[PATCH] Use mark_page_accessed() in follow_page()

Touching a page via follow_page() counts as a reference so we should be
either setting the referenced bit in the pte or running mark_page_accessed().

Altering the pte is tricky because we haven't implemented an atomic
pte_mkyoung().  And mark_page_accessed() is better anyway because it has more
aging state: it can move the page onto the active list.

mm/filemap.c

@@ -60,6 +60,7 @@
  *      ->swap_list_lock
  *        ->swap_device_lock	(exclusive_swap_page, others)
  *          ->mapping->page_lock
+ *
  *  ->mmap_sem
  *    ->i_shared_sem		(various places)
  *
@@ -69,10 +70,12 @@
  *  ->inode_lock
  *    ->sb_lock			(fs/fs-writeback.c)
  *    ->mapping->page_lock	(__sync_single_inode)
+ *
  *  ->page_table_lock
  *    ->swap_device_lock	(try_to_unmap_one)
  *    ->private_lock		(try_to_unmap_one)
  *    ->page_lock		(try_to_unmap_one)
+ *    ->zone.lru_lock		(follow_page->mark_page_accessed)
  */
 
 /*

mm/memory.c

@@ -646,8 +646,12 @@ follow_page(struct mm_struct *mm, unsigned long address, int write)
 	if (pte_present(pte)) {
 		if (!write || (pte_write(pte) && pte_dirty(pte))) {
 			pfn = pte_pfn(pte);
-			if (pfn_valid(pfn))
-				return pfn_to_page(pfn);
+			if (pfn_valid(pfn)) {
+				struct page *page = pfn_to_page(pfn);
+
+				mark_page_accessed(page);
+				return page;
+			}
 		}
 	}