mm: migrate: fix return value if all subpages of THPs are migrated successfully

During THP migration, if THPs are not migrated but they are split and all
subpages are migrated successfully, migrate_pages() will still return the
number of THP pages that were not migrated. This will confuse the callers
of migrate_pages(). For example, the longterm pinning will failed though
all pages are migrated successfully.

Thus we should return 0 to indicate that all pages are migrated in this
case

Link: https://lkml.kernel.org/r/de3[e-mail]
Fixes: b5bade978e9b ("mm: migrate: fix the return value of migrate_pages()")
Signed-off-by: Baolin Wang <[e-mail]>
Reviewed-by: Alistair Popple <[e-mail]>
Reviewed-by: Yang Shi <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: "Huang, Ying" <[e-mail]>
Cc: Zi Yan <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/migrate.c

···
 	 */
 	list_splice(&ret_pages, from);
 
+	/*
+	 * Return 0 in case all subpages of fail-to-migrate THPs are
+	 * migrated successfully.
+	 */
+	if (list_empty(from))
+		rc = 0;
+
 	count_vm_events(PGMIGRATE_SUCCESS, nr_succeeded);
 	count_vm_events(PGMIGRATE_FAIL, nr_failed_pages);
 	count_vm_events(THP_MIGRATION_SUCCESS, nr_thp_succeeded);