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

["Daniel P. Berrangé" <berrange@redhat.com>]

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.

>          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++;
    }


Introducing another local variable doesn't really add value IMHO.

With regards,
Daniel
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