[PATCH] mm: memcontrol: replace the loop with a list_for_each_entry()

[PATCH] mm: memcontrol: replace the loop with a list_for_each_entry()

[Muchun Song]

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@bytedance.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);
 }
 
-- 
2.11.0

Re: [PATCH] mm: memcontrol: replace the loop with a list_for_each_entry()

[Johannes Weiner]

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@bytedance.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

Re: [External] Re: [PATCH] mm: memcontrol: replace the loop with a list_for_each_entry()

[Muchun Song]

On Thu, Feb 4, 2021 at 11:14 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> 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@bytedance.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()?

Will do. Thanks.

>
> 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.

Right. It makes things more simple.

>
> Thanks