Re: [PATCH v6 2/2] QIOChannelSocket: flush zerocopy socket error queue on sendmsg failure due to ENOBUF

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

On Mon, Oct 13, 2025 at 09:21:22AM +0000, Tejus GK wrote:
> From: Manish Mishra <manish.mishra@nutanix.com>
> 
> The kernel allocates extra metadata SKBs in case of a zerocopy send,
> eventually used for zerocopy's notification mechanism. This metadata
> memory is accounted for in the OPTMEM limit. The kernel queues
> completion notifications on the socket error queue and this error queue
> is freed when userspace reads it.
> 
> Usually, in the case of in-order processing, the kernel will batch the
> notifications and merge the metadata into a single SKB and free the
> rest. As a result, it never exceeds the OPTMEM limit. However, if there
> is any out-of-order processing or intermittent zerocopy failures, this
> error chain can grow significantly, exhausting the OPTMEM limit. As a
> result, all new sendmsg requests fail to allocate any new SKB, leading
> to an ENOBUF error. Depending on the amount of data queued before the
> flush (i.e., large live migration iterations), even large OPTMEM limits
> are prone to failure.
> 
> To work around this, if we encounter an ENOBUF error with a zerocopy
> sendmsg, flush the error queue and retry once more.
> 
> Co-authored-by: Manish Mishra <manish.mishra@nutanix.com>
> Signed-off-by: Tejus GK <tejus.gk@nutanix.com>
> ---
>  include/io/channel-socket.h |  5 +++
>  io/channel-socket.c         | 75 ++++++++++++++++++++++++++++++-------
>  2 files changed, 66 insertions(+), 14 deletions(-)
> 
> diff --git a/include/io/channel-socket.h b/include/io/channel-socket.h
> index 26319fa98b..fcfd489c6c 100644
> --- a/include/io/channel-socket.h
> +++ b/include/io/channel-socket.h
> @@ -50,6 +50,11 @@ struct QIOChannelSocket {
>      ssize_t zero_copy_queued;
>      ssize_t zero_copy_sent;
>      bool blocking;
> +    /**
> +     * This flag indicates whether any new data was successfully sent with
> +     * zerocopy since the last qio_channel_socket_flush() call.
> +     */
> +    bool new_zero_copy_sent_success;
>  };
>  
>  
> diff --git a/io/channel-socket.c b/io/channel-socket.c
> index 8b30d5b7f7..7cd9f3666d 100644
> --- a/io/channel-socket.c
> +++ b/io/channel-socket.c
> @@ -37,6 +37,12 @@
>  
>  #define SOCKET_MAX_FDS 16
>  
> +#ifdef QEMU_MSG_ZEROCOPY
> +static int qio_channel_socket_flush_internal(QIOChannel *ioc,
> +                                             bool block,
> +                                             Error **errp);
> +#endif
> +
>  SocketAddress *
>  qio_channel_socket_get_local_address(QIOChannelSocket *ioc,
>                                       Error **errp)
> @@ -66,6 +72,7 @@ qio_channel_socket_new(void)
>      sioc->zero_copy_queued = 0;
>      sioc->zero_copy_sent = 0;
>      sioc->blocking = false;
> +    sioc->new_zero_copy_sent_success = FALSE;
>  
>      ioc = QIO_CHANNEL(sioc);
>      qio_channel_set_feature(ioc, QIO_CHANNEL_FEATURE_SHUTDOWN);
> @@ -618,6 +625,8 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
>      size_t fdsize = sizeof(int) * nfds;
>      struct cmsghdr *cmsg;
>      int sflags = 0;
> +    bool blocking = sioc->blocking;
> +    bool zerocopy_flushed_once = false;
>  
>      memset(control, 0, CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS));
>  
> @@ -664,9 +673,24 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
>              goto retry;
>          case ENOBUFS:
>              if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
> -                error_setg_errno(errp, errno,
> -                                 "Process can't lock enough memory for using MSG_ZEROCOPY");
> -                return -1;
> +                /**
> +                 * Socket error queueing may exhaust the OPTMEM limit. Try
> +                 * flushing the error queue once.
> +                 */
> +                if (!zerocopy_flushed_once) {
> +                    ret = qio_channel_socket_flush_internal(ioc, blocking,
> +                                                            errp);
> +                    if (ret < 0) {
> +                        return -1;
> +                    }
> +                    zerocopy_flushed_once = TRUE;
> +                    goto retry;
> +                } else {
> +                    error_setg_errno(errp, errno,
> +                                     "Process can't lock enough memory for "
> +                                     "using MSG_ZEROCOPY");
> +                    return -1;
> +                }
>              }
>              break;
>          }
> @@ -777,8 +801,9 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
>  
>  
>  #ifdef QEMU_MSG_ZEROCOPY
> -static int qio_channel_socket_flush(QIOChannel *ioc,
> -                                    Error **errp)
> +static int qio_channel_socket_flush_internal(QIOChannel *ioc,
> +                                             bool block,
> +                                             Error **errp)
>  {
>      QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(ioc);
>      struct msghdr msg = {};
> @@ -786,7 +811,6 @@ static int qio_channel_socket_flush(QIOChannel *ioc,
>      struct cmsghdr *cm;
>      char control[CMSG_SPACE(sizeof(*serr))];
>      int received;
> -    int ret;
>  
>      if (sioc->zero_copy_queued == sioc->zero_copy_sent) {
>          return 0;
> @@ -796,16 +820,20 @@ static int qio_channel_socket_flush(QIOChannel *ioc,
>      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) {
>              switch (errno) {
>              case EAGAIN:
> -                /* Nothing on errqueue, wait until something is available */
> -                qio_channel_wait(ioc, G_IO_ERR);
> -                continue;
> +                if (block) {
> +                    /*
> +                     * Nothing on errqueue, wait until something is
> +                     * available.
> +                     */
> +                    qio_channel_wait(ioc, G_IO_ERR);
> +                    continue;

Why G_IO_ERR ?  If we're waiting for recvmsg() to become ready, then
it would need to be G_IO_IN we're waiting for.

> +                }
> +                return 0;
>              case EINTR:
>                  continue;
>              default:

With regards,
Daniel
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