vmscan: split file huge pages before paging them out

This is preparation of vmscan for file huge pages.  We cannot write out
huge pages, so we need to split them on the way out.

Link: http://lkml.kernel.org/r/1466021202-61880-22-git-send-email-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -1055,8 +1055,14 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 
 			/* Adding to swap updated mapping */
 			mapping = page_mapping(page);
+		} else if (unlikely(PageTransHuge(page))) {
+			/* Split file THP */
+			if (split_huge_page_to_list(page, page_list))
+				goto keep_locked;
 		}
 
+		VM_BUG_ON_PAGE(PageTransHuge(page), page);
+
 		/*
 		 * The page is mapped into the page tables of one or more
 		 * processes. Try to unmap it here.