[PATCH v2] memcg: workingset: remove folio_memcg_rcu usage

[PATCH v2] 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 charge migration deprecation series.

Changes since v1:
- Fix for mem_cgroup_disabled(). (Yu Zhao)

 include/linux/memcontrol.h | 35 -----------------------------------
 mm/workingset.c            | 14 ++------------
 2 files changed, 2 insertions(+), 47 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..a4705e196545 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -591,22 +591,12 @@ 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.
-	 *
-	 * 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;
-	workingset_age_nonresident(folio_lruvec(folio), folio_nr_pages(folio));
-out:
-	rcu_read_unlock();
+	if (mem_cgroup_disabled() || folio_memcg_charged(folio))
+		workingset_age_nonresident(folio_lruvec(folio), folio_nr_pages(folio));
 }
 
 /*
-- 
2.43.5

Re: [PATCH v2] memcg: workingset: remove folio_memcg_rcu usage

[Shakeel Butt]

On Sat, Oct 26, 2024 at 09:37:07AM GMT, Shakeel Butt 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 charge migration deprecation series.
> 
> Changes since v1:
> - Fix for mem_cgroup_disabled(). (Yu Zhao)
> 

It seems like folio_memcg_charged() is not defined for CONFIG_MEMCG=n
config option. The following stub should fix the build for such config.


diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 89a1e9f10e1b..5502aa8e138e 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1055,6 +1055,11 @@ static inline struct mem_cgroup *folio_memcg(struct folio *folio)
 	return NULL;
 }
 
+static inline bool folio_memcg_charged(struct folio *folio)
+{
+	return false;
+}
+
 static inline struct mem_cgroup *folio_memcg_check(struct folio *folio)
 {
 	return NULL;

Re: [PATCH v2] memcg: workingset: remove folio_memcg_rcu usage

[SeongJae Park]

On Sat, 26 Oct 2024 23:55:14 -0700 Shakeel Butt <shakeel.butt@linux.dev> wrote:

[...]
 It seems like folio_memcg_charged() is not defined for CONFIG_MEMCG=n
> config option. The following stub should fix the build for such config.
> 
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 89a1e9f10e1b..5502aa8e138e 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1055,6 +1055,11 @@ static inline struct mem_cgroup *folio_memcg(struct folio *folio)
>  	return NULL;
>  }
>  
> +static inline bool folio_memcg_charged(struct folio *folio)
> +{
> +	return false;
> +}
> +
>  static inline struct mem_cgroup *folio_memcg_check(struct folio *folio)
>  {
>  	return NULL;

Tested-by: SeongJae Park <sj@kernel.org>


Thanks,
SJ