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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.07.13 at 16:53, Wei Liu <wei.liu2@xxxxxxxxxx> 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


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