[PATCH -mm -v9 3/3] mm, THP, swap: Enable THP swap optimization only if has compound map

["Huang, Ying" <ying.huang@intel.com>]

From: Huang Ying <ying.huang@intel.com>

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@cmpxchg.org>
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
---
 mm/swap_state.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 3a3217f68937..b025c9878e5e 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -192,9 +192,15 @@ int add_to_swap(struct page *page, struct list_head *list)
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 	VM_BUG_ON_PAGE(!PageUptodate(page), page);
 
-	/* cannot split, skip it */
-	if (unlikely(PageTransHuge(page)) && !can_split_huge_page(page, NULL))
-		return 0;
+	if (unlikely(PageTransHuge(page))) {
+		/* cannot split, skip it */
+		if (!can_split_huge_page(page, NULL))
+			return 0;
+		/* fallback to split huge page firstly if no PMD map */
+		if (!compound_mapcount(page) &&
+		    split_huge_page_to_list(page, list))
+			return 0;
+	}
 
 retry:
 	entry = get_swap_page(page);
-- 
2.11.0

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