the root cgroup is always 1, and a
+         * new cgroup will be assigned with a smallest available ID.
          *
          * Allocating/Removing ID must be protected by cgroup_mutex.
          */
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 21667f396a1e..3fa0463e74bb 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1531,7 +1531,7 @@ static int cgroup_setup_root(struct cgroup_root *root, unsigned int ss_mask)
         lockdep_assert_held(&cgroup_tree_mutex);
         lockdep_assert_held(&cgroup_mutex);
 
-        ret = idr_alloc(&root->cgroup_idr, root_cgrp, 0, 1, GFP_KERNEL);
+        ret = idr_alloc(&root->cgroup_idr, root_cgrp, 1, 2, GFP_KERNEL);
         if (ret < 0)
                 goto out;
         root_cgrp->id = ret;
@@ -4225,7 +4225,7 @@ static long cgroup_create(struct cgroup *parent, const char *name,
          * Temporarily set the pointer to NULL, so idr_find() won't return
          * a half-baked cgroup.
          */
-        cgrp->id = idr_alloc(&root->cgroup_idr, NULL, 1, 0, GFP_KERNEL);
+        cgrp->id = idr_alloc(&root->cgroup_idr, NULL, 2, 0, GFP_KERNEL);
         if (cgrp->id < 0) {
                 err = -ENOMEM;
                 goto err_unlock;
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);
 }