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 <[email protected]>
> ---
> 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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