Nouveau - Feb 2021 - [PATCH 1/9] mm/migrate.c: Always allow device private pages to migrate

Device private pages are used to represent device memory that is not
directly accessible from the CPU. Extra references to a device private
page are only used to ensure the struct page itself remains valid whilst
waiting for migration entries. Therefore extra references should not
prevent device private page migration as this can lead to failures to
migrate pages back to the CPU which are fatal to the user process.

Signed-off-by: Alistair Popple <apopple at nvidia.com>
---
 mm/migrate.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 20ca887ea769..053228559fd3 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -405,8 +405,13 @@ int migrate_page_move_mapping(struct address_space
*mapping,
 	int nr = thp_nr_pages(page);
 
 	if (!mapping) {
-		/* Anonymous page without mapping */
-		if (page_count(page) != expected_count)
+		/*
+		 * Anonymous page without mapping. Device private pages should
+		 * never have extra references except during migration, but it
+		 * is safe to ignore these.
+		 */
+		if (!is_device_private_page(page) &&
+			page_count(page) != expected_count)
 			return -EAGAIN;
 
 		/* No turning back from here */
-- 
2.20.1

On Tue, Feb 09, 2021 at 12:07:14PM +1100, Alistair Popple
wrote:
> Device private pages are used to represent device memory that is not
> directly accessible from the CPU. Extra references to a device private
> page are only used to ensure the struct page itself remains valid whilst
> waiting for migration entries. Therefore extra references should not
> prevent device private page migration as this can lead to failures to
> migrate pages back to the CPU which are fatal to the user process.
This should identify the extra references in expected_count, just
disabling this protection seems unsafe, ZONE_DEVICE is not so special
that the refcount means nothing

Is this a side effect of the extra refcounts that Ralph was trying to
get rid of? I'd rather see that work finished :)

Jason