[PATCH 3/5] mm: Remove references to folio in split_page_memcg()

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

We know that the passed in page is not part of a folio (it's a plain
page allocated with GFP_ACCOUNT), so we should get rid of the misleading
references to folios.

Introduce page_objcg() and page_set_objcg() helpers to make things more
clear.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 mm/memcontrol.c | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 07f0b42fee05..93102319409a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2697,6 +2697,23 @@ static int obj_cgroup_charge_pages(struct obj_cgroup *objcg, gfp_t gfp,
 	return ret;
 }
 
+static struct obj_cgroup *page_objcg(const struct page *first)
+{
+	unsigned long memcg_data = first->memcg_data;
+
+	if (!memcg_data)
+		return NULL;
+
+	VM_BUG_ON_PAGE((memcg_data & OBJEXTS_FLAGS_MASK) != MEMCG_DATA_KMEM,
+			first);
+	return (struct obj_cgroup *)(memcg_data - MEMCG_DATA_KMEM);
+}
+
+static void page_set_objcg(struct page *page, const struct obj_cgroup *objcg)
+{
+	page->memcg_data = (unsigned long)objcg | MEMCG_DATA_KMEM;
+}
+
 /**
  * __memcg_kmem_charge_page: charge a kmem page to the current memory cgroup
  * @page: page to charge
@@ -2715,8 +2732,7 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)
 		ret = obj_cgroup_charge_pages(objcg, gfp, 1 << order);
 		if (!ret) {
 			obj_cgroup_get(objcg);
-			page->memcg_data = (unsigned long)objcg |
-				MEMCG_DATA_KMEM;
+			page_set_objcg(page, objcg);
 			return 0;
 		}
 	}
@@ -3091,16 +3107,16 @@ void __memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
  */
 void split_page_memcg(struct page *first, unsigned order)
 {
-	struct folio *folio = page_folio(first);
+	struct obj_cgroup *objcg = page_objcg(first);
 	unsigned int i, nr = 1 << order;
 
-	if (mem_cgroup_disabled() || !folio_memcg_charged(folio))
+	if (!objcg)
 		return;
 
 	for (i = 1; i < nr; i++)
-		folio_page(folio, i)->memcg_data = folio->memcg_data;
+		page_set_objcg(&first[i], objcg);
 
-	obj_cgroup_get_many(__folio_objcg(folio), nr - 1);
+	obj_cgroup_get_many(objcg, nr - 1);
 }
 
 void folio_split_memcg(struct folio *folio, unsigned old_order,
-- 
2.47.2