Re: [PATCH v1 1/1] QIOChannelSocket: Fix zero-copy send so socket flush works

[Leonardo Bras Soares Passos]

Hello Daniel,

On Wed, Jun 8, 2022 at 3:46 PM Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> On Wed, Jun 08, 2022 at 03:18:09PM -0300, Leonardo Bras wrote:
> > Somewhere between v6 and v7 the of the zero-copy-send patchset a crucial
> > part of the flushing mechanism got missing: incrementing zero_copy_queued.
> >
> > Without that, the flushing interface becomes a no-op, and there is no
> > garantee the buffer is really sent.
> >
> > This can go as bad as causing a corruption in RAM during migration.
> >
> > Fixes: 2bc58ffc2926 ("QIOChannelSocket: Implement io_writev zero copy flag 
> > & io_flush for CONFIG_LINUX")
> > Reported-by: 徐闯 <xuchuangxclwt@bytedance.com>
> > Signed-off-by: Leonardo Bras <leobras@redhat.com>
> > ---
> >  io/channel-socket.c | 11 ++++++++---
> >  1 file changed, 8 insertions(+), 3 deletions(-)
> >
> > diff --git a/io/channel-socket.c b/io/channel-socket.c
> > index dc9c165de1..ca4cae930f 100644
> > --- a/io/channel-socket.c
> > +++ b/io/channel-socket.c
> > @@ -554,6 +554,7 @@ static ssize_t qio_channel_socket_writev(QIOChannel 
> > *ioc,
> >      size_t fdsize = sizeof(int) * nfds;
> >      struct cmsghdr *cmsg;
> >      int sflags = 0;
> > +    bool zero_copy_enabled = false;
> >
> >      memset(control, 0, CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS));
> >
> > @@ -581,6 +582,7 @@ static ssize_t qio_channel_socket_writev(QIOChannel 
> > *ioc,
> >  #ifdef QEMU_MSG_ZEROCOPY
> >      if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
> >          sflags = MSG_ZEROCOPY;
> > +        zero_copy_enabled = true;
> >      }
> >  #endif
> >
> > @@ -592,21 +594,24 @@ static ssize_t qio_channel_socket_writev(QIOChannel 
> > *ioc,
> >              return QIO_CHANNEL_ERR_BLOCK;
> >          case EINTR:
> >              goto retry;
> > -#ifdef QEMU_MSG_ZEROCOPY
>
> Removing this ifdef appears incidental to the change. If this is
> redundant just remove it in its own patch.

The idea is to reduce the amount of #ifdefs as Peter suggested,
because adding another ifdef here
would introduce extra noise. But sure, I see no problem adding this
change as a previous patch.

>
> >          case ENOBUFS:
> > -            if (sflags & MSG_ZEROCOPY) {
> > +            if (zero_copy_enabled) {
> >                  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;
> >      }
> > +
> > +    if (zero_copy_enabled) {
>
> What's wrong with
>
>    if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
>         sioc->zero_copy_queued++;
>     }

There is nothing wrong with it, but using zero_copy_enabled as
presented here will
compile-out this 'if()'  block if the user does not support MSG_ZEROCOPY.

Best regards,
Leo

>
>
> Introducing another local variable doesn't really add value IMHO.
>
> With regards,
> Daniel
> --
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