mm: use page->deferred_list

Now that we can represent the location of 'deferred_list' in C instead of
comments, make use of that ability.

Link: http://lkml.kernel.org/r/20180518194519.3820-9-willy@infradead.orgSigned-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/huge_memory.c

@@ -483,11 +483,8 @@ pmd_t maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma)
 
 static inline struct list_head *page_deferred_list(struct page *page)
 {
-	/*
-	 * ->lru in the tail pages is occupied by compound_head.
-	 * Let's use ->mapping + ->index in the second tail page as list_head.
-	 */
-	return (struct list_head *)&page[2].mapping;
+	/* ->lru in the tail pages is occupied by compound_head. */
+	return &page[2].deferred_list;
 }
 
 void prep_transhuge_page(struct page *page)

mm/page_alloc.c

@@ -952,7 +952,7 @@ static int free_tail_pages_check(struct page *head_page, struct page *page)
 	case 2:
 		/*
 		 * the second tail page: ->mapping is
-		 * page_deferred_list().next -- ignore value.
+		 * deferred_list.next -- ignore value.
 		 */
 		break;
 	default: