From ffa65753c43142f3b803486442813744da71cff2 Mon Sep 17 00:00:00 2001 From: Alistair Popple Date: Fri, 21 Jan 2022 22:10:46 -0800 Subject: mm/migrate.c: rework migration_entry_wait() to not take a pageref This fixes the FIXME in migrate_vma_check_page(). Before migrating a page migration code will take a reference and check there are no unexpected page references, failing the migration if there are. When a thread faults on a migration entry it will take a temporary reference to the page to wait for the page to become unlocked signifying the migration entry has been removed. This reference is dropped just prior to waiting on the page lock, however the extra reference can cause migration failures so it is desirable to avoid taking it. As migration code already has a reference to the migrating page an extra reference to wait on PG_locked is unnecessary so long as the reference can't be dropped whilst setting up the wait. When faulting on a migration entry the ptl is taken to check the migration entry. Removing a migration entry also requires the ptl, and migration code won't drop its page reference until after the migration entry has been removed. Therefore retaining the ptl of a migration entry is sufficient to ensure the page has a reference. Reworking migration_entry_wait() to hold the ptl until the wait setup is complete means the extra page reference is no longer needed. [apopple@nvidia.com: v5] Link: https://lkml.kernel.org/r/20211213033848.1973946-1-apopple@nvidia.com Link: https://lkml.kernel.org/r/20211118020754.954425-1-apopple@nvidia.com Signed-off-by: Alistair Popple Acked-by: David Hildenbrand Cc: David Howells Cc: Hugh Dickins Cc: Jason Gunthorpe Cc: Jerome Glisse Cc: John Hubbard Cc: Matthew Wilcox (Oracle) Cc: Ralph Campbell Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/migrate.h | 2 ++ 1 file changed, 2 insertions(+) (limited to 'include/linux') diff --git a/include/linux/migrate.h b/include/linux/migrate.h index 4850cc5bf813..db96e10eb8da 100644 --- a/include/linux/migrate.h +++ b/include/linux/migrate.h @@ -40,6 +40,8 @@ extern int migrate_huge_page_move_mapping(struct address_space *mapping, struct page *newpage, struct page *page); extern int migrate_page_move_mapping(struct address_space *mapping, struct page *newpage, struct page *page, int extra_count); +void migration_entry_wait_on_locked(swp_entry_t entry, pte_t *ptep, + spinlock_t *ptl); void folio_migrate_flags(struct folio *newfolio, struct folio *folio); void folio_migrate_copy(struct folio *newfolio, struct folio *folio); int folio_migrate_mapping(struct address_space *mapping, -- cgit v1.2.3-71-gd317