mm-memcontrol-make-obj_cgroup_memcg-handle-null-objcg.patch removed from -mm tree

The quilt patch titled
Subject: mm: memcontrol: make obj_cgroup_memcg() handle NULL objcg
has been removed from the -mm tree. Its filename was
mm-memcontrol-make-obj_cgroup_memcg-handle-null-objcg.patch

This patch was dropped because it was merged into the mm-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Qi Zheng <[email protected]>
Subject: mm: memcontrol: make obj_cgroup_memcg() handle NULL objcg
Date: Mon, 17 Aug 2026 17:03:26 +0800

Patch series "make unused huge shrinker memcg aware", v4.

The shmem unused huge shrinker maintains a per-superblock list of inodes whose tail huge folio extends beyond i_size. Because this list is not memcg aware, reclaim triggered by memcg A can scan inodes across the entire superblock and split huge folios charged to unrelated memcg B, causing unexpected impact on it.

In the worst case, memcg A has no reclaimable shmem at all, making the reclaim entirely useless and incurring unnecessary latency. We observed this in production, where page lock contention during split caused multi-hundred-millisecond stalls:

tid 11340 comm scanner locked a page for 182264 us! kstack:

          unlock_page+1
          split_huge_page_to_list+3135
          shmem_unused_huge_shrink+767
          super_cache_scan+329
          do_shrink_slab+291
          shrink_slab+533
          shrink_node+400
          do_try_to_free_pages+206
          try_to_free_mem_cgroup_pages+262
          try_charge_memcg+591
          mem_cgroup_charge+136
          __handle_mm_fault+2431
          handle_mm_fault+194
          do_user_addr_fault+462
          __do_page_fault+176
          do_page_fault+48
          page_fault+62

Usama's recent patch [1] prevents the shmem unused shrinker from being invoked during memcg-level reclaim altogether, but this is overly
conservative: we can do better by reclaiming only the shmem charged to the reclaiming memcg.

This series converts the shrinker list to a memcg-aware list_lru, so that non-root memcg reclaim walks only candidates charged to the reclaiming memcg. Global reclaim, root memcg reclaim and shmem quota reclaim retain their existing global semantics.

To avoid pinning a dying memcg through a long-lived CSS reference, each inode stores an obj_cgroup reference instead of a mem_cgroup reference. The list_lru add/delete paths resolve the current memcg from the objcg under RCU, staying consistent with list_lru's own memcg migration on offline.

This patch (of 3):

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.

---

 include/linux/memcontrol.h |   11 ++++++++---
 mm/page_owner.c            |    2 +-
 mm/zswap.c                 |   17 ++---------------
 3 files changed, 11 insertions(+), 19 deletions(-)

@@ -380,7 +380,7 @@ 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;
 }
 
 /*

@@ -433,7 +433,7 @@ static inline struct mem_cgroup *folio_m
 {
 	struct obj_cgroup *objcg = folio_objcg(folio);
 
-	return objcg ? obj_cgroup_memcg(objcg) : NULL;
+	return obj_cgroup_memcg(objcg);
 }
 
 /*

@@ -476,7 +476,7 @@ static inline struct mem_cgroup *folio_m
 
 	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)

@@ -1050,6 +1050,11 @@ 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)

@@ -575,7 +575,7 @@ static inline int print_page_owner_memcg
 	}
 
 	objcg = (void *)(memcg_data & ~OBJEXTS_FLAGS_MASK);
-	memcg = objcg ? obj_cgroup_memcg(objcg) : NULL;
+	memcg = obj_cgroup_memcg(objcg);
 	if (!memcg)
 		goto out_unlock;
 

@@ -647,19 +647,6 @@ static int zswap_enabled_param_set(const
 * 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_e
 	 * 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_e
 	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();

_

Patches currently in -mm which might be from [email protected] are