netdev - Re: [PATCH net-next] virtio-net: invoke zerocopy callback on xmit path if no tx napi

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Date:   Sun, 20 Aug 2017 16:49:15 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Koichiro Den <den@...ipeden.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] virtio-net: invoke zerocopy callback on xmit
 path if no tx napi

On Sat, Aug 19, 2017 at 2:38 AM, Koichiro Den <den@...ipeden.com> wrote:
> Facing the possible unbounded delay relying on freeing on xmit path,
> we also better to invoke and clear the upper layer zerocopy callback
> beforehand to keep them from waiting for unbounded duration in vain.

Good point.

> For instance, this removes the possible deadlock in the case that the
> upper layer is a zerocopy-enabled vhost-net.
> This does not apply if napi_tx is enabled since it will be called in
> reasonale time.

Indeed. Btw, I am gathering data to eventually make napi the default
mode. But that is taking some time.

>
> Signed-off-by: Koichiro Den <den@...ipeden.com>
> ---
>  drivers/net/virtio_net.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 4302f313d9a7..f7deaa5b7b50 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1290,6 +1290,14 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>
>         /* Don't wait up for transmitted skbs to be freed. */
>         if (!use_napi) {
> +               if (skb_shinfo(skb)->tx_flags & SKBTX_DEV_ZEROCOPY) {
> +                       struct ubuf_info *uarg;
> +                       uarg = skb_shinfo(skb)->destructor_arg;
> +                       if (uarg->callback)
> +                           uarg->callback(uarg, true);
> +                       skb_shinfo(skb)->destructor_arg = NULL;
> +                       skb_shinfo(skb)->tx_flags &= ~SKBTX_DEV_ZEROCOPY;
> +               }

Instead of open coding, this can use skb_zcopy_clear.

>                 skb_orphan(skb);
>                 nf_reset(skb);
>         }
> --
> 2.9.4
>
>