[PATCH -mm -v8 3/3] 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/swap_state.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 612fb2418df6..528af29327c9 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -205,7 +205,9 @@ int add_to_swap(struct page *page, struct list_head *list)
 		/* cannot split, skip it */
 		if (!can_split_huge_page(page, NULL))
 			return 0;
-		huge = true;
+		/* fallback to split huge page firstly if no PMD map */
+		if (compound_mapcount(page))
+			huge = true;
 	}
 #endif
 
-- 
2.11.0