BLK_TN_MAX_MSG
bt->msg_data = __alloc_percpu(BLK_TN_MAX_MSG, __alignof__(char));
n = vscnprintf(buf, BLK_TN_MAX_MSG, fmt, args);
if (count >= BLK_TN_MAX_MSG)
bt->msg_data = __alloc_percpu(BLK_TN_MAX_MSG, __alignof__(char));