It seems that multiprocessing is fairly conservative wrt. 64-bit support. For example, conn_send_string has a string limit of 0x7fffffff. Therefore, several of the warnings are harmless; the respective lengths fit into int just fine.

I recommend that the size_t parameters are converted to int in the places where this is actually known to be correct from control flow (e.g. both cases of conn_send_string), along with an assert() that the value being cast is <= INT_MAX. 

IIUC, _conn_recvall can legitimately return int, no need to return Py_ssize_t: it will only return error codes (possibly MP_SUCCESS). Also, it might be sufficient to restrict the length of _conn_recvall to MAX_INT.

The alternate route would be to actually widen socket_connection to support 64 bit transmits (*); this would be Jesse's call. It's probably acceptable to defer this until somebody needs this enough to contribute a patch. Copying 2GiB over a pipe takes 1.5s on my system.

(*) My protocol proposal would be this: a length with the highest bit set (i.e. >= 0x80000000) indicates a huge message, where another four bytes of of length follow, for a total of 63 bits of length (MSB is unavailable as it serves as the length-of-length indicator). This would not waste bytes for small messages, and be backwards compatible.