[088/192] mm: migrate: check mapcount for THP instead of refcount

Commit Message

Andrew Morton July 1, 2021, 1:51 a.m. UTC

From: Yang Shi <[e-mail]>
Subject: 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/[e-mail]
Signed-off-by: Yang Shi <[e-mail]>
Cc: Christian Borntraeger <[e-mail]>
Cc: Gerald Schaefer <[e-mail]>
Cc: Heiko Carstens <[e-mail]>
Cc: Huang Ying <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Kirill A. Shutemov <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Vasily Gorbik <[e-mail]>
Cc: Zi Yan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/migrate.c |   16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)

Patch

--- a/mm/migrate.c~mm-migrate-check-mapcount-for-thp-instead-of-refcount
+++ a/mm/migrate.c
@@ -2073,6 +2073,10 @@  static int numamigrate_isolate_page(pg_d
 
 	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_d
 	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));