diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 7ada5e74346d..ff02d763cc97 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -3105,11 +3105,32 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 	if (ret)
 		goto out_unlock;
 
+	/*
+	 * The attach_ctx.old_cs is used mainly by cpuset_migrate_mm() to get
+	 * the old_mems_allowed value. There are two ways that many-to-one
+	 * cpuset migration can happen:
+	 * 1) A multithread application with threads in different cpusets is
+	 *    wholely migrated to a new cpuset.
+	 * 2) Disabling v2 cpuset controller will move all the tasks in child
+	 *    cpusets to the parent cpuset.
+	 *
+	 * In the former case, it is the mm setting of the group leader that
+	 * really matters. So attach_ctx.old_cs should track the oldcs of the
+	 * group leader. It falls back to the oldcs of the first task if there
+	 * is no group leader in the taskset. In the latter case, effective_mems
+	 * of child cpusets must always be a subset of the parent. So no real
+	 * page migration will be necessary no matter which child cpuset is
+	 * selected as attach_ctx.old_cs.
+	 */
 	cgroup_taskset_for_each(task, css, tset) {
 		ret = task_can_attach(task);
 		if (ret)
 			goto out_unlock;
 
+		/* Update attach_ctx.old_cs to the latest group leader */
+		if (task == task->group_leader)
+			attach_ctx.old_cs = task_cs(task);
+
 		if (setsched_check) {
 			ret = security_task_setscheduler(task);
 			if (ret)