mm, THP, swap: enable THP swap optimization only if has compound map

If there is no compound map for a THP (Transparent Huge Page), it is
possible that the map count of some sub-pages of the THP is 0.  So it is
better to split the THP before swapping out.  In this way, the sub-pages
not mapped will be freed, and we can avoid the unnecessary swap out
operations for these sub-pages.

Link: http://lkml.kernel.org/r/20170515112522.32457-6-ying.huang@intel.comSigned-off-by: N"Huang, Ying" <ying.huang@intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Ebru Akagunduz <ebru.akagunduz@gmail.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Shaohua Li <shli@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -1125,10 +1125,19 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 		    !PageSwapCache(page)) {
 			if (!(sc->gfp_mask & __GFP_IO))
 				goto keep_locked;
-			/* cannot split THP, skip it */
-			if (PageTransHuge(page) &&
-			    !can_split_huge_page(page, NULL))
-				goto activate_locked;
+			if (PageTransHuge(page)) {
+				/* cannot split THP, skip it */
+				if (!can_split_huge_page(page, NULL))
+					goto activate_locked;
+				/*
+				 * Split pages without a PMD map right
+				 * away. Chances are some or all of the
+				 * tail pages can be freed without IO.
+				 */
+				if (!compound_mapcount(page) &&
+				    split_huge_page_to_list(page, page_list))
+					goto activate_locked;
+			}
 			if (!add_to_swap(page)) {
 				if (!PageTransHuge(page))
 					goto activate_locked;