Re: [PATCH 2/3] mm/memcg: set pos to prev unconditionally

[Johannes Weiner <hannes@xxxxxxxxxxx>]

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@xxxxxxxxx>
> ---
>  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;
	}