[v2 PATCH] mm: move_pages: report the number of non-attempted pages

[Yang Shi]

Since commit a49bd4d71637 ("mm, numa: rework do_pages_move"),
the semantic of move_pages() was changed to return the number of
non-migrated pages (failed to migration) and the call would be aborted
immediately if migrate_pages() returns positive value.  But it didn't
report the number of pages that we even haven't attempted to migrate.
So, fix it by including non-attempted pages in the return value.

Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
Suggested-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>    [4.17+]
Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
---
v2: Rebased on top of the latest mainline kernel per Andrew

 mm/migrate.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 86873b6..9b8eb5d 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1627,8 +1627,18 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
 			start = i;
 		} else if (node != current_node) {
 			err = do_move_pages_to_node(mm, &pagelist, current_node);
-			if (err)
+			if (err) {
+				/*
+				 * Positive err means the number of failed
+				 * pages to migrate.  Since we are going to
+				 * abort and return the number of non-migrated
+				 * pages, so need incude the rest of the
+				 * nr_pages that have not attempted as well.
+				 */
+				if (err > 0)
+					err += nr_pages - i - 1;
 				goto out;
+			}
 			err = store_status(status, start, current_node, i - start);
 			if (err)
 				goto out;
@@ -1659,8 +1669,11 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
 			goto out_flush;
 
 		err = do_move_pages_to_node(mm, &pagelist, current_node);
-		if (err)
+		if (err) {
+			if (err > 0)
+				err += nr_pages - i - 1;
 			goto out;
+		}
 		if (i > start) {
 			err = store_status(status, start, current_node, i - start);
 			if (err)
@@ -1674,6 +1687,13 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
 
 	/* Make sure we do not overwrite the existing error */
 	err1 = do_move_pages_to_node(mm, &pagelist, current_node);
+	/*
+	 * Don't have to report non-attempted pages here since:
+	 *     - If the above loop is done gracefully there is not non-attempted
+	 *       page.
+	 *     - If the above loop is aborted to it means more fatal error
+	 *       happened, should return err.
+	 */
 	if (!err1)
 		err1 = store_status(status, start, current_node, i - start);
 	if (!err)
-- 
1.8.3.1