linux-kernel - Re: [PATCH] memcg: workingset: remove folio_memcg

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Message-ID: <CAOUHufYjLWJ6WgHjmRrBTNLFyFSaMG8tSV4JExqMTKADnd=sDg@mail.gmail.com>
Date: Sat, 26 Oct 2024 09:37:00 -0600
From: Yu Zhao <yuzhao@...gle.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Hocko <mhocko@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Muchun Song <muchun.song@...ux.dev>, Hugh Dickins <hughd@...gle.com>, 
	Yosry Ahmed <yosryahmed@...gle.com>, linux-mm@...ck.org, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH] memcg: workingset: remove folio_memcg_rcu usage

On Sat, Oct 26, 2024 at 1:12 AM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
>
> The function workingset_activation() is called from
> folio_mark_accessed() with the guarantee that the given folio can not be
> freed under us in workingset_activation(). In addition, the association
> of the folio and its memcg can not be broken here because charge
> migration is no more. There is no need to use folio_memcg_rcu. Simply
> use folio_memcg_charged() because that is what this function cares
> about.
>
> Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>
> Suggested-by: Yu Zhao <yuzhao@...gle.com>
> ---
>
> Andrew, please put this patch after the charge migration deprecation
> series.
>
>  include/linux/memcontrol.h | 35 -----------------------------------
>  mm/workingset.c            | 10 ++--------
>  2 files changed, 2 insertions(+), 43 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 932534291ca2..89a1e9f10e1b 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -443,35 +443,6 @@ static inline bool folio_memcg_charged(struct folio *folio)
>         return __folio_memcg(folio) != NULL;
>  }
>
> -/**
> - * folio_memcg_rcu - Locklessly get the memory cgroup associated with a folio.
> - * @folio: Pointer to the folio.
> - *
> - * This function assumes that the folio is known to have a
> - * proper memory cgroup pointer. It's not safe to call this function
> - * against some type of folios, e.g. slab folios or ex-slab folios.
> - *
> - * Return: A pointer to the memory cgroup associated with the folio,
> - * or NULL.
> - */
> -static inline struct mem_cgroup *folio_memcg_rcu(struct folio *folio)
> -{
> -       unsigned long memcg_data = READ_ONCE(folio->memcg_data);
> -
> -       VM_BUG_ON_FOLIO(folio_test_slab(folio), folio);
> -
> -       if (memcg_data & MEMCG_DATA_KMEM) {
> -               struct obj_cgroup *objcg;
> -
> -               objcg = (void *)(memcg_data & ~OBJEXTS_FLAGS_MASK);
> -               return obj_cgroup_memcg(objcg);
> -       }
> -
> -       WARN_ON_ONCE(!rcu_read_lock_held());
> -
> -       return (struct mem_cgroup *)(memcg_data & ~OBJEXTS_FLAGS_MASK);
> -}
> -
>  /*
>   * folio_memcg_check - Get the memory cgroup associated with a folio.
>   * @folio: Pointer to the folio.
> @@ -1084,12 +1055,6 @@ static inline struct mem_cgroup *folio_memcg(struct folio *folio)
>         return NULL;
>  }
>
> -static inline struct mem_cgroup *folio_memcg_rcu(struct folio *folio)
> -{
> -       WARN_ON_ONCE(!rcu_read_lock_held());
> -       return NULL;
> -}
> -
>  static inline struct mem_cgroup *folio_memcg_check(struct folio *folio)
>  {
>         return NULL;
> diff --git a/mm/workingset.c b/mm/workingset.c
> index 4b58ef535a17..c47aa482fad5 100644
> --- a/mm/workingset.c
> +++ b/mm/workingset.c
> @@ -591,9 +591,6 @@ void workingset_refault(struct folio *folio, void *shadow)
>   */
>  void workingset_activation(struct folio *folio)
>  {
> -       struct mem_cgroup *memcg;
> -
> -       rcu_read_lock();
>         /*
>          * Filter non-memcg pages here, e.g. unmap can call
>          * mark_page_accessed() on VDSO pages.
> @@ -601,12 +598,9 @@ void workingset_activation(struct folio *folio)
>          * XXX: See workingset_refault() - this should return
>          * root_mem_cgroup even for !CONFIG_MEMCG.

The "XXX" part of the comments doesn't apply anymore.

>          */
> -       memcg = folio_memcg_rcu(folio);
> -       if (!mem_cgroup_disabled() && !memcg)
> -               goto out;
> +       if (mem_cgroup_disabled() || !folio_memcg_charged(folio))
> +               return;
>         workingset_age_nonresident(folio_lruvec(folio), folio_nr_pages(folio));

 if (mem_cgroup_disabled() || folio_memcg_charged(folio))
    workingset_age_nonresident()

We still want to call workingset_age_nonresident() when memcg is
disabled. (We just want to "Filter non-memcg pages here" when memcg is
enabled, as the comment above says, which I assume still applies?)

> -out:
> -       rcu_read_unlock();
>  }
>
>  /*
> --
> 2.43.5
>