Re: [PATCH v1 11/29] mm/migrate: move movable_ops page handling out of move_to_new_folio()

Lorenzo Stoakes
2025-07-01T09:39:17Z

On Mon, Jun 30, 2025 at 02:59:52PM +0200, David Hildenbrand wrote:
> Let's move that handling directly into migrate_folio_move(), so we can
> simplify move_to_new_folio(). While at it, fixup the documentation a
> bit.
>
> Note that unmap_and_move_huge_page() does not care, because it only
> deals with actual folios. (we only support migration of
> individual movable_ops pages)

Important caveat here :)

>
> Reviewed-by: Zi Yan <ziy@nvidia.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
> mm/migrate.c | 63 +++++++++++++++++++++++++---------------------------
> 1 file changed, 30 insertions(+), 33 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 0898ddd2f661f..22c115710d0e2 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1024,11 +1024,12 @@ static int fallback_migrate_folio(struct address_space *mapping,
> }
>
> /*
> - * Move a page to a newly allocated page
> - * The page is locked and all ptes have been successfully removed.
> + * Move a src folio to a newly allocated dst folio.
> *
> - * The new page will have replaced the old page if this function
> - * is successful.
> + * The src and dst folios are locked and the src folios was unmapped from
> + * the page tables.
> + *
> + * On success, the src folio was replaced by the dst folio.
> *
> * Return value:
> * < 0 - error code
> @@ -1037,34 +1038,30 @@ static int fallback_migrate_folio(struct address_space *mapping,
> static int move_to_new_folio(struct folio *dst, struct folio *src,
> enum migrate_mode mode)
> {
> + struct address_space *mapping = folio_mapping(src);
> int rc = -EAGAIN;
> - bool is_lru = !__folio_test_movable(src);

This is_lru was already sketchy, !movable_ops doesn't imply on lru...

>
> VM_BUG_ON_FOLIO(!folio_test_locked(src), src);
> VM_BUG_ON_FOLIO(!folio_test_locked(dst), dst);
>
> - if (likely(is_lru)) {
> - struct address_space *mapping = folio_mapping(src);
> -
> - if (!mapping)
> - rc = migrate_folio(mapping, dst, src, mode);
> - else if (mapping_inaccessible(mapping))
> - rc = -EOPNOTSUPP;
> - else if (mapping->a_ops->migrate_folio)
> - /*
> - * Most folios have a mapping and most filesystems
> - * provide a migrate_folio callback. Anonymous folios
> - * are part of swap space which also has its own
> - * migrate_folio callback. This is the most common path
> - * for page migration.
> - */
> - rc = mapping->a_ops->migrate_folio(mapping, dst, src,
> - mode);
> - else
> - rc = fallback_migrate_folio(mapping, dst, src, mode);
> + if (!mapping)
> + rc = migrate_folio(mapping, dst, src, mode);
> + else if (mapping_inaccessible(mapping))
> + rc = -EOPNOTSUPP;
> + else if (mapping->a_ops->migrate_folio)
> + /*
> + * Most folios have a mapping and most filesystems
> + * provide a migrate_folio callback. Anonymous folios
> + * are part of swap space which also has its own
> + * migrate_folio callback. This is the most common path
> + * for page migration.
> + */
> + rc = mapping->a_ops->migrate_folio(mapping, dst, src,
> + mode);
> + else
> + rc = fallback_migrate_folio(mapping, dst, src, mode);
>
> - if (rc != MIGRATEPAGE_SUCCESS)
> - goto out;
> + if (rc == MIGRATEPAGE_SUCCESS) {
> /*
> * For pagecache folios, src->mapping must be cleared before src
> * is freed. Anonymous folios must stay anonymous until freed.
> @@ -1074,10 +1071,7 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
>
> if (likely(!folio_is_zone_device(dst)))
> flush_dcache_folio(dst);
> - } else {
> - rc = migrate_movable_ops_page(&dst->page, &src->page, mode);
> }
> -out:
> return rc;
> }
>
> @@ -1328,20 +1322,23 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
> int rc;
> int old_page_state = 0;
> struct anon_vma *anon_vma = NULL;
> - bool is_lru = !__folio_test_movable(src);
> struct list_head *prev;
>
> __migrate_folio_extract(dst, &old_page_state, &anon_vma);
> prev = dst->lru.prev;
> list_del(&dst->lru);
>
> + if (unlikely(__folio_test_movable(src))) {
> + rc = migrate_movable_ops_page(&dst->page, &src->page, mode);
> + if (rc)
> + goto out;
> + goto out_unlock_both;
> + }
> +
> rc = move_to_new_folio(dst, src, mode);
> if (rc)
> goto out;
>
> - if (unlikely(!is_lru))
> - goto out_unlock_both;
> -
> /*
> * When successful, push dst to LRU immediately: so that if it
> * turns out to be an mlocked page, remove_migration_ptes() will
> --
> 2.49.0
>