[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>
---
The patch is based off Wei Yang's cleanup patchset:
https://lore.kernel.org/linux-mm/20200122011647.13636-1-richardw.yang@xxxxxxxxxxxxxxx/T/#t

 mm/migrate.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 591f2e5..51b1b76 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1582,7 +1582,7 @@ static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
 
 static int move_pages_and_store_status(struct mm_struct *mm, int node,
 		struct list_head *pagelist, int __user *status,
-		int start, int nr)
+		int start, int nr, unsigned long total)
 {
 	int err;
 
@@ -1590,8 +1590,17 @@ static int move_pages_and_store_status(struct mm_struct *mm, int node,
 		return 0;
 
 	err = do_move_pages_to_node(mm, pagelist, node);
-	if (err)
+	if (err) {
+		/*
+		 * Possitive 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 are not attempted as well.
+		 */
+		if (err > 0)
+			err += total - start - nr - 1;
 		return err;
+	}
 	err = store_status(status, start, node, nr);
 	return err;
 }
@@ -1640,7 +1649,8 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
 			start = i;
 		} else if (node != current_node) {
 			err = move_pages_and_store_status(mm, current_node,
-					&pagelist, status, start, i - start);
+					&pagelist, status, start, i - start,
+					nr_pages);
 			if (err)
 				goto out;
 			start = i;
@@ -1670,7 +1680,7 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
 			goto out_flush;
 
 		err = move_pages_and_store_status(mm, current_node, &pagelist,
-				status, start, i - start);
+				status, start, i - start, nr_pages);
 		if (err)
 			goto out;
 		current_node = NUMA_NO_NODE;
@@ -1678,7 +1688,7 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
 out_flush:
 	/* Make sure we do not overwrite the existing error */
 	err1 = move_pages_and_store_status(mm, current_node, &pagelist,
-				status, start, i - start);
+				status, start, i - start, nr_pages);
 	if (err >= 0)
 		err = err1;
 out:
-- 
1.8.3.1