netdev - Re: [PATCH net 3/4] virtio-net: reset num_buf to 1 after linearizing packet

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Date:   Mon, 21 May 2018 17:59:14 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 3/4] virtio-net: reset num_buf to 1 after linearizing
 packet

On Mon, May 21, 2018 at 04:35:05PM +0800, Jason Wang wrote:
> If we successfully linearize the packets, num_buf were set to zero
> which was wrong since we now have only 1 buffer to be used for e.g in
> the error path of receive_mergeable(). Zero num_buf will lead the code
> try to pop the buffers of next packet and drop it. Fixing this by set
> num_buf to 1 if we successfully linearize the packet.
> 
> Fixes: 4941d472bf95 ("virtio-net: do not reset during XDP set")
> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
>  drivers/net/virtio_net.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 6260d65..165a922 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -722,6 +722,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>  						      &len);
>  			if (!xdp_page)
>  				goto err_xdp;
> +			num_buf = 1;

So this is tweaked here for the benefit of err_skb below.
That's confusing and we won't remember to change it
if we change the error handling.

How about fixing the error path?


-        while (--num_buf) {
+        while (num_buf-- > 1) {

Seems more robust to me.


>  			offset = VIRTIO_XDP_HEADROOM;
>  		} else {
>  			xdp_page = page;
> -- 
> 2.7.4