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

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Date:   Thu, 4 Feb 2021 23:55:45 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Cgroups <cgroups@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH] mm: memcontrol: replace the loop with a list_for_each_entry()

On Thu, Feb 4, 2021 at 11:14 PM Johannes Weiner <hannes@...xchg.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@...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()?

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