[PATCH v1 09/29] mm/migrate: factor out movable_ops page handling into migrate_movable_ops_page()

David Hildenbrand
2025-06-30T13:00:47Z

Let's factor it out, simplifying the calling code.

The assumption is that flush_dcache_page() is not required for
movable_ops pages: as documented for flush_dcache_folio(), it really
only applies when the kernel wrote to pagecache pages / pages in
highmem. movable_ops callbacks should be handling flushing
caches if ever required.

Note that we can now change folio_mapping_flags() to folio_test_anon()
to make it clearer, because movable_ops pages will never take that path.

Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
mm/migrate.c | 82 ++++++++++++++++++++++++++++------------------------
1 file changed, 45 insertions(+), 37 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index d97f7cd137e63..0898ddd2f661f 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -159,6 +159,45 @@ static void putback_movable_ops_page(struct page *page)
folio_put(folio);
}

+/**
+ * migrate_movable_ops_page - migrate an isolated movable_ops page
+ * @page: The isolated page.
+ *
+ * Migrate an isolated movable_ops page.
+ *
+ * If the src page was already released by its owner, the src page is
+ * un-isolated (putback) and migration succeeds; the migration core will be the
+ * owner of both pages.
+ *
+ * If the src page was not released by its owner and the migration was
+ * successful, the owner of the src page and the dst page are swapped and
+ * the src page is un-isolated.
+ *
+ * If migration fails, the ownership stays unmodified and the src page
+ * remains isolated: migration may be retried later or the page can be putback.
+ *
+ * TODO: migration core will treat both pages as folios and lock them before
+ * this call to unlock them after this call. Further, the folio refcounts on
+ * src and dst are also released by migration core. These pages will not be
+ * folios in the future, so that must be reworked.
+ *
+ * Returns MIGRATEPAGE_SUCCESS on success, otherwise a negative error
+ * code.
+ */
+static int migrate_movable_ops_page(struct page *dst, struct page *src,
+ enum migrate_mode mode)
+{
+ int rc = MIGRATEPAGE_SUCCESS;
+
+ VM_WARN_ON_ONCE_PAGE(!PageIsolated(src), src);
+ /* If the page was released by it's owner, there is nothing to do. */
+ if (PageMovable(src))
+ rc = page_movable_ops(src)->migrate_page(dst, src, mode);
+ if (rc == MIGRATEPAGE_SUCCESS)
+ ClearPageIsolated(src);
+ return rc;
+}
+
/*
* Put previously isolated pages back onto the appropriate lists
* from where they were once taken off for compaction/migration.
@@ -1023,51 +1062,20 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
mode);
else
rc = fallback_migrate_folio(mapping, dst, src, mode);
- } else {
- const struct movable_operations *mops;

- /*
- * In case of non-lru page, it could be released after
- * isolation step. In that case, we shouldn't try migration.
- */
- VM_BUG_ON_FOLIO(!folio_test_isolated(src), src);
- if (!folio_test_movable(src)) {
- rc = MIGRATEPAGE_SUCCESS;
- folio_clear_isolated(src);
+ if (rc != MIGRATEPAGE_SUCCESS)
goto out;
- }
-
- mops = folio_movable_ops(src);
- rc = mops->migrate_page(&dst->page, &src->page, mode);
- WARN_ON_ONCE(rc == MIGRATEPAGE_SUCCESS &&
- !folio_test_isolated(src));
- }
-
- /*
- * When successful, old pagecache src->mapping must be cleared before
- * src is freed; but stats require that PageAnon be left as PageAnon.
- */
- if (rc == MIGRATEPAGE_SUCCESS) {
- if (__folio_test_movable(src)) {
- VM_BUG_ON_FOLIO(!folio_test_isolated(src), src);
-
- /*
- * We clear PG_movable under page_lock so any compactor
- * cannot try to migrate this page.
- */
- folio_clear_isolated(src);
- }
-
/*
- * Anonymous and movable src->mapping will be cleared by
- * free_pages_prepare so don't reset it here for keeping
- * the type to work PageAnon, for example.
+ * For pagecache folios, src->mapping must be cleared before src
+ * is freed. Anonymous folios must stay anonymous until freed.
*/
- if (!folio_mapping_flags(src))
+ if (!folio_test_anon(src))
src->mapping = NULL;

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;
--
2.49.0