mm/migrate: make migrate_pages_batch() stats consistent

As Ying pointed out in [1], stats->nr_thp_failed needs to be updated to
avoid stats inconsistency between MIGRATE_SYNC and MIGRATE_ASYNC when
calling migrate_pages_batch().

Because if not, when migrate_pages_batch() is called via
migrate_pages(MIGRATE_ASYNC), nr_thp_failed will not be increased and when
migrate_pages_batch() is called via migrate_pages(MIGRATE_SYNC*),
nr_thp_failed will be increase in migrate_pages_sync() by
stats->nr_thp_failed += astats.nr_thp_split.

[1] https://lore.kernel.org/linux-mm/[email protected]/

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 7262f20 ("mm/migrate: split source folio if it is on deferred split list")
Signed-off-by: Zi Yan <[email protected]>
Suggested-by: "Huang, Ying" <[email protected]>
Reviewed-by: "Huang, Ying" <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Yin Fengwei <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: x-y-z

mm/migrate.c

@@ -1659,6 +1659,10 @@ static int migrate_pages_batch(struct list_head *from,
 			 * migrate_pages() may report success with (split but
 			 * unmigrated) pages still on its fromlist; whereas it
 			 * always reports success when its fromlist is empty.
+			 * stats->nr_thp_failed should be increased too,
+			 * otherwise stats inconsistency will happen when
+			 * migrate_pages_batch is called via migrate_pages()
+			 * with MIGRATE_SYNC and MIGRATE_ASYNC.
 			 *
 			 * Only check it without removing it from the list.
 			 * Since the folio can be on deferred_split_scan()
@@ -1675,6 +1679,7 @@ static int migrate_pages_batch(struct list_head *from,
 			   !list_empty(&folio->_deferred_list)) {
 				if (try_split_folio(folio, split_folios) == 0) {
 					nr_failed++;
+					stats->nr_thp_failed += is_thp;
 					stats->nr_thp_split += is_thp;
 					stats->nr_split++;
 					continue;