Re: [External] Re: [PATCH] mm: memcontrol: remove rcu_read_lock from get_mem_cgroup_from_page

[Muchun Song]

On Sat, Feb 6, 2021 at 2:15 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> On Fri, Feb 05, 2021 at 11:32:24AM +0100, Michal Hocko wrote:
> > On Fri 05-02-21 17:14:30, Muchun Song wrote:
> > > On Fri, Feb 5, 2021 at 4:36 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > > >
> > > > On Fri 05-02-21 14:27:19, Muchun Song wrote:
> > > > > The get_mem_cgroup_from_page() is called under page lock, so the page
> > > > > memcg cannot be changed under us.
> > > >
> > > > Where is the page lock enforced?
> > >
> > > Because it is called from alloc_page_buffers(). This path is under
> > > page lock.
> >
> > I do not see any page lock enforecement there. There is not even a
> > comment requiring that. Can we grow more users where this is not the
> > case? There is no actual relation between alloc_page_buffers and
> > get_mem_cgroup_from_page except that the former is the only _current_
> > existing user. I would be careful to dictate locking based solely on
> > that.
>
> Since alloc_page_buffers() holds the page lock throughout the entire
> time it uses the memcg, there is no actual reason for it to use RCU or
> even acquire an additional reference on the css. We know it's pinned,
> the charge pins it, and the page lock pins the charge. It can neither
> move to a different cgroup nor be uncharged.

Thanks for your patient explanation.

>
> So what do you say we switch alloc_page_buffers() to page_memcg()?

It's better than mine.

>
> And because that removes the last user of get_mem_cgroup_from_page(),
> we can kill it off and worry about a good interface once a consumer
> materializes for it.
>
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 96c7604f69b3..12a10f461b81 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -847,7 +847,7 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
>         if (retry)
>                 gfp |= __GFP_NOFAIL;
>
> -       memcg = get_mem_cgroup_from_page(page);
> +       memcg = page_memcg(page);
>         old_memcg = set_active_memcg(memcg);
>
>         head = NULL;
> @@ -868,7 +868,6 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
>         }
>  out:
>         set_active_memcg(old_memcg);
> -       mem_cgroup_put(memcg);
>         return head;
>  /*
>   * In case anything failed, we just free everything we got.
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index a8c7a0ccc759..a44b2d51aecc 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -687,8 +687,6 @@ struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p);
>
>  struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm);
>
> -struct mem_cgroup *get_mem_cgroup_from_page(struct page *page);
> -
>  struct lruvec *lock_page_lruvec(struct page *page);
>  struct lruvec *lock_page_lruvec_irq(struct page *page);
>  struct lruvec *lock_page_lruvec_irqsave(struct page *page,
> @@ -1169,11 +1167,6 @@ static inline struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
>         return NULL;
>  }
>
> -static inline struct mem_cgroup *get_mem_cgroup_from_page(struct page *page)
> -{
> -       return NULL;
> -}
> -
>  static inline void mem_cgroup_put(struct mem_cgroup *memcg)
>  {
>  }
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 490357945f2c..ff52550d2f65 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1048,29 +1048,6 @@ struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
>  }
>  EXPORT_SYMBOL(get_mem_cgroup_from_mm);
>
> -/**
> - * get_mem_cgroup_from_page: Obtain a reference on given page's memcg.
> - * @page: page from which memcg should be extracted.
> - *
> - * Obtain a reference on page->memcg and returns it if successful. Otherwise
> - * root_mem_cgroup is returned.
> - */
> -struct mem_cgroup *get_mem_cgroup_from_page(struct page *page)
> -{
> -       struct mem_cgroup *memcg = page_memcg(page);
> -
> -       if (mem_cgroup_disabled())
> -               return NULL;
> -
> -       rcu_read_lock();
> -       /* Page should not get uncharged and freed memcg under us. */
> -       if (!memcg || WARN_ON_ONCE(!css_tryget(&memcg->css)))
> -               memcg = root_mem_cgroup;
> -       rcu_read_unlock();
> -       return memcg;
> -}
> -EXPORT_SYMBOL(get_mem_cgroup_from_page);
> -
>  static __always_inline struct mem_cgroup *active_memcg(void)
>  {
>         if (in_interrupt())