+	switch (attr->propagation) {
+	case 0:
+		kattr->propagation = 0;
+		break;
+	case MS_UNBINDABLE:
+		kattr->propagation = MS_UNBINDABLE;
+		break;
+	case MS_PRIVATE:
+		kattr->propagation = MS_PRIVATE;
+		break;
+	case MS_SLAVE:
+		kattr->propagation = MS_SLAVE;
+		break;
+	case MS_SHARED:
+		kattr->propagation = MS_SHARED;
+		break;
+	default:
+		return -EINVAL;
+	}

This can be shortened to:

#define MOUNT_SETATTR_PROPAGATION_FLAGS \
	(MS_UNBINDABLE | MS_PRIVATE | MS_SLAVE | MS_SHARED)

	if (attr->propagation & ~MOUNT_SETATTR_PROPAGATION_FLAGS)
		return -EINVAL;
	if (hweight32(attr->propagation & MOUNT_SETATTR_PROPAGATION_FLAGS) > 1)
		return -EINVAL;
	kattr->propagation = attr->propagation;

+asmlinkage long sys_mount_setattr(int dfd, const char __user *path, unsigned int flags,

Overly long line.

Otherwise looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>