[PATCH 2/3] mm: Simplify split_page_memcg()

["Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>]

The last argument to split_page_memcg() is now always 0, so remove it,
effectively reverting commit b8791381d7ed.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 include/linux/memcontrol.h |  4 ++--
 mm/memcontrol.c            | 26 ++++++++++++++------------
 mm/page_alloc.c            |  4 ++--
 3 files changed, 18 insertions(+), 16 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 5502aa8e138e..a787080f814f 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1044,7 +1044,7 @@ static inline void memcg_memory_event_mm(struct mm_struct *mm,
 	rcu_read_unlock();
 }
 
-void split_page_memcg(struct page *head, int old_order, int new_order);
+void split_page_memcg(struct page *first, int order);
 
 #else /* CONFIG_MEMCG */
 
@@ -1463,7 +1463,7 @@ void count_memcg_event_mm(struct mm_struct *mm, enum vm_event_item idx)
 {
 }
 
-static inline void split_page_memcg(struct page *head, int old_order, int new_order)
+static inline void split_page_memcg(struct page *first, int order)
 {
 }
 #endif /* CONFIG_MEMCG */
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5e44d6e7591e..506439a5dcfe 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3034,25 +3034,27 @@ void __memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
 }
 
 /*
- * Because folio_memcg(head) is not set on tails, set it now.
+ * The memcg data is only set on the first page, now transfer it to all the
+ * other pages.
  */
-void split_page_memcg(struct page *head, int old_order, int new_order)
+void split_page_memcg(struct page *first, int order)
 {
-	struct folio *folio = page_folio(head);
+	unsigned long memcg_data = first->memcg_data;
+	struct obj_cgroup *objcg;
 	int i;
-	unsigned int old_nr = 1 << old_order;
-	unsigned int new_nr = 1 << new_order;
+	unsigned int nr = 1 << order;
 
-	if (mem_cgroup_disabled() || !folio_memcg_charged(folio))
+	if (!memcg_data)
 		return;
 
-	for (i = new_nr; i < old_nr; i += new_nr)
-		folio_page(folio, i)->memcg_data = folio->memcg_data;
+	VM_BUG_ON_PAGE((memcg_data & OBJEXTS_FLAGS_MASK) != MEMCG_DATA_KMEM,
+			first);
+	objcg = (void *)(memcg_data & ~OBJEXTS_FLAGS_MASK);
 
-	if (folio_memcg_kmem(folio))
-		obj_cgroup_get_many(__folio_objcg(folio), old_nr / new_nr - 1);
-	else
-		css_get_many(&folio_memcg(folio)->css, old_nr / new_nr - 1);
+	for (i = 1; i < nr; i++)
+		first[i].memcg_data = memcg_data;
+
+	obj_cgroup_get_many(objcg, nr - 1);
 }
 
 unsigned long mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 47048b39b8ca..5523654c9759 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2823,7 +2823,7 @@ void split_page(struct page *page, unsigned int order)
 		set_page_refcounted(page + i);
 	split_page_owner(page, order, 0);
 	pgalloc_tag_split(page_folio(page), order, 0);
-	split_page_memcg(page, order, 0);
+	split_page_memcg(page, order);
 }
 EXPORT_SYMBOL_GPL(split_page);
 
@@ -5020,7 +5020,7 @@ static void *make_alloc_exact(unsigned long addr, unsigned int order,
 
 		split_page_owner(page, order, 0);
 		pgalloc_tag_split(page_folio(page), order, 0);
-		split_page_memcg(page, order, 0);
+		split_page_memcg(page, order);
 		while (page < --last)
 			set_page_refcounted(last);
 
-- 
2.43.0