Re: [Xen-devel] [PATCH v2] xen-netfront: pull on receive skb may need to happen earlier

[Dion Kant <g.w.kant@xxxxxxxxxx>]

On 07/16/2013 12:25 PM, Wei Liu wrote:
> On Tue, Jul 16, 2013 at 10:46:01AM +0100, Jan Beulich wrote:
>> Due to commit 3683243b ("xen-netfront: use __pskb_pull_tail to ensure
>> linear area is big enough on RX") xennet_fill_frags() may end up
>> filling MAX_SKB_FRAGS + 1 fragments in a receive skb, and only reduce
>> the fragment count subsequently via __pskb_pull_tail(). That's a
>> result of xennet_get_responses() allowing a maximum of one more slot to
>> be consumed (and intermediately transformed into a fragment) if the
>> head slot has a size less than or equal to RX_COPY_THRESHOLD.
>>
>> Hence we need to adjust xennet_fill_frags() to pull earlier if we
>> reached the maximum fragment count - due to the described behavior of
>> xennet_get_responses() this guarantees that at least the first fragment
>> will get completely consumed, and hence the fragment count reduced.
>>
>> In order to not needlessly call __pskb_pull_tail() twice, make the
>> original call conditional upon the pull target not having been reached
>> yet, and defer the newly added one as much as possible (an alternative
>> would have been to always call the function right before the call to
>> xennet_fill_frags(), but that would imply more frequent cases of
>> needing to call it twice).
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
>> Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
>> Cc: stable@xxxxxxxxxxxxxxx (3.6 onwards)
>> ---
>> v2: Use skb_add_rx_frag() to keep all accounting fields up to date as
>>     we go (skb->len needing intermediate updating was pointed out by
>>     Wei Liu and David Miller, shinfo->nr_frags needing updating before
>>     calling __pskb_pull_tail() was spotted out by Dion Kant).
> Jan and Dion, is this a confirmed fix for SuSE kernel?
Dear Wei,

I did thorough testing on the system which revealed the original problem
and I can confirm that it does fix the issue.

Dion


>
> I complied and tested it, at least it didn't break things for me. The
> tests I ran were 1) scp large_file to domU; 2) iperf from Dom0 to DomU;
> 3) netperf from Dom0 to DomU.
>
> On the basis that this patch 1) fixes the bug for SuSE kernel (to be
> confirmed with Jan); 2) doesn't break upstream (tested by me).
>
> Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>
>
>> ---
>>  drivers/net/xen-netfront.c |   32 +++++++++++++-------------------
>>  1 file changed, 13 insertions(+), 19 deletions(-)
>>
>> --- 3.11-rc1/drivers/net/xen-netfront.c
>> +++ 3.11-rc1-xen-netfront-pull-earlier/drivers/net/xen-netfront.c
>> @@ -286,8 +286,7 @@ no_skb:
>>                      break;
>>              }
>>  
>> -            __skb_fill_page_desc(skb, 0, page, 0, 0);
>> -            skb_shinfo(skb)->nr_frags = 1;
>> +            skb_add_rx_frag(skb, 0, page, 0, 0, PAGE_SIZE);
>>              __skb_queue_tail(&np->rx_batch, skb);
>>      }
>>  
>> @@ -831,7 +830,6 @@ static RING_IDX xennet_fill_frags(struct
>>                                struct sk_buff_head *list)
>>  {
>>      struct skb_shared_info *shinfo = skb_shinfo(skb);
>> -    int nr_frags = shinfo->nr_frags;
>>      RING_IDX cons = np->rx.rsp_cons;
>>      struct sk_buff *nskb;
>>  
>> @@ -840,19 +838,21 @@ static RING_IDX xennet_fill_frags(struct
>>                      RING_GET_RESPONSE(&np->rx, ++cons);
>>              skb_frag_t *nfrag = &skb_shinfo(nskb)->frags[0];
>>  
>> -            __skb_fill_page_desc(skb, nr_frags,
>> -                                 skb_frag_page(nfrag),
>> -                                 rx->offset, rx->status);
>> +            if (shinfo->nr_frags == MAX_SKB_FRAGS) {
>> +                    unsigned int pull_to = NETFRONT_SKB_CB(skb)->pull_to;
>>  
>> -            skb->data_len += rx->status;
>> +                    BUG_ON(pull_to <= skb_headlen(skb));
>> +                    __pskb_pull_tail(skb, pull_to - skb_headlen(skb));
>> +            }
>> +            BUG_ON(shinfo->nr_frags >= MAX_SKB_FRAGS);
>> +
>> +            skb_add_rx_frag(skb, shinfo->nr_frags, skb_frag_page(nfrag),
>> +                            rx->offset, rx->status, PAGE_SIZE);
>>  
>>              skb_shinfo(nskb)->nr_frags = 0;
>>              kfree_skb(nskb);
>> -
>> -            nr_frags++;
>>      }
>>  
>> -    shinfo->nr_frags = nr_frags;
>>      return cons;
>>  }
>>  
>> @@ -933,7 +933,8 @@ static int handle_incoming_queue(struct 
>>      while ((skb = __skb_dequeue(rxq)) != NULL) {
>>              int pull_to = NETFRONT_SKB_CB(skb)->pull_to;
>>  
>> -            __pskb_pull_tail(skb, pull_to - skb_headlen(skb));
>> +            if (pull_to > skb_headlen(skb))
>> +                    __pskb_pull_tail(skb, pull_to - skb_headlen(skb));
>>  
>>              /* Ethernet work: Delayed to here as it peeks the header. */
>>              skb->protocol = eth_type_trans(skb, dev);
>> @@ -1018,17 +1019,10 @@ err:
>>  
>>              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;
>> +            skb->len += 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;
>> -
>>              if (rx->flags & XEN_NETRXF_csum_blank)
>>                      skb->ip_summed = CHECKSUM_PARTIAL;
>>              else if (rx->flags & XEN_NETRXF_data_validated)
>>


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