[RFC PATCH 1/3] mm: migrate: Fix the return value of migrate_pages()

[Baolin Wang]

As Zi Yan pointed out, the syscall move_pages() can return a non-migrated
number larger than the number of pages the users tried to migrate, when a
THP page is failed to migrate. This is confusing for users.

Since other migration scenarios do not care about the actual non-migrated
number of pages except the memory compaction migration which will fix in
following patch. Thus we can change the return value to return the number
of {normal page, THP, hugetlb} instead to avoid this issue, meanwhile we
should still keep the migration counters using the number of normal pages.

Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
 mm/migrate.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index a11e948..00b8922 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1428,7 +1428,7 @@ static inline int try_split_thp(struct page *page, struct page **page2,
  * @mode:		The migration mode that specifies the constraints for
  *			page migration, if any.
  * @reason:		The reason for page migration.
- * @ret_succeeded:	Set to the number of pages migrated successfully if
+ * @ret_succeeded:	Set to the number of normal pages migrated successfully if
  *			the caller passes a non-NULL pointer.
  *
  * The function returns after 10 attempts or if no pages are movable any more
@@ -1436,7 +1436,7 @@ static inline int try_split_thp(struct page *page, struct page **page2,
  * It is caller's responsibility to call putback_movable_pages() to return pages
  * to the LRU or free list only if ret != 0.
  *
- * Returns the number of pages that were not migrated, or an error code.
+ * Returns the number of {normal page, THP} that were not migrated, or an error code.
  */
 int migrate_pages(struct list_head *from, new_page_t get_new_page,
 		free_page_t put_new_page, unsigned long private,
@@ -1445,6 +1445,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 	int retry = 1;
 	int thp_retry = 1;
 	int nr_failed = 0;
+	int nr_failed_pages = 0;
 	int nr_succeeded = 0;
 	int nr_thp_succeeded = 0;
 	int nr_thp_failed = 0;
@@ -1517,7 +1518,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 					}
 
 					nr_thp_failed++;
-					nr_failed += nr_subpages;
+					nr_failed_pages += nr_subpages;
 					break;
 				}
 
@@ -1537,7 +1538,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 					}
 
 					nr_thp_failed++;
-					nr_failed += nr_subpages;
+					nr_failed_pages += nr_subpages;
 					goto out;
 				}
 				nr_failed++;
@@ -1566,7 +1567,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 				 */
 				if (is_thp) {
 					nr_thp_failed++;
-					nr_failed += nr_subpages;
+					nr_failed_pages += nr_subpages;
 					break;
 				}
 				nr_failed++;
@@ -1575,8 +1576,9 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 		}
 	}
 	nr_failed += retry + thp_retry;
+	nr_failed_pages += nr_failed;
 	nr_thp_failed += thp_retry;
-	rc = nr_failed;
+	rc = nr_failed + nr_thp_failed;
 out:
 	/*
 	 * Put the permanent failure page back to migration list, they
@@ -1585,11 +1587,11 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 	list_splice(&ret_pages, from);
 
 	count_vm_events(PGMIGRATE_SUCCESS, nr_succeeded);
-	count_vm_events(PGMIGRATE_FAIL, nr_failed);
+	count_vm_events(PGMIGRATE_FAIL, nr_failed_pages);
 	count_vm_events(THP_MIGRATION_SUCCESS, nr_thp_succeeded);
 	count_vm_events(THP_MIGRATION_FAIL, nr_thp_failed);
 	count_vm_events(THP_MIGRATION_SPLIT, nr_thp_split);
-	trace_mm_migrate_pages(nr_succeeded, nr_failed, nr_thp_succeeded,
+	trace_mm_migrate_pages(nr_succeeded, nr_failed_pages, nr_thp_succeeded,
 			       nr_thp_failed, nr_thp_split, mode, reason);
 
 	if (!swapwrite)
-- 
1.8.3.1