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

[Michal Hocko <mhocko@suse.cz>]

On Thu 24-04-14 17:02:09, Tejun Heo wrote:
> 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>

Sorry for a late response.
Thanks for removing this hack from memcg!
cgroup code change looks good at first glance but I am not familiar with
all the recent changes so I do not feel to add my Reviewed-by.

Anyway
Acked-by: Michal Hocko <mhocko@suse.cz>

for the change in the semantic and the memcg change.

Thanks!

> ---
>  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
> 

-- 
Michal Hocko
SUSE Labs