Re: [PATCH] memcg: oom: fix totalpages calculation for memory.swappiness==0

[Michal Hocko <mhocko@suse.cz>]

On Thu 11-10-12 10:57:39, Michal Hocko wrote:
> oom_badness takes totalpages argument which says how many pages are
> available and it uses it as a base for the score calculation. The value
> is calculated by mem_cgroup_get_limit which considers both limit and
> total_swap_pages (resp. memsw portion of it).
> 
> This is usually correct but since fe35004f (mm: avoid swapping out
> with swappiness==0) we do not swap when swappiness is 0 which means
> that we cannot really use up all the totalpages pages. This in turn
> confuses oom score calculation if the memcg limit is much smaller than
> the available swap because the used memory (capped by the limit) is
> negligible comparing to totalpages so the resulting score is too small
> if adj!=0 (typically task with CAP_SYS_ADMIN or non zero oom_score_adj).
> A wrong process might be selected as result.
> 
> The same issue exists for the global oom killer as well but it is not
> that problematic as the amount of the RAM is usually much bigger than
> the swap space.
> 
> The problem can be worked around by checking mem_cgroup_swappiness==0
> and not considering swap at all in such a case.
> 
> Signed-off-by: Michal Hocko <mhocko@suse.cz>
> Acked-by: David Rientjes <rientjes@google.com>
> Cc: stable [3.5+]

I have just realized that fe35004f (introduced in 3.5-rc1) has been
backported to 3.2 and 3.4 stable kernels so this should be [3.2+]

> ---
>  mm/memcontrol.c |   21 +++++++++++++++------
>  1 file changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 7acf43b..93a7e36 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1452,17 +1452,26 @@ static int mem_cgroup_count_children(struct mem_cgroup *memcg)
>  static u64 mem_cgroup_get_limit(struct mem_cgroup *memcg)
>  {
>  	u64 limit;
> -	u64 memsw;
>  
>  	limit = res_counter_read_u64(&memcg->res, RES_LIMIT);
> -	limit += total_swap_pages << PAGE_SHIFT;
>  
> -	memsw = res_counter_read_u64(&memcg->memsw, RES_LIMIT);
>  	/*
> -	 * If memsw is finite and limits the amount of swap space available
> -	 * to this memcg, return that limit.
> +	 * Do not consider swap space if we cannot swap due to swappiness
>  	 */
> -	return min(limit, memsw);
> +	if (mem_cgroup_swappiness(memcg)) {
> +		u64 memsw;
> +
> +		limit += total_swap_pages << PAGE_SHIFT;
> +		memsw = res_counter_read_u64(&memcg->memsw, RES_LIMIT);
> +
> +		/*
> +		 * If memsw is finite and limits the amount of swap space
> +		 * available to this memcg, return that limit.
> +		 */
> +		limit = min(limit, memsw);
> +	}
> +
> +	return limit;
>  }
>  
>  void mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
> -- 
> 1.7.10.4
> 
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-- 
Michal Hocko
SUSE Labs

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