migrate_pages_batch: fix statistics for longterm pin retry

commit 851ae64 upstream.

In commit fd4a7ac ("mm: migrate: try again if THP split is failed due
to page refcnt"), if the THP splitting fails due to page reference count,
we will retry to improve migration successful rate.  But the failed
splitting is counted as migration failure and migration retry, which will
cause duplicated failure counting.  So, in this patch, this is fixed via
undoing the failure counting if we decide to retry.  The patch is tested
via failure injection.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: fd4a7ac ("mm: migrate: try again if THP split is failed due to page refcnt")
Signed-off-by: "Huang, Ying" <[email protected]>
Reviewed-by: Baolin Wang <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Zi Yan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

mm/migrate.c

@@ -1700,6 +1700,9 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
 							large_retry++;
 							thp_retry += is_thp;
 							nr_retry_pages += nr_pages;
+							/* Undo duplicated failure counting. */
+							nr_large_failed--;
+							stats->nr_thp_failed -= is_thp;
 							break;
 						}
 					}