Re: [PATCH] memcg: put soft limit reclaim out of way if the excess tree is empty

[Vladimir Davydov]

On Mon, Aug 01, 2016 at 12:00:21PM +0200, Michal Hocko wrote:
...
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c265212bec8c..eb7e39c2d948 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2543,6 +2543,11 @@ static int mem_cgroup_resize_memsw_limit(struct mem_cgroup *memcg,
>  	return ret;
>  }
>  
> +static inline bool soft_limit_tree_empty(struct mem_cgroup_tree_per_node *mctz)
> +{
> +	return rb_last(&mctz->rb_root) == NULL;
> +}
> +

I don't think traversing rb tree as rb_last() does w/o holding the lock
is a good idea. Why is RB_EMPTY_ROOT() insufficient here?