Re: [PATCH v7 2/5] QIOChannelSocket: Implement io_writev zero copy flag & io_flush for CONFIG_LINUX

[Daniel P . Berrangé]

On Thu, Jan 13, 2022 at 02:48:15PM +0800, Peter Xu wrote:
> On Thu, Jan 06, 2022 at 07:13:39PM -0300, Leonardo Bras wrote:
> > @@ -558,15 +575,26 @@ static ssize_t qio_channel_socket_writev(QIOChannel 
> > *ioc,
> >          memcpy(CMSG_DATA(cmsg), fds, fdsize);
> >      }
> >  
> > +    if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
> > +        sflags = MSG_ZEROCOPY;
> > +    }
> > +
> >   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;
> > +        case ENOBUFS:
> > +            if (sflags & MSG_ZEROCOPY) {
> > +                error_setg_errno(errp, errno,
> > +                                 "Process can't lock enough memory for 
> > using MSG_ZEROCOPY");
> > +                return -1;
> > +            }
> 
> I have no idea whether it'll make a real differnece, but - should we better 
> add
> a "break" here?  If you agree and with that fixed, feel free to add:
> 
> Reviewed-by: Peter Xu <peterx@redhat.com>
> 
> I also wonder whether you hit ENOBUFS in any of the environments.  On Fedora
> here it's by default unlimited, but just curious when we should keep an eye.

Fedora doesn't allow unlimited locked memory by default

$ grep "locked memory" /proc/self/limits 
Max locked memory         65536                65536                bytes     

And  regardless of Fedora defaults, libvirt will set a limit
for the guest. It will only be unlimited if requiring certain
things like VFIO.

Regards,
Daniel
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