From: Yang Shi <shy828301@xxxxxxxxx> 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/20210518200801.7413-7-shy828301@xxxxxxxxx Signed-off-by: Yang Shi <shy828301@xxxxxxxxx> Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx> Cc: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx> Cc: Heiko Carstens <hca@xxxxxxxxxxxxx> Cc: Huang Ying <ying.huang@xxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: Mel Gorman <mgorman@xxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxx> Cc: Vasily Gorbik <gor@xxxxxxxxxxxxx> Cc: Zi Yan <ziy@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/migrate.c | 16 ++++------------ 1 file changed, 4 insertions(+), 12 deletions(-) --- 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)); _
