[v2 PATCH 6/7] mm: migrate: check mapcount for THP instead of ref count

[Yang Shi]

The generic migration path will check refcount, so no need check refcount here.
But the old code actually prevents from migrating shared THP (mapped by multiple
processes), so bail out early if mapcount is > 1 to keep the behavior.

Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
---
 mm/migrate.c | 16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index a72994c68ec6..dc7cc7f3a124 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2067,6 +2067,10 @@ static int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)
 
 	VM_BUG_ON_PAGE(compound_order(page) && !PageTransHuge(page), page);
 
+	/* Do not migrate THP mapped by multiple processes */
+	if (PageTransHuge(page) && page_mapcount(page) > 1)
+		return 0;
+
 	/* Avoid migrating to a node that is nearly full */
 	if (!migrate_balanced_pgdat(pgdat, compound_nr(page)))
 		return 0;
@@ -2074,18 +2078,6 @@ static int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)
 	if (isolate_lru_page(page))
 		return 0;
 
-	/*
-	 * migrate_misplaced_transhuge_page() skips page migration's usual
-	 * check on page_count(), so we must do it here, now that the page
-	 * has been isolated: a GUP pin, or any other pin, prevents migration.
-	 * The expected page count is 3: 1 for page's mapcount and 1 for the
-	 * caller's pin and 1 for the reference taken by isolate_lru_page().
-	 */
-	if (PageTransHuge(page) && page_count(page) != 3) {
-		putback_lru_page(page);
-		return 0;
-	}
-
 	page_lru = page_is_file_lru(page);
 	mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON + page_lru,
 				thp_nr_pages(page));
-- 
2.26.2