Re: [PATCH 06/51] memcg: add mem_cgroup_root_css

[Michal Hocko]

On Fri 22-05-15 17:13:20, Tejun Heo wrote:
> Add global mem_cgroup_root_css which points to the root memcg css.

Is there any reason to using css rather than mem_cgroup other than the
structure is not visible outside of memcontrol.c? Because I have a
patchset which exports it. It is not merged yet so a move to mem_cgroup
could be done later. I am just interested whether there is a stronger
reason.

> This will be used by cgroup writeback support.  If memcg is disabled,
> it's defined as ERR_PTR(-EINVAL).

Hmm. Why EINVAL? I can see only mm/backing-dev.c (in
review-cgroup-writeback-switch-20150528 branch) which uses it and that
shouldn't even try to compile if !CONFIG_MEMCG no? Otherwise we would
simply blow up.

> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> aCc: Michal Hocko <mhocko@xxxxxxx>
> ---
>  include/linux/memcontrol.h | 4 ++++
>  mm/memcontrol.c            | 2 ++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 5fe6411..294498f 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -68,6 +68,8 @@ enum mem_cgroup_events_index {
>  };
>  
>  #ifdef CONFIG_MEMCG
> +extern struct cgroup_subsys_state *mem_cgroup_root_css;
> +
>  void mem_cgroup_events(struct mem_cgroup *memcg,
>  		       enum mem_cgroup_events_index idx,
>  		       unsigned int nr);
> @@ -196,6 +198,8 @@ void mem_cgroup_split_huge_fixup(struct page *head);
>  #else /* CONFIG_MEMCG */
>  struct mem_cgroup;
>  
> +#define mem_cgroup_root_css ((struct cgroup_subsys_state *)ERR_PTR(-EINVAL))
> +
>  static inline void mem_cgroup_events(struct mem_cgroup *memcg,
>  				     enum mem_cgroup_events_index idx,
>  				     unsigned int nr)
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c23c1a3..b22a92b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -77,6 +77,7 @@ EXPORT_SYMBOL(memory_cgrp_subsys);
>  
>  #define MEM_CGROUP_RECLAIM_RETRIES	5
>  static struct mem_cgroup *root_mem_cgroup __read_mostly;
> +struct cgroup_subsys_state *mem_cgroup_root_css __read_mostly;
>  
>  /* Whether the swap controller is active */
>  #ifdef CONFIG_MEMCG_SWAP
> @@ -4441,6 +4442,7 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
>  	/* root ? */
>  	if (parent_css == NULL) {
>  		root_mem_cgroup = memcg;
> +		mem_cgroup_root_css = &memcg->css;
>  		page_counter_init(&memcg->memory, NULL);
>  		memcg->high = PAGE_COUNTER_MAX;
>  		memcg->soft_limit = PAGE_COUNTER_MAX;
> -- 
> 2.4.0
> 

-- 
Michal Hocko
SUSE Labs
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