Re: [Xen-devel] [PATCH net-next RESEND] xen-netfront: avoid packet loss when ethernet header crosses page boundary

[Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>]

David Miller <davem@xxxxxxxxxxxxx> writes:

> From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Date: Fri, 16 Sep 2016 12:59:14 +0200
>
>> @@ -595,6 +596,19 @@ static int xennet_start_xmit(struct sk_buff *skb, 
>> struct net_device *dev)
>>      offset = offset_in_page(skb->data);
>>      len = skb_headlen(skb);
>>  
>> +    /* The first req should be at least ETH_HLEN size or the packet will be
>> +     * dropped by netback.
>> +     */
>> +    if (unlikely(PAGE_SIZE - offset < ETH_HLEN)) {
>> +            nskb = skb_copy(skb, GFP_ATOMIC);
>> +            if (!nskb)
>> +                    goto drop;
>> +            dev_kfree_skb_any(skb);
>> +            skb = nskb;
>> +            page = virt_to_page(skb->data);
>> +            offset = offset_in_page(skb->data);
>> +    }
>> +
>>      spin_lock_irqsave(&queue->tx_lock, flags);
>
> I think you also have to recalculate 'len' in this case too, as
> skb_headlen() will definitely be different for nskb.
>
> In fact, I can't see how this code can work properly without that fix.

Thank you for your feedback David,

in my testing (even when I tried doing skb_copy() for all skbs
unconditionally) skb_headlen(nskb) always equals 'len' so I was under an
impression that both 'skb->len' and 'skb->data_len' remain the same when
we do skb_copy(). However, in case you think there are cases when
headlen changes, I see no problem with re-calculating 'len' as it won't
bring any significant performace penalty compared to the already added
skb_copy().

I'll send 'v2'.

-- 
  Vitaly

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel