On 30.06.25 19:05, Lorenzo Stoakes wrote:

On Mon, Jun 30, 2025 at 02:59:50PM +0200, David Hildenbrand wrote:

quoted

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.

But we've enot changed this have we? The flush_dcache_folio() invocation seems
to happen the same way now as before? Did I miss something?

I think, before this change we would have called it also for movable_ops 
pages


if (rc == MIGRATEPAGE_SUCCESS) {
	if (__folio_test_movable(src)) {
		...
	}

	...

	if (likely(!folio_is_zone_device(dst)))
		flush_dcache_folio(dst);
}

Now, we no longer do that for movable_ops pages.

For balloon pages, we're not copying anything, so we never possibly have 
to flush the dcache.

For zsmalloc, we do the copy in zs_object_copy() through kmap_local.

I think we could have HIGHMEM, so I wonder if we should just do a 
flush_dcache_page() in zs_object_copy().

At least, staring at highmem.h with memcpy_to_page(), it looks like that 
might be the right thing to do.


So likely I'll add a patch before this one that will do the 
flush_dcache_page() in there.

quoted

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 <redacted>

Have scrutinised this a lot and it seems correct to me, so:

Reviewed-by: Lorenzo Stoakes <redacted>

quoted

---
  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.
+ */

Love these comments you're adding!!

quoted

+static int migrate_movable_ops_page(struct page *dst, struct page *src,
+		enum migrate_mode mode)
+{
+	int rc = MIGRATEPAGE_SUCCESS;

Maybe worth asserting src, dst locking?

We do have these sanity checks right now in move_to_new_folio() already. 
(next patch moves it further out)

Not sure how reasonable these sanity checks are in these internal 
helpers: E.g., after we called move_to_new_folio() we will unlock both 
folios, which should blow up if the folios wouldn't be locked.

-- 
Cheers,

David / dhildenb