[PATCH] mm, thp: Do not queue fully unmapped pages for deferred split

["Kirill A. Shutemov" <kirill@shutemov.name>]

Adding fully unmapped pages into deferred split queue is not productive:
these pages are about to be freed or they are pinned and cannot be split
anyway.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/rmap.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 003377e24232..45388f1bf317 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1271,12 +1271,20 @@ static void page_remove_anon_compound_rmap(struct page *page)
 	if (TestClearPageDoubleMap(page)) {
 		/*
 		 * Subpages can be mapped with PTEs too. Check how many of
-		 * themi are still mapped.
+		 * them are still mapped.
 		 */
 		for (i = 0, nr = 0; i < HPAGE_PMD_NR; i++) {
 			if (atomic_add_negative(-1, &page[i]._mapcount))
 				nr++;
 		}
+
+		/*
+		 * Queue the page for deferred split if at least one small
+		 * page of the compound page is unmapped, but at least one
+		 * small page is still mapped.
+		 */
+		if (nr && nr < HPAGE_PMD_NR)
+			deferred_split_huge_page(page);
 	} else {
 		nr = HPAGE_PMD_NR;
 	}
@@ -1284,10 +1292,8 @@ static void page_remove_anon_compound_rmap(struct page *page)
 	if (unlikely(PageMlocked(page)))
 		clear_page_mlock(page);
 
-	if (nr) {
+	if (nr)
 		__mod_node_page_state(page_pgdat(page), NR_ANON_MAPPED, -nr);
-		deferred_split_huge_page(page);
-	}
 }
 
 /**
-- 
2.21.0