fault_in_safe_writeable
return fault_in_safe_writeable((char __user *)dest, size) == 0;
size_t fault_in_safe_writeable(const char __user *uaddr, size_t size);
n -= fault_in_safe_writeable(i->ubuf + i->iov_offset, n);
ret = fault_in_safe_writeable(p->iov_base + skip, len);
EXPORT_SYMBOL(fault_in_safe_writeable);