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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-07-16 at 10:46 +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).
> 
> ---
>  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;

Of the top of my head I have no confidence in my understanding of what
this is actually going to end up assigned to ->len or ->data_len after
this.

For my benefit, can we do the dumb version please?

Also ref CodingStyle:
        Don't put multiple assignments on a single line either.  Kernel coding 
style
        is super simple.  Avoid tricky expressions.

The rest of it looked OK to me.

Ian.


>  
>               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)
> 
> 



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel