memcg: workingset: remove folio_memcg_rcu usage

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.

[akpm@linux-foundation.org: provide folio_memcg_charged stub for CONFIG_MEMCG=n]
Link: https://lkml.kernel.org/r/20241026163707.2479526-1-shakeel.butt@linux.dev
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Suggested-by: Yu Zhao <yuzhao@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: shakeelb

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.
@@ -1084,10 +1055,9 @@ static inline struct mem_cgroup *folio_memcg(struct folio *folio)
 	return NULL;
 }
 
-static inline struct mem_cgroup *folio_memcg_rcu(struct folio *folio)
+static inline bool folio_memcg_charged(struct folio *folio)
 {
-	WARN_ON_ONCE(!rcu_read_lock_held());
-	return NULL;
+	return false;
 }
 
 static inline struct mem_cgroup *folio_memcg_check(struct folio *folio)

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));
 }
 
 /*