Re: [RFC][PATCH] memcg: get/put parents at create/free

[KAMEZAWA Hiroyuki]

On Thu, 15 Jan 2009 17:13:15 +0900
Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:

> From: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> 
> mem_cgroup_get ensures that the memcg that has been got can be accessed
> even after the directory has been removed, but it doesn't ensure that parents
> of it can be accessed: parents might have been freed already by rmdir.
> 
> This causes a bug in case of use_hierarchy==1, because res_counter_uncharge
> climb up the tree.
> 
> This patch tries to fix this probrem by getting the parent at create, and
> putting it at freeing.
> 
> Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>

Seems very simple and promissive.
But one nitpick

> ---
>  mm/memcontrol.c |   29 ++++++++++++++++++++++++++++-
>  1 files changed, 28 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index fb62b43..a80ba68 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -202,6 +202,8 @@ pcg_default_flags[NR_CHARGE_TYPE] = {
>  
>  static void mem_cgroup_get(struct mem_cgroup *mem);
>  static void mem_cgroup_put(struct mem_cgroup *mem);
> +static void mem_cgroup_get_parent(struct mem_cgroup *mem);
> +static void mem_cgroup_put_parent(struct mem_cgroup *mem);
>  
>  static void mem_cgroup_charge_statistics(struct mem_cgroup *mem,
>  					 struct page_cgroup *pc,
> @@ -2185,10 +2187,34 @@ static void mem_cgroup_get(struct mem_cgroup *mem)
>  
>  static void mem_cgroup_put(struct mem_cgroup *mem)
>  {
> -	if (atomic_dec_and_test(&mem->refcnt))
> +	if (atomic_dec_and_test(&mem->refcnt)) {
> +		mem_cgroup_put_parent(mem);
>  		__mem_cgroup_free(mem);
> +	}
> +}

Here, parent is freed before children is freed. Then,

==
	if (atomic_dec_and_test(&mem->refcnt)) {
		struct mem_cgroup *parent = parent_mem_cgroup(mem);
		__mem_cgroup_free(mem);
		mem_cgroup_put(parent);
	}
==

Is maybe usual way.

-Kame




> +
> +static struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *mem)
> +{
> +	if (!mem->res.parent)
> +		return NULL;
> +	return mem_cgroup_from_res_counter(mem->res.parent, res);
> +}
> +
> +static void mem_cgroup_get_parent(struct mem_cgroup *mem)
> +{
> +	struct mem_cgroup *parent = parent_mem_cgroup(mem);
> +
> +	if (parent)
> +		mem_cgroup_get(parent);
>  }
>  
> +static void mem_cgroup_put_parent(struct mem_cgroup *mem)
> +{
> +	struct mem_cgroup *parent = parent_mem_cgroup(mem);
> +
> +	if (parent)
> +		mem_cgroup_put(parent);
> +}
>  
>  #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
>  static void __init enable_swap_cgroup(void)
> @@ -2237,6 +2263,7 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
>  	if (parent)
>  		mem->swappiness = get_swappiness(parent);
>  	atomic_set(&mem->refcnt, 1);
> +	mem_cgroup_get_parent(mem);
>  	return &mem->css;
>  free_out:
>  	__mem_cgroup_free(mem);
> 

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