Re: [PATCH v3 1/3] QIOChannelSocket: Fix zero-copy flush returning code 1 when nothing sent

[Leonardo Bras Soares Passos]

Hello Peter,

On Thu, Jul 7, 2022 at 2:47 PM Peter Xu <peterx@redhat.com> wrote:
>
> Hi, Leo,
>
> On Mon, Jul 04, 2022 at 05:23:13PM -0300, Leonardo Bras wrote:
> > If flush is called when no buffer was sent with MSG_ZEROCOPY, it currently
> > returns 1. This return code should be used only when Linux fails to use
> > MSG_ZEROCOPY on a lot of sendmsg().
> >
> > Fix this by returning early from flush if no sendmsg(...,MSG_ZEROCOPY)
> > was attempted.
> >
> > Fixes: 2bc58ffc2926 ("QIOChannelSocket: Implement io_writev zero copy flag 
> > & io_flush for CONFIG_LINUX")
> > Signed-off-by: Leonardo Bras <leobras@redhat.com>
> > ---
> >  io/channel-socket.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/io/channel-socket.c b/io/channel-socket.c
> > index 4466bb1cd4..698c086b70 100644
> > --- a/io/channel-socket.c
> > +++ b/io/channel-socket.c
> > @@ -716,12 +716,18 @@ static int qio_channel_socket_flush(QIOChannel *ioc,
> >      struct cmsghdr *cm;
> >      char control[CMSG_SPACE(sizeof(*serr))];
> >      int received;
> > -    int ret = 1;
> > +    int ret;
> > +
> > +    if (!sioc->zero_copy_queued) {
>
> I think I asked this in the downstream review but didn't get a
> response.. shouldn't this check be "queued == sent"?

This is just supposed to skip flush if nothing was queued for sending.
queued == sent is tested bellow in the while part.

Without this, the function could return 1 if nothing was sent with zero-copy,
and it would be confusing, because the QIOChannel API says 1 should be
returned only if all zero-copy sends fell back to copying.

Best regards,
Leo

>
> > +        return 0;
> > +    }
> >
> >      msg.msg_control = control;
> >      msg.msg_controllen = sizeof(control);
> >      memset(control, 0, sizeof(control));
> >
> > +    ret = 1;
> > +
> >      while (sioc->zero_copy_sent < sioc->zero_copy_queued) {
> >          received = recvmsg(sioc->fd, &msg, MSG_ERRQUEUE);
> >          if (received < 0) {
> > --
> > 2.36.1
> >
>
> --
> Peter Xu
>