* chuang xu (xuchuangxc...@bytedance.com) wrote:
>
> On 2022/5/13 下午2:28, Leonardo Bras wrote:
> > @@ -557,15 +578,31 @@ static ssize_t qio_channel_socket_writev(QIOChannel
> > *ioc,
> > memcpy(CMSG_DATA(cmsg), fds, fdsize);
> > }
> > +#ifdef QEMU_MSG_ZEROCOPY
> > + if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
> > + sflags = MSG_ZEROCOPY;
> > + }
> > +#endif
> > +
> > retry:
> > - ret = sendmsg(sioc->fd, &msg, 0);
> > + ret = sendmsg(sioc->fd, &msg, sflags);
> > if (ret <= 0) {
> > - if (errno == EAGAIN) {
> > + switch (errno) {
> > + case EAGAIN:
> > return QIO_CHANNEL_ERR_BLOCK;
> > - }
> > - if (errno == EINTR) {
> > + case EINTR:
> > goto retry;
> > +#ifdef QEMU_MSG_ZEROCOPY
> > + case ENOBUFS:
> > + if (sflags & MSG_ZEROCOPY) {
> > + error_setg_errno(errp, errno,
> > + "Process can't lock enough memory for
> > using MSG_ZEROCOPY");
> > + return -1;
> > + }
> > + break;
> > +#endif
> > }
> > +
> > error_setg_errno(errp, errno,
> > "Unable to write to socket");
> > return -1;
>
> Hi, Leo.
>
> There are some other questions I would like to discuss with you.
>
> I tested the multifd zero_copy migration and found that sometimes even if
> max locked memory of qemu was set to 16GB（much greater than
> `MULTIFD_PACKET_SIZE`）, the error "Process can't lock enough memory for
> using MSG_ZEROCOPY" would still be reported.
>
> I noticed that the
> doc(https://www.kernel.org/doc/html/v5.12/networking/msg_zerocopy.html) says
> "A zerocopy failure will return -1 with errno ENOBUFS. This happens if the
> socket option was not set, _the socket exceeds its optmem limit_ or the user
> exceeds its ulimit on locked pages."
>
> I also found that the RFC(https://lwn.net/Articles/715279/) says _"__The
> change to allocate notification skbuffs from optmem requires__ensuring that
> net.core.optmem is at least a few 100KB."_
Interesting.
> On my host, optmem was initially set to 20KB, I tried to change it to 100KB
> (echo 102400 > /proc/sys/net/core/optmem_max) as the RFC says.Then I tested
> the multifd zero_copy migration repeatedly，and the error disappeared.
>
> So when sendmsg returns -1 with errno ENOBUFS, should we distinguish between
> error ''socket exceeds optmem limit" and error "user exceeds ulimit on
> locked pages"? Or is there any better way to avoid this problem?
I don't think we can tell which one of them triggered the error; so the
only thing I can suggest is that we document the need for optmem_max
setting; I wonder how we get a better answer than 'a few 100KB'?
I guess it's something like the number of packets inflight *
sizeof(cmsghdr) ?
Dave
> Best Regards,
>
> chuang xu
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
