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

[Peter Xu]

On Thu, Jul 07, 2022 at 04:44:21PM -0300, Leonardo Bras Soares Passos wrote:
> 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.

I know it won't happen in practise, but.. what if we call flush() without
sending anything zero-copy-wise at all (so zero_copy_sent > 0,
zero_copy_queued > 0, meanwhile zero_copy_sent == zero_copy_queued)?  Then
IIUC we'll return 1 even if we didn't do any fallback, or am I wrong?

> 
> 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
> >
> 

-- 
Peter Xu