Re: [RFC][PATCH 5/4] memcg: don't call res_counter_uncharge when obsolete

["KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>]

Daisuke Nishimura さんは書きました：
> This is a new one. Please review.
>
> ===
> From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
>
> 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.
>
> Check if the memcg is obsolete, and don't call res_counter_uncharge when
> obsole.
>
Hmm, did you see panic ?
To handle the problem "parent may be obsolete",

call mem_cgroup_get(parent) at create()
call mem_cgroup_put(parent) at freeing memcg.
     (regardless of use_hierarchy.)

is clearer way to go, I think.

I wonder whether there is  mis-accounting problem or not..

So, adding css_tryget() around problematic code can be a fix.
--
  mem = swap_cgroup_record();
  if (css_tryget(&mem->css)) {
      res_counter_uncharge(&mem->memsw, PAZE_SIZE);
      css_put(&mem->css)
  }
--
I like css_tryget() rather than mem_cgroup_obsolete().
To be honest, I'd like to remove memcg special stuff when I can.

Thanks,
-Kame

> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> ---
>  mm/memcontrol.c |    9 ++++++---
>  1 files changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index fb62b43..4ee95a8 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1182,7 +1182,8 @@ int mem_cgroup_cache_charge(struct page *page,
> struct mm_struct *mm,
>  		/* avoid double counting */
>  		mem = swap_cgroup_record(ent, NULL);
>  		if (mem) {
> -			res_counter_uncharge(&mem->memsw, PAGE_SIZE);
> +			if (!mem_cgroup_is_obsolete(mem))
> +				res_counter_uncharge(&mem->memsw, PAGE_SIZE);
>  			mem_cgroup_put(mem);
>  		}
>  	}
> @@ -1252,7 +1253,8 @@ void mem_cgroup_commit_charge_swapin(struct page
> *page, struct mem_cgroup *ptr)
>  		struct mem_cgroup *memcg;
>  		memcg = swap_cgroup_record(ent, NULL);
>  		if (memcg) {
> -			res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
> +			if (!mem_cgroup_is_obsolete(memcg))
> +				res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
>  			mem_cgroup_put(memcg);
>  		}
>
> @@ -1397,7 +1399,8 @@ void mem_cgroup_uncharge_swap(swp_entry_t ent)
>
>  	memcg = swap_cgroup_record(ent, NULL);
>  	if (memcg) {
> -		res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
> +		if (!mem_cgroup_is_obsolete(memcg))
> +			res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
>  		mem_cgroup_put(memcg);
>  	}
>  }
>
> --
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