Re: [PATCH] mm: memcontrol: clarify the uncharge_list() loop

[Michal Hocko]

On Thu 10-03-16 15:50:15, Johannes Weiner wrote:
> uncharge_list() does an unusual list walk because the function can
> take regular lists with dedicated list_heads as well as singleton
> lists where a single page is passed via the page->lru list node.
> 
> This can sometimes lead to confusion as well as suggestions to replace
> the loop with a list_for_each_entry(), which wouldn't work.

Yes this confused at least me 2 times AFAIR.

> 
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks!

> ---
>  mm/memcontrol.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8614e0d750e5..fa7bf354ae32 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5420,6 +5420,10 @@ static void uncharge_list(struct list_head *page_list)
>  	struct list_head *next;
>  	struct page *page;
>  
> +	/*
> +	 * Note that the list can be a single page->lru; hence the
> +	 * do-while loop instead of a simple list_for_each_entry().
> +	 */
>  	next = page_list->next;
>  	do {
>  		unsigned int nr_pages = 1;
> -- 
> 2.7.2

-- 
Michal Hocko
SUSE Labs