[PATCH 2/6] cgroup, memcg: allocate cgroup ID from 1

[Tejun Heo <tj@kernel.org>]

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>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
---
 include/linux/cgroup.h | 4 ++--
 kernel/cgroup.c        | 4 ++--
 mm/memcontrol.c        | 8 ++------
 3 files changed, 6 insertions(+), 10 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index c6c703f..793f70a 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 21667f3..3fa0463 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 29501f0..1d0b297 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);
 }
 
-- 
1.9.0