os.read_into() may be used by the following functions.

subprocess.Popen._execute_child():

    # Wait for exec to fail or succeed; possibly raising an
    # exception (limited in size)
    errpipe_data = bytearray()
    while True:
        part = os.read(errpipe_read, 50000)
        errpipe_data += part
        if not part or len(errpipe_data) > 50000:
            break

subprocess.Popen.communicate():

    self._fileobj2output = {}
    if self.stdout:
        self._fileobj2output[self.stdout] = []
    ...
    data = os.read(key.fd, 32768)
    if not data:
        ...
    self._fileobj2output[key.fileobj].append(data)
    ...
    stdout = b''.join(...)

multiprocessing.Connection._recv():

    def _recv(self, size, read=_read):
        buf = io.BytesIO()
        handle = self._handle
        remaining = size
        while remaining > 0:
            chunk = read(handle, remaining)
            n = len(chunk)
            if n == 0:
                if remaining == size:
                    raise EOFError
                else:
                    raise OSError("got end of file during message")
            buf.write(chunk)
            remaining -= n
        return buf

multiprocessing.read_unsigned():

    def read_unsigned(fd):
        data = b''
        length = UNSIGNED_STRUCT.size
        while len(data) < length:
            s = os.read(fd, length - len(data))
            if not s:
                raise EOFError('unexpected EOF')
            data += s
        return UNSIGNED_STRUCT.unpack(data)[0]

The problem is that some functions still require to return a bytes, not a bytearray or something else. Converting a bytearray to a bytes still require a memory copy...