[PATCH] memcg: workingset: remove folio_memcg_rcu usage

[PATCH] memcg: workingset: remove folio_memcg_rcu usage

[Shakeel Butt]

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@linux.dev>
Suggested-by: Yu Zhao <yuzhao@google.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.
 	 */
-	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));
-out:
-	rcu_read_unlock();
 }
 
 /*
-- 
2.43.5

Re: [PATCH] memcg: workingset: remove folio_memcg_rcu usage

[Yu Zhao]

On Sat, Oct 26, 2024 at 1:12 AM Shakeel Butt <shakeel.butt@linux.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@linux.dev>
> Suggested-by: Yu Zhao <yuzhao@google.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
>

Re: [PATCH] memcg: workingset: remove folio_memcg_rcu usage

[Shakeel Butt]

On Sat, Oct 26, 2024 at 09:37:00AM GMT, Yu Zhao wrote:
> On Sat, Oct 26, 2024 at 1:12 AM Shakeel Butt <shakeel.butt@linux.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@linux.dev>
> > Suggested-by: Yu Zhao <yuzhao@google.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?)
> 

Yes, you are right. Let me send the v2 with the fix.