mm: migrate: check mapcount for THP instead of refcount

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.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Yang Shi <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Gerald Schaefer <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Cc: Zi Yan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 4 insertions, 12 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index 6337b0fdc098..8810c1421f5d 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2073,6 +2073,10 @@ 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) && total_mapcount(page) > 1)
+		return 0;
+
 	/* Avoid migrating to a node that is nearly full */
 	if (!migrate_balanced_pgdat(pgdat, compound_nr(page)))
 		return 0;
@@ -2080,18 +2084,6 @@ 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));