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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Jul 08, 2013 at 03:20:26PM +0100, Jan Beulich wrote:
> >>> On 08.07.13 at 11:59, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
> >>>> 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.
> 
> Okay, so with that done (see below) Dion is now seeing the
> WARN_ON_ONCE(delta < len) in skb_try_coalesce() triggering. Of
> course, with it having crashed before, it's hard to tell whether the
> triggering now is an effect of the patch, or just got unmasked by it.
> 

I just ported your below patch to upstream kernel and I didn't see the
WARN_ON_ONCE. I only did iperf and netperf tests.

If the work load to trigger this bug is simple enough I can give it a
shot...


Wei.

> Looking over the ->truesize handling, I can't see how the change
> here could break things: RX_COPY_THRESHOLD is already
> accounted for by how alloc_skb() gets called, and the increment
> right after the call to xennet_fill_frags() should now be even more
> correct than before (since __pskb_pull_tail() can drop fragments,
> which would then have made this an over-estimation afaict).
> 
> That all said with me knowing pretty little about the networking
> code, so I'd appreciate if you could point out anything wrong with
> my idea of how things work. Additionally - is my fundamental (for
> this patch) assumption right that multiple __pskb_pull_tail() call
> are cumulative (i.e. calling it twice with a delta of
> pull_to - skb_headlen(skb) would indeed end up pulling up to
> pull_to, provided there is enough data)?
> 
> Jan
> 
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -831,10 +831,20 @@ 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));
> +                     nr_frags = shinfo->nr_frags;
> +             }
> +             BUG_ON(nr_frags >= MAX_SKB_FRAGS);
> +
>               __skb_fill_page_desc(skb, nr_frags,
>                                    skb_frag_page(nfrag),
>                                    rx->offset, rx->status);
>  
> +             skb->len += rx->status;
>               skb->data_len += rx->status;
>  
>               skb_shinfo(nskb)->nr_frags = 0;
> @@ -929,7 +939,8 @@ 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);
> @@ -1014,7 +1025,7 @@ 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);
>  
> @@ -1023,7 +1034,6 @@ err:
>                   * 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;

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