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

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 16 Jan 2009 10:50:09 +0900 Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> This version works well in my test.
> 
> Andrew, please pick up this one.
> 
> ===
> From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> 
> The lifetime of struct cgroup and struct mem_cgroup is different and
> mem_cgroup has its own reference count for handling references from swap_cgroup.
> 
> This causes strange problem that the parent mem_cgroup dies while
> child mem_cgroup alive, and this problem causes a bug in case of use_hierarchy==1
> because res_counter_uncharge climbs up the tree.
> 
> This patch is for avoiding it by getting the parent at create, and
> putting it at freeing.
> 
> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> Reviewed-by; KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
>  mm/memcontrol.c |   23 ++++++++++++++++++++++-
>  1 files changed, 22 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index fb62b43..45e1b51 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 struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *mem);
> +static void mem_cgroup_get_parent(struct mem_cgroup *mem);
>  
>  static void mem_cgroup_charge_statistics(struct mem_cgroup *mem,
>  					 struct page_cgroup *pc,
> @@ -2185,10 +2187,28 @@ 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)) {
> +		struct mem_cgroup *parent = parent_mem_cgroup(mem);
>  		__mem_cgroup_free(mem);
> +		if (parent)
> +			mem_cgroup_put(parent);
> +	}
> +}
> +
> +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);
> +}
>  
>  #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
>  static void __init enable_swap_cgroup(void)
> @@ -2237,6 +2257,7 @@ 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);

It seems strange that we add a little helper function for the get(),
but open-code the put()?