[PATCH v2 1/4] mm: memcontrol: use helpers to access page's memcg data

[Roman Gushchin]

Currently there are many open-coded reads and writes of the
page->mem_cgroup pointer, as well as a couple of read helpers,
which are barely used.

It creates an obstacle on a way to reuse some bits of the pointer
for storing additional bits of information. In fact, we already do
this for slab pages, where the last bit indicates that a pointer has
an attached vector of objcg pointers instead of a regular memcg
pointer.

This commits introduces 4 new helper functions and converts all
raw accesses to page->mem_cgroup to calls of these helpers:
  struct mem_cgroup *page_mem_cgroup(struct page *page);
  struct mem_cgroup *page_mem_cgroup_check(struct page *page);
  void set_page_mem_cgroup(struct page *page, struct mem_cgroup *memcg);
  void clear_page_mem_cgroup(struct page *page);

page_mem_cgroup_check() is intended to be used in cases when the page
can be a slab page and have a memcg pointer pointing at objcg vector.
It does check the lowest bit, and if set, returns NULL.
page_mem_cgroup() contains a VM_BUG_ON_PAGE() check for the page not
being a slab page. So do set_page_mem_cgroup() and clear_page_mem_cgroup().

To make sure nobody uses a direct access, struct page's
mem_cgroup/obj_cgroups is converted to unsigned long memcg_data.
Only new helpers and a couple of slab-accounting related functions
access this field directly.

page_memcg() and page_memcg_rcu() helpers defined in mm.h are removed.
New page_mem_cgroup() is a direct analog of page_memcg(), while
page_memcg_rcu() has a single call site in a small rcu-read-lock
section, so it's just not worth it to have a separate helper. So
it's replaced with page_mem_cgroup() too.

Signed-off-by: Roman Gushchin <guro@xxxxxx>
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
---
 fs/buffer.c                      |   2 +-
 fs/iomap/buffered-io.c           |   2 +-
 include/linux/memcontrol.h       | 103 +++++++++++++++++++++++---
 include/linux/mm.h               |  22 ------
 include/linux/mm_types.h         |   5 +-
 include/trace/events/writeback.h |   2 +-
 kernel/fork.c                    |   7 +-
 mm/debug.c                       |   4 +-
 mm/huge_memory.c                 |   4 +-
 mm/memcontrol.c                  | 119 ++++++++++++++-----------------
 mm/migrate.c                     |   2 +-
 mm/page_alloc.c                  |   4 +-
 mm/page_io.c                     |   4 +-
 mm/slab.h                        |   9 ++-
 mm/workingset.c                  |   6 +-
 15 files changed, 170 insertions(+), 125 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index bf4d8037f91b..64564ac7dcc5 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -657,7 +657,7 @@ int __set_page_dirty_buffers(struct page *page)
 		} while (bh != head);
 	}
 	/*
-	 * Lock out page->mem_cgroup migration to keep PageDirty
+	 * Lock out page's memcg migration to keep PageDirty
 	 * synchronized with per-memcg dirty page counters.
 	 */
 	lock_page_memcg(page);
diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 897ab9a26a74..71381931f2c3 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -669,7 +669,7 @@ iomap_set_page_dirty(struct page *page)
 		return !TestSetPageDirty(page);
 
 	/*
-	 * Lock out page->mem_cgroup migration to keep PageDirty
+	 * Lock out page's memcg migration to keep PageDirty
 	 * synchronized with per-memcg dirty page counters.
 	 */
 	lock_page_memcg(page);
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index e391e3c56de5..3313e7c21534 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -343,6 +343,72 @@ struct mem_cgroup {
 
 extern struct mem_cgroup *root_mem_cgroup;
 
+/*
+ * page_mem_cgroup - get the memory cgroup associated with a page
+ * @page: a pointer to the page struct
+ *
+ * Returns a pointer to the memory cgroup associated with the page,
+ * or NULL. This function assumes that the page is known to have a
+ * proper memory cgroup pointer. It's not safe to call this function
+ * against some type of pages, e.g. slab pages or ex-slab pages.
+ */
+static inline struct mem_cgroup *page_mem_cgroup(struct page *page)
+{
+	VM_BUG_ON_PAGE(PageSlab(page), page);
+	return (struct mem_cgroup *)page->memcg_data;
+}
+
+/*
+ * page_mem_cgroup_check - get the memory cgroup associated with a page
+ * @page: a pointer to the page struct
+ *
+ * Returns a pointer to the memory cgroup associated with the page,
+ * or NULL. This function unlike page_mem_cgroup() can take any  page
+ * as an argument. It has to be used in cases when it's not known if a page
+ * has an associated memory cgroup pointer or an object cgroups vector.
+ */
+static inline struct mem_cgroup *page_mem_cgroup_check(struct page *page)
+{
+	unsigned long memcg_data = page->memcg_data;
+
+	/*
+	 * The lowest bit set means that memcg isn't a valid
+	 * memcg pointer, but a obj_cgroups pointer.
+	 * In this case the page is shared and doesn't belong
+	 * to any specific memory cgroup.
+	 */
+	if (memcg_data & 0x1UL)
+		return NULL;
+
+	return (struct mem_cgroup *)memcg_data;
+}
+
+/*
+ * set_page_mem_cgroup - associate a page with a memory cgroup
+ * @page: a pointer to the page struct
+ * @memcg: a pointer to the memory cgroup
+ *
+ * Associates a page with a memory cgroup.
+ */
+static inline void set_page_mem_cgroup(struct page *page,
+				       struct mem_cgroup *memcg)
+{
+	VM_BUG_ON_PAGE(PageSlab(page), page);
+	page->memcg_data = (unsigned long)memcg;
+}
+
+/*
+ * clear_page_mem_cgroup - clear an association of a page with a memory cgroup
+ * @page: a pointer to the page struct
+ *
+ * Clears an association of a page with a memory cgroup.
+ */
+static inline void clear_page_mem_cgroup(struct page *page)
+{
+	VM_BUG_ON_PAGE(PageSlab(page), page);
+	page->memcg_data = 0;
+}
+
 static __always_inline bool memcg_stat_item_in_bytes(int idx)
 {
 	if (idx == MEMCG_PERCPU_B)
@@ -743,15 +809,15 @@ static inline void mod_memcg_state(struct mem_cgroup *memcg,
 static inline void __mod_memcg_page_state(struct page *page,
 					  int idx, int val)
 {
-	if (page->mem_cgroup)
-		__mod_memcg_state(page->mem_cgroup, idx, val);
+	if (page_mem_cgroup(page))
+		__mod_memcg_state(page_mem_cgroup(page), idx, val);
 }
 
 static inline void mod_memcg_page_state(struct page *page,
 					int idx, int val)
 {
-	if (page->mem_cgroup)
-		mod_memcg_state(page->mem_cgroup, idx, val);
+	if (page_mem_cgroup(page))
+		mod_memcg_state(page_mem_cgroup(page), idx, val);
 }
 
 static inline unsigned long lruvec_page_state(struct lruvec *lruvec,
@@ -838,12 +904,12 @@ static inline void __mod_lruvec_page_state(struct page *page,
 	struct lruvec *lruvec;
 
 	/* Untracked pages have no memcg, no lruvec. Update only the node */
-	if (!head->mem_cgroup) {
+	if (!page_mem_cgroup(head)) {
 		__mod_node_page_state(pgdat, idx, val);
 		return;
 	}
 
-	lruvec = mem_cgroup_lruvec(head->mem_cgroup, pgdat);
+	lruvec = mem_cgroup_lruvec(page_mem_cgroup(head), pgdat);
 	__mod_lruvec_state(lruvec, idx, val);
 }
 
@@ -878,8 +944,8 @@ static inline void count_memcg_events(struct mem_cgroup *memcg,
 static inline void count_memcg_page_event(struct page *page,
 					  enum vm_event_item idx)
 {
-	if (page->mem_cgroup)
-		count_memcg_events(page->mem_cgroup, idx, 1);
+	if (page_mem_cgroup(page))
+		count_memcg_events(page_mem_cgroup(page), idx, 1);
 }
 
 static inline void count_memcg_event_mm(struct mm_struct *mm,
@@ -941,6 +1007,25 @@ void mem_cgroup_split_huge_fixup(struct page *head);
 
 struct mem_cgroup;
 
+static inline struct mem_cgroup *page_mem_cgroup(struct page *page)
+{
+	return NULL;
+}
+
+static inline struct mem_cgroup *page_mem_cgroup_check(struct page *page)
+{
+	return NULL;
+}
+
+static inline void set_page_mem_cgroup(struct page *page,
+				       struct mem_cgroup *memcg)
+{
+}
+
+static inline void clear_page_mem_cgroup(struct page *page)
+{
+}
+
 static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
 {
 	return true;
@@ -1430,7 +1515,7 @@ static inline void mem_cgroup_track_foreign_dirty(struct page *page,
 	if (mem_cgroup_disabled())
 		return;
 
-	if (unlikely(&page->mem_cgroup->css != wb->memcg_css))
+	if (unlikely(&page_mem_cgroup(page)->css != wb->memcg_css))
 		mem_cgroup_track_foreign_dirty_slowpath(page, wb);
 }
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 17e712207d74..5e24ff2ffec9 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1476,28 +1476,6 @@ static inline void set_page_links(struct page *page, enum zone_type zone,
 #endif
 }
 
-#ifdef CONFIG_MEMCG
-static inline struct mem_cgroup *page_memcg(struct page *page)
-{
-	return page->mem_cgroup;
-}
-static inline struct mem_cgroup *page_memcg_rcu(struct page *page)
-{
-	WARN_ON_ONCE(!rcu_read_lock_held());
-	return READ_ONCE(page->mem_cgroup);
-}
-#else
-static inline struct mem_cgroup *page_memcg(struct page *page)
-{
-	return NULL;
-}
-static inline struct mem_cgroup *page_memcg_rcu(struct page *page)
-{
-	WARN_ON_ONCE(!rcu_read_lock_held());
-	return NULL;
-}
-#endif
-
 /*
  * Some inline functions in vmstat.h depend on page_zone()
  */
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 496c3ff97cce..4856d23b1161 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -199,10 +199,7 @@ struct page {
 	atomic_t _refcount;
 
 #ifdef CONFIG_MEMCG
-	union {
-		struct mem_cgroup *mem_cgroup;
-		struct obj_cgroup **obj_cgroups;
-	};
+	unsigned long memcg_data;
 #endif
 
 	/*
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index e7cbccc7c14c..b1fa3ac64fa5 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -257,7 +257,7 @@ TRACE_EVENT(track_foreign_dirty,
 		__entry->ino		= inode ? inode->i_ino : 0;
 		__entry->memcg_id	= wb->memcg_css->id;
 		__entry->cgroup_ino	= __trace_wb_assign_cgroup(wb);
-		__entry->page_cgroup_ino = cgroup_ino(page->mem_cgroup->css.cgroup);
+		__entry->page_cgroup_ino = cgroup_ino(page_mem_cgroup(page)->css.cgroup);
 	),
 
 	TP_printk("bdi %s[%llu]: ino=%lu memcg_id=%u cgroup_ino=%lu page_cgroup_ino=%lu",
diff --git a/kernel/fork.c b/kernel/fork.c
index 138cd6ca50da..ecbd44831130 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -403,9 +403,10 @@ static int memcg_charge_kernel_stack(struct task_struct *tsk)
 
 		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) {
 			/*
-			 * If memcg_kmem_charge_page() fails, page->mem_cgroup
-			 * pointer is NULL, and memcg_kmem_uncharge_page() in
-			 * free_thread_stack() will ignore this page.
+			 * If memcg_kmem_charge_page() fails, page's
+			 * memory cgroup pointer is NULL, and
+			 * memcg_kmem_uncharge_page() in free_thread_stack()
+			 * will ignore this page.
 			 */
 			ret = memcg_kmem_charge_page(vm->pages[i], GFP_KERNEL,
 						     0);
diff --git a/mm/debug.c b/mm/debug.c
index ccca576b2899..8a40b3fefbeb 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -182,8 +182,8 @@ void __dump_page(struct page *page, const char *reason)
 		pr_warn("page dumped because: %s\n", reason);
 
 #ifdef CONFIG_MEMCG
-	if (!page_poisoned && page->mem_cgroup)
-		pr_warn("page->mem_cgroup:%px\n", page->mem_cgroup);
+	if (!page_poisoned && page->memcg_data)
+		pr_warn("pages's memcg:%lx\n", page->memcg_data);
 #endif
 }
 
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index b1c7dc8a6f96..8e9b2749ef21 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -470,7 +470,7 @@ pmd_t maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma)
 #ifdef CONFIG_MEMCG
 static inline struct deferred_split *get_deferred_split_queue(struct page *page)
 {
-	struct mem_cgroup *memcg = compound_head(page)->mem_cgroup;
+	struct mem_cgroup *memcg = page_mem_cgroup(compound_head(page));
 	struct pglist_data *pgdat = NODE_DATA(page_to_nid(page));
 
 	if (memcg)
@@ -2735,7 +2735,7 @@ void deferred_split_huge_page(struct page *page)
 {
 	struct deferred_split *ds_queue = get_deferred_split_queue(page);
 #ifdef CONFIG_MEMCG
-	struct mem_cgroup *memcg = compound_head(page)->mem_cgroup;
+	struct mem_cgroup *memcg = page_mem_cgroup(compound_head(page));
 #endif
 	unsigned long flags;
 
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 9c4a0851348f..40220b7bf96d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -533,7 +533,7 @@ struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page)
 {
 	struct mem_cgroup *memcg;
 
-	memcg = page->mem_cgroup;
+	memcg = page_mem_cgroup(page);
 
 	if (!memcg || !cgroup_subsys_on_dfl(memory_cgrp_subsys))
 		memcg = root_mem_cgroup;
@@ -560,16 +560,7 @@ ino_t page_cgroup_ino(struct page *page)
 	unsigned long ino = 0;
 
 	rcu_read_lock();
-	memcg = page->mem_cgroup;
-
-	/*
-	 * The lowest bit set means that memcg isn't a valid
-	 * memcg pointer, but a obj_cgroups pointer.
-	 * In this case the page is shared and doesn't belong
-	 * to any specific memory cgroup.
-	 */
-	if ((unsigned long) memcg & 0x1UL)
-		memcg = NULL;
+	memcg = page_mem_cgroup_check(page);
 
 	while (memcg && !(memcg->css.flags & CSS_ONLINE))
 		memcg = parent_mem_cgroup(memcg);
@@ -1050,7 +1041,7 @@ EXPORT_SYMBOL(get_mem_cgroup_from_mm);
  */
 struct mem_cgroup *get_mem_cgroup_from_page(struct page *page)
 {
-	struct mem_cgroup *memcg = page->mem_cgroup;
+	struct mem_cgroup *memcg = page_mem_cgroup(page);
 
 	if (mem_cgroup_disabled())
 		return NULL;
@@ -1335,7 +1326,7 @@ int mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
  * @page: the page
  * @pgdat: pgdat of the page
  *
- * This function relies on page->mem_cgroup being stable - see the
+ * This function relies on page and memcg binding being stable - see the
  * access rules in commit_charge().
  */
 struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct pglist_data *pgdat)
@@ -1349,7 +1340,7 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct pglist_data *pgd
 		goto out;
 	}
 
-	memcg = page->mem_cgroup;
+	memcg = page_mem_cgroup(page);
 	/*
 	 * Swapcache readahead pages are added to the LRU - and
 	 * possibly migrated - before they are charged.
@@ -2105,7 +2096,7 @@ void mem_cgroup_print_oom_group(struct mem_cgroup *memcg)
 }
 
 /**
- * lock_page_memcg - lock a page->mem_cgroup binding
+ * lock_page_memcg - lock a page and memcg binding
  * @page: the page
  *
  * This function protects unlocked LRU pages from being moved to
@@ -2137,7 +2128,7 @@ struct mem_cgroup *lock_page_memcg(struct page *page)
 	if (mem_cgroup_disabled())
 		return NULL;
 again:
-	memcg = head->mem_cgroup;
+	memcg = page_mem_cgroup(head);
 	if (unlikely(!memcg))
 		return NULL;
 
@@ -2145,7 +2136,7 @@ struct mem_cgroup *lock_page_memcg(struct page *page)
 		return memcg;
 
 	spin_lock_irqsave(&memcg->move_lock, flags);
-	if (memcg != head->mem_cgroup) {
+	if (memcg != page_mem_cgroup(head)) {
 		spin_unlock_irqrestore(&memcg->move_lock, flags);
 		goto again;
 	}
@@ -2183,14 +2174,14 @@ void __unlock_page_memcg(struct mem_cgroup *memcg)
 }
 
 /**
- * unlock_page_memcg - unlock a page->mem_cgroup binding
+ * unlock_page_memcg - unlock a page and memcg binding
  * @page: the page
  */
 void unlock_page_memcg(struct page *page)
 {
 	struct page *head = compound_head(page);
 
-	__unlock_page_memcg(head->mem_cgroup);
+	__unlock_page_memcg(page_mem_cgroup(head));
 }
 EXPORT_SYMBOL(unlock_page_memcg);
 
@@ -2880,16 +2871,16 @@ static void cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages)
 
 static void commit_charge(struct page *page, struct mem_cgroup *memcg)
 {
-	VM_BUG_ON_PAGE(page->mem_cgroup, page);
+	VM_BUG_ON_PAGE(page_mem_cgroup(page), page);
 	/*
-	 * Any of the following ensures page->mem_cgroup stability:
+	 * Any of the following ensures page and memcg binding stability:
 	 *
 	 * - the page lock
 	 * - LRU isolation
 	 * - lock_page_memcg()
 	 * - exclusive reference
 	 */
-	page->mem_cgroup = memcg;
+	set_page_mem_cgroup(page, memcg);
 }
 
 #ifdef CONFIG_MEMCG_KMEM
@@ -2904,8 +2895,7 @@ int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
 	if (!vec)
 		return -ENOMEM;
 
-	if (cmpxchg(&page->obj_cgroups, NULL,
-		    (struct obj_cgroup **) ((unsigned long)vec | 0x1UL)))
+	if (cmpxchg(&page->memcg_data, 0, (unsigned long)vec | 0x1UL))
 		kfree(vec);
 	else
 		kmemleak_not_leak(vec);
@@ -2928,17 +2918,6 @@ struct mem_cgroup *mem_cgroup_from_obj(void *p)
 
 	page = virt_to_head_page(p);
 
-	/*
-	 * If page->mem_cgroup is set, it's either a simple mem_cgroup pointer
-	 * or a pointer to obj_cgroup vector. In the latter case the lowest
-	 * bit of the pointer is set.
-	 * The page->mem_cgroup pointer can be asynchronously changed
-	 * from NULL to (obj_cgroup_vec | 0x1UL), but can't be changed
-	 * from a valid memcg pointer to objcg vector or back.
-	 */
-	if (!page->mem_cgroup)
-		return NULL;
-
 	/*
 	 * Slab objects are accounted individually, not per-page.
 	 * Memcg membership data for each individual object is saved in
@@ -2956,8 +2935,14 @@ struct mem_cgroup *mem_cgroup_from_obj(void *p)
 		return NULL;
 	}
 
-	/* All other pages use page->mem_cgroup */
-	return page->mem_cgroup;
+	/*
+	 * page_mem_cgroup_check() is used here, because page_has_obj_cgroups()
+	 * check above could fail because the object cgroups vector wasn't set
+	 * at that moment, but it can be set concurrently.
+	 * page_mem_cgroup_check(page) will guarantee tat a proper memory
+	 * cgroup pointer or NULL will be returned.
+	 */
+	return page_mem_cgroup_check(page);
 }
 
 __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void)
@@ -3095,7 +3080,7 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)
 	if (memcg && !mem_cgroup_is_root(memcg)) {
 		ret = __memcg_kmem_charge(memcg, gfp, 1 << order);
 		if (!ret) {
-			page->mem_cgroup = memcg;
+			set_page_mem_cgroup(page, memcg);
 			__SetPageKmemcg(page);
 			return 0;
 		}
@@ -3111,7 +3096,7 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)
  */
 void __memcg_kmem_uncharge_page(struct page *page, int order)
 {
-	struct mem_cgroup *memcg = page->mem_cgroup;
+	struct mem_cgroup *memcg = page_mem_cgroup(page);
 	unsigned int nr_pages = 1 << order;
 
 	if (!memcg)
@@ -3119,7 +3104,7 @@ void __memcg_kmem_uncharge_page(struct page *page, int order)
 
 	VM_BUG_ON_PAGE(mem_cgroup_is_root(memcg), page);
 	__memcg_kmem_uncharge(memcg, nr_pages);
-	page->mem_cgroup = NULL;
+	clear_page_mem_cgroup(page);
 	css_put(&memcg->css);
 
 	/* slab pages do not have PageKmemcg flag set */
@@ -3270,7 +3255,7 @@ void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size)
  */
 void mem_cgroup_split_huge_fixup(struct page *head)
 {
-	struct mem_cgroup *memcg = head->mem_cgroup;
+	struct mem_cgroup *memcg = page_mem_cgroup(head);
 	int i;
 
 	if (mem_cgroup_disabled())
@@ -3278,7 +3263,7 @@ void mem_cgroup_split_huge_fixup(struct page *head)
 
 	for (i = 1; i < HPAGE_PMD_NR; i++) {
 		css_get(&memcg->css);
-		head[i].mem_cgroup = memcg;
+		set_page_mem_cgroup(&head[i], memcg);
 	}
 }
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
@@ -4654,7 +4639,7 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
 void mem_cgroup_track_foreign_dirty_slowpath(struct page *page,
 					     struct bdi_writeback *wb)
 {
-	struct mem_cgroup *memcg = page->mem_cgroup;
+	struct mem_cgroup *memcg = page_mem_cgroup(page);
 	struct memcg_cgwb_frn *frn;
 	u64 now = get_jiffies_64();
 	u64 oldest_at = now;
@@ -5623,14 +5608,14 @@ static int mem_cgroup_move_account(struct page *page,
 
 	/*
 	 * Prevent mem_cgroup_migrate() from looking at
-	 * page->mem_cgroup of its source page while we change it.
+	 * page's memory cgroup of its source page while we change it.
 	 */
 	ret = -EBUSY;
 	if (!trylock_page(page))
 		goto out;
 
 	ret = -EINVAL;
-	if (page->mem_cgroup != from)
+	if (page_mem_cgroup(page) != from)
 		goto out_unlock;
 
 	pgdat = page_pgdat(page);
@@ -5685,13 +5670,13 @@ static int mem_cgroup_move_account(struct page *page,
 	/*
 	 * All state has been migrated, let's switch to the new memcg.
 	 *
-	 * It is safe to change page->mem_cgroup here because the page
+	 * It is safe to change page's memcg here because the page
 	 * is referenced, charged, isolated, and locked: we can't race
 	 * with (un)charging, migration, LRU putback, or anything else
-	 * that would rely on a stable page->mem_cgroup.
+	 * that would rely on a stable page's memory cgroup.
 	 *
 	 * Note that lock_page_memcg is a memcg lock, not a page lock,
-	 * to save space. As soon as we switch page->mem_cgroup to a
+	 * to save space. As soon as we switch page's memory cgroup to a
 	 * new memcg that isn't locked, the above state can change
 	 * concurrently again. Make sure we're truly done with it.
 	 */
@@ -5700,7 +5685,7 @@ static int mem_cgroup_move_account(struct page *page,
 	css_get(&to->css);
 	css_put(&from->css);
 
-	page->mem_cgroup = to;
+	set_page_mem_cgroup(page, to);
 
 	__unlock_page_memcg(from);
 
@@ -5766,7 +5751,7 @@ static enum mc_target_type get_mctgt_type(struct vm_area_struct *vma,
 		 * mem_cgroup_move_account() checks the page is valid or
 		 * not under LRU exclusion.
 		 */
-		if (page->mem_cgroup == mc.from) {
+		if (page_mem_cgroup(page) == mc.from) {
 			ret = MC_TARGET_PAGE;
 			if (is_device_private_page(page))
 				ret = MC_TARGET_DEVICE;
@@ -5810,7 +5795,7 @@ static enum mc_target_type get_mctgt_type_thp(struct vm_area_struct *vma,
 	VM_BUG_ON_PAGE(!page || !PageHead(page), page);
 	if (!(mc.flags & MOVE_ANON))
 		return ret;
-	if (page->mem_cgroup == mc.from) {
+	if (page_mem_cgroup(page) == mc.from) {
 		ret = MC_TARGET_PAGE;
 		if (target) {
 			get_page(page);
@@ -6793,12 +6778,12 @@ int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask)
 		/*
 		 * Every swap fault against a single page tries to charge the
 		 * page, bail as early as possible.  shmem_unuse() encounters
-		 * already charged pages, too.  page->mem_cgroup is protected
-		 * by the page lock, which serializes swap cache removal, which
-		 * in turn serializes uncharging.
+		 * already charged pages, too.  page and memcg binding is
+		 * protected by the page lock, which serializes swap cache
+		 * removal, which in turn serializes uncharging.
 		 */
 		VM_BUG_ON_PAGE(!PageLocked(page), page);
-		if (compound_head(page)->mem_cgroup)
+		if (page_mem_cgroup(compound_head(page)))
 			goto out;
 
 		id = lookup_swap_cgroup_id(ent);
@@ -6882,21 +6867,21 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)
 
 	VM_BUG_ON_PAGE(PageLRU(page), page);
 
-	if (!page->mem_cgroup)
+	if (!page_mem_cgroup(page))
 		return;
 
 	/*
 	 * Nobody should be changing or seriously looking at
-	 * page->mem_cgroup at this point, we have fully
+	 * page_mem_cgroup(page) at this point, we have fully
 	 * exclusive access to the page.
 	 */
 
-	if (ug->memcg != page->mem_cgroup) {
+	if (ug->memcg != page_mem_cgroup(page)) {
 		if (ug->memcg) {
 			uncharge_batch(ug);
 			uncharge_gather_clear(ug);
 		}
-		ug->memcg = page->mem_cgroup;
+		ug->memcg = page_mem_cgroup(page);
 
 		/* pairs with css_put in uncharge_batch */
 		css_get(&ug->memcg->css);
@@ -6913,7 +6898,7 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)
 	}
 
 	ug->dummy_page = page;
-	page->mem_cgroup = NULL;
+	clear_page_mem_cgroup(page);
 	css_put(&ug->memcg->css);
 }
 
@@ -6956,7 +6941,7 @@ void mem_cgroup_uncharge(struct page *page)
 		return;
 
 	/* Don't touch page->lru of any random page, pre-check: */
-	if (!page->mem_cgroup)
+	if (!page_mem_cgroup(page))
 		return;
 
 	uncharge_gather_clear(&ug);
@@ -7006,11 +6991,11 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage)
 		return;
 
 	/* Page cache replacement: new page already charged? */
-	if (newpage->mem_cgroup)
+	if (page_mem_cgroup(newpage))
 		return;
 
 	/* Swapcache readahead pages can get replaced before being charged */
-	memcg = oldpage->mem_cgroup;
+	memcg = page_mem_cgroup(oldpage);
 	if (!memcg)
 		return;
 
@@ -7205,7 +7190,7 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
 	if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
 		return;
 
-	memcg = page->mem_cgroup;
+	memcg = page_mem_cgroup(page);
 
 	/* Readahead page, never charged */
 	if (!memcg)
@@ -7226,7 +7211,7 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
 	VM_BUG_ON_PAGE(oldid, page);
 	mod_memcg_state(swap_memcg, MEMCG_SWAP, nr_entries);
 
-	page->mem_cgroup = NULL;
+	clear_page_mem_cgroup(page);
 
 	if (!mem_cgroup_is_root(memcg))
 		page_counter_uncharge(&memcg->memory, nr_entries);
@@ -7269,7 +7254,7 @@ int mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry)
 	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
 		return 0;
 
-	memcg = page->mem_cgroup;
+	memcg = page_mem_cgroup(page);
 
 	/* Readahead page, never charged */
 	if (!memcg)
@@ -7350,7 +7335,7 @@ bool mem_cgroup_swap_full(struct page *page)
 	if (cgroup_memory_noswap || !cgroup_subsys_on_dfl(memory_cgrp_subsys))
 		return false;
 
-	memcg = page->mem_cgroup;
+	memcg = page_mem_cgroup(page);
 	if (!memcg)
 		return false;
 
diff --git a/mm/migrate.c b/mm/migrate.c
index 3ab965f83029..54c198c97b64 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -493,7 +493,7 @@ int migrate_page_move_mapping(struct address_space *mapping,
 		struct lruvec *old_lruvec, *new_lruvec;
 		struct mem_cgroup *memcg;
 
-		memcg = page_memcg(page);
+		memcg = page_mem_cgroup(page);
 		old_lruvec = mem_cgroup_lruvec(memcg, oldzone->zone_pgdat);
 		new_lruvec = mem_cgroup_lruvec(memcg, newzone->zone_pgdat);
 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6b1b4a331792..d4d181e15e7c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1056,7 +1056,7 @@ static inline bool page_expected_state(struct page *page,
 	if (unlikely((unsigned long)page->mapping |
 			page_ref_count(page) |
 #ifdef CONFIG_MEMCG
-			(unsigned long)page->mem_cgroup |
+			(unsigned long)page_mem_cgroup(page) |
 #endif
 			(page->flags & check_flags)))
 		return false;
@@ -1081,7 +1081,7 @@ static const char *page_bad_reason(struct page *page, unsigned long flags)
 			bad_reason = "PAGE_FLAGS_CHECK_AT_FREE flag(s) set";
 	}
 #ifdef CONFIG_MEMCG
-	if (unlikely(page->mem_cgroup))
+	if (unlikely(page_mem_cgroup(page)))
 		bad_reason = "page still charged to cgroup";
 #endif
 	return bad_reason;
diff --git a/mm/page_io.c b/mm/page_io.c
index dc6de6962612..ffa3a7d20c58 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -282,11 +282,11 @@ static void bio_associate_blkg_from_page(struct bio *bio, struct page *page)
 {
 	struct cgroup_subsys_state *css;
 
-	if (!page->mem_cgroup)
+	if (!page_mem_cgroup(page))
 		return;
 
 	rcu_read_lock();
-	css = cgroup_e_css(page->mem_cgroup->css.cgroup, &io_cgrp_subsys);
+	css = cgroup_e_css(page_mem_cgroup(page)->css.cgroup, &io_cgrp_subsys);
 	bio_associate_blkg_from_css(bio, css);
 	rcu_read_unlock();
 }
diff --git a/mm/slab.h b/mm/slab.h
index 4a24e1702923..5ac89260f329 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -242,18 +242,17 @@ static inline bool kmem_cache_debug_flags(struct kmem_cache *s, slab_flags_t fla
 static inline struct obj_cgroup **page_obj_cgroups(struct page *page)
 {
 	/*
-	 * page->mem_cgroup and page->obj_cgroups are sharing the same
+	 * Page's memory cgroup and obj_cgroups vector are sharing the same
 	 * space. To distinguish between them in case we don't know for sure
 	 * that the page is a slab page (e.g. page_cgroup_ino()), let's
 	 * always set the lowest bit of obj_cgroups.
 	 */
-	return (struct obj_cgroup **)
-		((unsigned long)page->obj_cgroups & ~0x1UL);
+	return (struct obj_cgroup **)(page->memcg_data & ~0x1UL);
 }
 
 static inline bool page_has_obj_cgroups(struct page *page)
 {
-	return ((unsigned long)page->obj_cgroups & 0x1UL);
+	return page->memcg_data & 0x1UL;
 }
 
 int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
@@ -262,7 +261,7 @@ int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
 static inline void memcg_free_page_obj_cgroups(struct page *page)
 {
 	kfree(page_obj_cgroups(page));
-	page->obj_cgroups = NULL;
+	page->memcg_data = 0;
 }
 
 static inline size_t obj_full_size(struct kmem_cache *s)
diff --git a/mm/workingset.c b/mm/workingset.c
index 8ed8e6296d8c..8a56f4ca4a2e 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -257,7 +257,7 @@ void *workingset_eviction(struct page *page, struct mem_cgroup *target_memcg)
 	struct lruvec *lruvec;
 	int memcgid;
 
-	/* Page is fully exclusive and pins page->mem_cgroup */
+	/* Page is fully exclusive and pins page's memory cgroup pointer */
 	VM_BUG_ON_PAGE(PageLRU(page), page);
 	VM_BUG_ON_PAGE(page_count(page), page);
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
@@ -345,7 +345,7 @@ void workingset_refault(struct page *page, void *shadow)
 	 * However, the cgroup that will own the page is the one that
 	 * is actually experiencing the refault event.
 	 */
-	memcg = page_memcg(page);
+	memcg = page_mem_cgroup(page);
 	lruvec = mem_cgroup_lruvec(memcg, pgdat);
 
 	inc_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + file);
@@ -407,7 +407,7 @@ void workingset_activation(struct page *page)
 	 * XXX: See workingset_refault() - this should return
 	 * root_mem_cgroup even for !CONFIG_MEMCG.
 	 */
-	memcg = page_memcg_rcu(page);
+	memcg = page_mem_cgroup(page);
 	if (!mem_cgroup_disabled() && !memcg)
 		goto out;
 	lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
-- 
2.26.2