mm/vmscan.c: support removing arbitrary sized pages from mapping

__remove_mapping() assumes that pages can only be either base pages or
HPAGE_PMD_SIZE.  Ask the page what size it is.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 99cb0db ("mm,thp: add read-only THP support for (non-shmem) FS")
Signed-off-by: William Kucharski <[email protected]>
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Song Liu <[email protected]>
Acked-by: Yang Shi <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Srikar Dronamraju <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: William Kucharski

mm/vmscan.c

@@ -933,10 +933,7 @@ static int __remove_mapping(struct address_space *mapping, struct page *page,
 	 * Note that if SetPageDirty is always performed via set_page_dirty,
 	 * and thus under the i_pages lock, then this ordering is not required.
 	 */
-	if (unlikely(PageTransHuge(page)) && PageSwapCache(page))
-		refcount = 1 + HPAGE_PMD_NR;
-	else
-		refcount = 2;
+	refcount = 1 + compound_nr(page);
 	if (!page_ref_freeze(page, refcount))
 		goto cannot_free;
 	/* note: atomic_cmpxchg in page_ref_freeze provides the smp_rmb */