[PATCH v2 1/2] mm: migrate: Fix return value if all subpages of THPs are migrated successfully

[Baolin Wang <baolin.wang@linux.alibaba.com>]

When THP migration, if THPs are split and all subpages are migrated successfully
, the migrate_pages() will still return the number of THP that were not migrated.
That will confuse the callers of migrate_pages(), for example, which will make
the longterm pinning failed though all pages are migrated successfully.

Thus we should return 0 to indicate all pages are migrated in this case.

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
Changes from v1:
- Fix the return value of migrate_pages() instead of fixing the
  callers' validation.
---
 mm/migrate.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index 8e5eb6e..1da0dbc 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1582,6 +1582,13 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 	 */
 	list_splice(&ret_pages, from);
 
+	/*
+	 * Return 0 in case all subpages of fail-to-migrate THPs are
+	 * migrated successfully.
+	 */
+	if (nr_thp_split && list_empty(from))
+		rc = 0;
+
 	count_vm_events(PGMIGRATE_SUCCESS, nr_succeeded);
 	count_vm_events(PGMIGRATE_FAIL, nr_failed_pages);
 	count_vm_events(THP_MIGRATION_SUCCESS, nr_thp_succeeded);
-- 
1.8.3.1