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@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxx>
---
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
--
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