From mboxrd@z Thu Jan  1 00:00:00 1970
From: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
Subject: Re: [PATCH 2/3] mm/memcg: set pos to prev unconditionally
Date: Tue, 29 Mar 2022 14:48:05 -0400
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References: <20220225003437.12620-1-richard.weiyang@gmail.com>
 <20220225003437.12620-3-richard.weiyang@gmail.com>
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To: Wei Yang <richard.weiyang-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, vdavydov.dev-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org

On Fri, Feb 25, 2022 at 12:34:36AM +0000, Wei Yang wrote:
> Current code set pos to prev based on condition (prev && !reclaim),
> while we can do this unconditionally.
> 
> Since:
> 
>   * If !reclaim, pos is the same as prev no matter it is NULL or not.
>   * If reclaim, pos would be set properly from iter->position.
> 
> Signed-off-by: Wei Yang <richard.weiyang-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> ---
>  mm/memcontrol.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 9464fe2aa329..03399146168f 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -980,7 +980,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>  	struct mem_cgroup_reclaim_iter *iter;
>  	struct cgroup_subsys_state *css = NULL;
>  	struct mem_cgroup *memcg = NULL;
> -	struct mem_cgroup *pos = NULL;
> +	struct mem_cgroup *pos = prev;

I don't like this so much. It suggests pos always starts with prev, no
matter what. But this isn't true for reclaim mode, which overrides the
initialized value again.

>  	if (mem_cgroup_disabled())
>  		return NULL;
> @@ -988,9 +988,6 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>  	if (!root)
>  		root = root_mem_cgroup;
>  
> -	if (prev && !reclaim)
> -		pos = prev;

How about making the reclaim vs non-reclaim mode explicit and do:

	if (reclaim) {
		...
		pos = iter->position;
		...
	} else {
		pos = prev;
	}