Re: [PATCH] xen-netfront: pull on receive skb may need to happen earlier

[Dion Kant <g.w.kant@hunenet.nl>]

On 07/08/2013 11:59 AM, Jan Beulich wrote:
>>>> On 05.07.13 at 16:53, Wei Liu <wei.liu2@citrix.com> wrote:
>> On Fri, Jul 05, 2013 at 10:32:41AM +0100, Jan Beulich wrote:
>>> --- a/drivers/net/xen-netfront.c
>>> +++ b/drivers/net/xen-netfront.c
>>> @@ -831,6 +831,15 @@ static RING_IDX xennet_fill_frags(struct
>>>  			RING_GET_RESPONSE(&np->rx, ++cons);
>>>  		skb_frag_t *nfrag = &skb_shinfo(nskb)->frags[0];
>>>  
>>> +		if (nr_frags == MAX_SKB_FRAGS) {
>>> +			unsigned int pull_to = NETFRONT_SKB_CB(skb)->pull_to;
>>> +
>>> +			BUG_ON(pull_to <= skb_headlen(skb));
>>> +			__pskb_pull_tail(skb, pull_to - skb_headlen(skb));
>>
>> skb_headlen is in fact "skb->len - skb->data_len". Looking at the
>> caller code:
>>
>>     while loop {
>>         skb_shinfo(skb)->frags[0].page_offset = rx->offset;
>> 	skb_frag_size_set(&skb_shinfo(skb)->frags[0], rx->status);
>> 	skb->data_len = rx->status;
>>
>> 	i = xennet_fill_frags(np, skb, &tmpq);
>>
>> 	/*                                                                           
>>                                                                   
>> 	 * Truesize is the actual allocation size, even if the                       
>>                                                                   
>> 	 * allocation is only partially used.                                        
>>                                                                   
>> 	 */
>> 	skb->truesize += PAGE_SIZE * skb_shinfo(skb)->nr_frags;
>> 	skb->len += skb->data_len;
>>     }
>>
>>     handle_incoming_packet();
>>
>> You seem to be altering the behavior of the original code, because in
>> your patch the skb->len is incremented before use, while in the original
>> code (which calls skb_headlen in handle_incoming_packet) the skb->len is
>> correctly set.
> 
> Right. So I basically need to keep skb->len up-to-date along with
> ->data_len. Just handed a patch to Dion with that done; I'll defer
> sending a v2 for the upstream code until I know the change works
> for our kernel.
> 
> Jan
> 

Jan,

I was wondering about the following.

In netif_poll(struct napi_struct *napi, int budget) the skb->cb is assigend,
but it may be clipped to RX_COPY_THRESHOLD

		NETFRONT_SKB_CB(skb)->pull_to = rx->status;
		if (NETFRONT_SKB_CB(skb)->pull_to > RX_COPY_THRESHOLD)
		      NETFRONT_SKB_CB(skb)->pull_to = RX_COPY_THRESHOLD;

How does this modification of NETFRONT_SKB_CB(skb)->pull_to propagates
or is this irrelevant?

Dion