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

[Peter Xu]

On Thu, Jan 13, 2022 at 10:06:14AM +0000, Daniel P. Berrangé wrote:
> 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.

Thanks, I obviously checked up the wrong host..

Leo, do you know how much locked memory will be needed by zero copy?  Will
there be a limit?  Is it linear to the number of sockets/channels?

It'll be better if we can fail at enabling the feature when we detected that
the specified locked memory limit may not be suffice.

-- 
Peter Xu