[PATCH v4 2/4] mm: memcontrol: make obj_cgroup_memcg() handle NULL objcg

[Qi Zheng]

From: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>

obj_cgroup_memcg() currently requires a non-NULL objcg, so callers that
may hold a NULL objcg must guard the call with an explicit NULL check.
This pattern is duplicated in folio_memcg(), folio_memcg_check(),
mm/page_owner.c, and mm/zswap.c.

Teach obj_cgroup_memcg() to accept NULL and return NULL in that case, then
remove the redundant NULL checks at the call sites.  Also remove the
mem_cgroup_from_entry() wrapper in zswap, which existed solely to provide
this NULL-safe behaviour, and replace its two callers with direct
obj_cgroup_memcg() calls.

No functional change intended.

Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
---
 include/linux/memcontrol.h | 11 ++++++++---
 mm/page_owner.c            |  2 +-
 mm/zswap.c                 | 17 ++---------------
 3 files changed, 11 insertions(+), 19 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index cba35690c21aa..5107c227ae9b8 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -375,7 +375,7 @@ enum objext_flags {
 static inline struct mem_cgroup *obj_cgroup_memcg(struct obj_cgroup *objcg)
 {
 	lockdep_assert_once(rcu_read_lock_held() || lockdep_is_held(&cgroup_mutex));
-	return READ_ONCE(objcg->memcg);
+	return objcg ? READ_ONCE(objcg->memcg) : NULL;
 }
 
 /*
@@ -428,7 +428,7 @@ static inline struct mem_cgroup *folio_memcg(struct folio *folio)
 {
 	struct obj_cgroup *objcg = folio_objcg(folio);
 
-	return objcg ? obj_cgroup_memcg(objcg) : NULL;
+	return obj_cgroup_memcg(objcg);
 }
 
 /*
@@ -471,7 +471,7 @@ static inline struct mem_cgroup *folio_memcg_check(struct folio *folio)
 
 	objcg = (void *)(memcg_data & ~OBJEXTS_FLAGS_MASK);
 
-	return objcg ? obj_cgroup_memcg(objcg) : NULL;
+	return obj_cgroup_memcg(objcg);
 }
 
 static inline struct mem_cgroup *page_memcg_check(struct page *page)
@@ -1045,6 +1045,11 @@ void mem_cgroup_flush_workqueue(void);
 extern int mem_cgroup_init(void);
 #else /* CONFIG_MEMCG */
 
+static inline struct mem_cgroup *obj_cgroup_memcg(struct obj_cgroup *objcg)
+{
+	return NULL;
+}
+
 #define MEM_CGROUP_ID_SHIFT	0
 
 #define root_mem_cgroup		(NULL)
diff --git a/mm/page_owner.c b/mm/page_owner.c
index fbbda7ba914ba..3fc37d9b908ef 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -575,7 +575,7 @@ static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret,
 	}
 
 	objcg = (void *)(memcg_data & ~OBJEXTS_FLAGS_MASK);
-	memcg = objcg ? obj_cgroup_memcg(objcg) : NULL;
+	memcg = obj_cgroup_memcg(objcg);
 	if (!memcg)
 		goto out_unlock;
 
diff --git a/mm/zswap.c b/mm/zswap.c
index 37f34e406c8e3..c1dc60926bad9 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -647,19 +647,6 @@ static int zswap_enabled_param_set(const char *val,
 * lru functions
 **********************************/
 
-/* should be called under RCU */
-#ifdef CONFIG_MEMCG
-static inline struct mem_cgroup *mem_cgroup_from_entry(struct zswap_entry *entry)
-{
-	return entry->objcg ? obj_cgroup_memcg(entry->objcg) : NULL;
-}
-#else
-static inline struct mem_cgroup *mem_cgroup_from_entry(struct zswap_entry *entry)
-{
-	return NULL;
-}
-#endif
-
 static inline int entry_to_nid(struct zswap_entry *entry)
 {
 	return page_to_nid(virt_to_page(entry));
@@ -682,7 +669,7 @@ static void zswap_lru_add(struct zswap_entry *entry)
 	 * Similar reasoning holds for list_lru_del().
 	 */
 	rcu_read_lock();
-	memcg = mem_cgroup_from_entry(entry);
+	memcg = obj_cgroup_memcg(entry->objcg);
 	/* will always succeed */
 	list_lru_add(&zswap_list_lru, &entry->lru, nid, memcg);
 	rcu_read_unlock();
@@ -694,7 +681,7 @@ static void zswap_lru_del(struct zswap_entry *entry)
 	struct mem_cgroup *memcg;
 
 	rcu_read_lock();
-	memcg = mem_cgroup_from_entry(entry);
+	memcg = obj_cgroup_memcg(entry->objcg);
 	/* will always succeed */
 	list_lru_del(&zswap_list_lru, &entry->lru, nid, memcg);
 	rcu_read_unlock();
-- 
2.54.0