linux-kernel - Re: [PATCH] mm: memcontrol: replace the loop with a list_for_each

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Message-ID: <YBwPQUwLQ6hAbdSV@cmpxchg.org>
Date:   Thu, 4 Feb 2021 10:14:09 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     mhocko@...nel.org, vdavydov.dev@...il.com,
        akpm@...ux-foundation.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: memcontrol: replace the loop with a
 list_for_each_entry()

On Thu, Feb 04, 2021 at 06:53:20PM +0800, Muchun Song wrote:
> The rule of list walk has gone since:
> 
>  commit a9d5adeeb4b2 ("mm/memcontrol: allow to uncharge page without using page->lru field")
> 
> So remove the strange comment and replace the loop with a
> list_for_each_entry().
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  mm/memcontrol.c | 17 ++---------------
>  1 file changed, 2 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6c7f1ea3955e..43341bd7ea1c 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6891,24 +6891,11 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)
>  static void uncharge_list(struct list_head *page_list)
>  {
>  	struct uncharge_gather ug;
> -	struct list_head *next;
> +	struct page *page;
>  
>  	uncharge_gather_clear(&ug);
> -
> -	/*
> -	 * 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 {
> -		struct page *page;
> -
> -		page = list_entry(next, struct page, lru);
> -		next = page->lru.next;
> -
> +	list_for_each_entry(page, page_list, lru)
>  		uncharge_page(page, &ug);
> -	} while (next != page_list);
> -
>  	uncharge_batch(&ug);

Good catch, this makes things much simpler.

Looking at the surrounding code, there also seems to be no reason
anymore to have uncharge_list() as a separate function: there is only
one caller after the mentioned commit, and it's trivial after your
change. Would you mind folding it into mem_cgroup_uncharge_list()?

The list_empty() check in that one is also unnecessary now: the
do-while loop required at least one page to be on the list or it would
crash, but list_for_each() will be just fine on an empty list.

Thanks