Subject: Re: [PATCH net-next v3 6/9] xen-netback: Handle guests with too many frags

From: Zoltan Kiss <[email-masked]>
Date: Wed, 8 Jan 2014 14:13:12 +0000

On 08/01/14 13:54, Eric Dumazet wrote:
> On Wed, 2014-01-08 at 13:49 +0000, Zoltan Kiss wrote:
>> On 08/01/14 02:12, Eric Dumazet wrote:
>>> On Wed, 2014-01-08 at 00:10 +0000, Zoltan Kiss wrote:
>>>
>>>>
>>>> +		if (skb_shinfo(skb)->frag_list) {
>>>> +			nskb = skb_shinfo(skb)->frag_list;
>>>> +			xenvif_fill_frags(vif, nskb, INVALID_PENDING_IDX);
>>>> +			skb->len += nskb->len;
>>>> +			skb->data_len += nskb->len;
>>>> +			skb->truesize += nskb->truesize;
>>>> +			skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
>>>> +			skb_shinfo(nskb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
>>>> +			vif->tx_zerocopy_sent += 2;
>>>> +			nskb = skb;
>>>> +
>>>> +			skb = skb_copy_expand(skb,
>>>> +					0,
>>>> +					0,
>>>> +					GFP_ATOMIC | __GFP_NOWARN);
>>>
>>> skb can be NULL here
>>
>> Thanks, fixed that.
>
> BTW, I am not sure why you copy the skb.
>
> Is it to get rid of frag_list, and why ?

Yes, it is to get rid of the frag_list, just to be on the safe side. I'm 
not sure if it is normal to send a big skb with MAX_SKB_FRAGS frags plus 
an empty skb on the frag_list with one frag, so I just consolidate them 
here. This scenario shouldn't happen very often anyway, even guests 
which can send more than MAX_SKB_FRAGS slots tends to do it rarely.

Zoli