>> Hi all, >> >> I have one question on the behavior of TCP/IPv4 sending using the >> MSG_ZEROCOPY flag, >> the kernel version is 4.19.18. >> >> What happens if I close the sending socket immediately after performing a >> socket >> send() or sendmsg() call (called with the MSG_ZEROCOPY flag)? >> I.e. in this situation not all messages have been sent yet, however - as the >> socket is closed - it is no longer possible to retrieve the completion >> notification via the error channel. >> >> Is it fine for the user space program to free all outstanding messages after >> the >> socket close() has returned? >> Or is there anything else that has to be considered? > > If closing the socket while user memory is still in transmission, it > will not be possible to safely reuse the memory, as the process has no > way of discovering when the kernel has finished transmission. > > Depending on type of memory, there may be workarounds to avoid > unbound virtual memory growth, such as unmapping the virtual > address range in the case of mmap()ed data. > > But in general, the right approach is to wait for all completions > before closing a socket. > > If this takes a long time, say due to the TCP stack hold on to data for > retransmission in the case a peer does not properly close its side, > disconnect (connect() AF_UNSPEC) can be used to purge the > queues and trigger notifications. Again, this is a last resort and > usually not needed. Hi Willem! Thank you for the clarification. Do you think you could add this to the documentation of MSG_ZEROCOPY? I think this could be really helpful for all users of the zerocopy feature.
Thanks a lot Mathias
