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

From: Vitaly Kuznetsov <[email-masked]>
Date: Mon, 19 Sep 2016 12:36:55 +0200

David Vrabel <[email-masked]> writes:

> On 19/09/16 11:22, Vitaly Kuznetsov wrote:
>> David Miller <[email-masked]> writes:
>> 
>>> From: Vitaly Kuznetsov <[email-masked]>
>>> 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 think you can move the len = skb_headlen(skb) after the if, no need to
> recalculate it.

Sorry, I was too fast with sending 'v2' and did the other way
around. I'll do v3.

-- 
  Vitaly