[PATCH -mm -v11 5/5] mm, THP, swap: Enable THP swap optimization only if has compound map

[Huang, Ying]

From: Huang Ying <ying.huang@xxxxxxxxx>

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.

Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
---
 mm/vmscan.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index a5355022dc2f..f7e949ac9756 100644
--- a/mm/vmscan.c
+++ b/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;
-- 
2.11.0