Re: [PATCH v4] QIOChannelSocket: Flush zerocopy socket error queue on sendmsg failure due to ENOBUF

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

On Thu, Apr 03, 2025 at 04:21:21AM -0400, Manish Mishra wrote:
> We allocate extra metadata SKBs in case of a zerocopy send. This metadata
> memory is accounted for in the OPTMEM limit. If there is any error while
> sending zerocopy packets or if zerocopy is skipped, these metadata SKBs are
> queued in the socket error queue. This error queue is freed when userspace
> reads it.
> 
> Usually, if there are continuous failures, we merge the metadata into a single
> SKB and free another one. 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,
> we flush the error queue and retry once more.
> 
> Signed-off-by: Manish Mishra <manish.mishra@nutanix.com>
> ---
>  include/io/channel-socket.h |  5 +++
>  io/channel-socket.c         | 74 ++++++++++++++++++++++++++++++-------
>  2 files changed, 65 insertions(+), 14 deletions(-)
> 
> V2:
>   1. Removed the dirty_sync_missed_zero_copy migration stat.
>   2. Made the call to qio_channel_socket_flush_internal() from
>      qio_channel_socket_writev() non-blocking.
> 
> V3:
>   1. Add the dirty_sync_missed_zero_copy migration stat again.
> 
> V4:
>   1. Minor nit to rename s/zero_copy_flush_pending/zerocopy_flushed_once.
> 
> diff --git a/include/io/channel-socket.h b/include/io/channel-socket.h
> index ab15577d38..2c48b972e8 100644
> --- a/include/io/channel-socket.h
> +++ b/include/io/channel-socket.h
> @@ -49,6 +49,11 @@ struct QIOChannelSocket {
>      socklen_t remoteAddrLen;
>      ssize_t zero_copy_queued;
>      ssize_t zero_copy_sent;
> +    /**
> +     * 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 608bcf066e..d5882c16fe 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)
> @@ -65,6 +71,7 @@ qio_channel_socket_new(void)
>      sioc->fd = -1;
>      sioc->zero_copy_queued = 0;
>      sioc->zero_copy_sent = 0;
> +    sioc->new_zero_copy_sent_success = FALSE;
>  
>      ioc = QIO_CHANNEL(sioc);
>      qio_channel_set_feature(ioc, QIO_CHANNEL_FEATURE_SHUTDOWN);
> @@ -566,6 +573,7 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
>      size_t fdsize = sizeof(int) * nfds;
>      struct cmsghdr *cmsg;
>      int sflags = 0;
> +    bool zerocopy_flushed_once = FALSE;
>  
>      memset(control, 0, CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS));
>  
> @@ -612,9 +620,25 @@ 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, false, errp);

Passing in 'errp', so the error will be set on failure of this method...

> +                    if (ret < 0) {
> +                        error_setg_errno(errp, errno,
> +                                         "Zerocopy flush failed");

...but we now try to overwrite the already set error. This error_setg_errno
call should be removed.

> +                        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;
>          }

With regards,
Daniel
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