Re: [Xen-devel] [PATCH net] xen-netback: Fix slot estimation

[David Laight <David.Laight@xxxxxxxxxx>]

From: netdev-owner@xxxxxxxxxxxxxxx
> A recent commit (a02eb4 "xen-netback: worse-case estimate in xenvif_rx_action 
> is
> underestimating") capped the slot estimation to MAX_SKB_FRAGS, but that 
> triggers
> the next BUG_ON a few lines down, as the packet consumes more slots than
> estimated.
> This patch remove that cap, and if the frontend doesn't provide enough slot,
> put back the skb to the top of the queue and caps rx_last_skb_slots. When the
> next try also fails, it drops the packet.
> Capping rx_last_skb_slots is needed because if the frontend never gives enough
> slots, the ring gets stalled.
> 
> Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
> Cc: Paul Durrant <paul.durrant@xxxxxxxxxx>
> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
> Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
> 
> diff --git a/drivers/net/xen-netback/netback.c 
> b/drivers/net/xen-netback/netback.c
> index da85ffb..7164157 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -600,13 +600,6 @@ static void xenvif_rx_action(struct xenvif *vif)
>                                                        PAGE_SIZE);
>               }
> 
> -             /* To avoid the estimate becoming too pessimal for some
> -              * frontends that limit posted rx requests, cap the estimate
> -              * at MAX_SKB_FRAGS.
> -              */
> -             if (max_slots_needed > MAX_SKB_FRAGS)
> -                     max_slots_needed = MAX_SKB_FRAGS;
> -
>               /* We may need one more slot for GSO metadata */
>               if (skb_is_gso(skb) &&
>                  (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV4 ||
> @@ -615,9 +608,27 @@ static void xenvif_rx_action(struct xenvif *vif)
> 
>               /* If the skb may not fit then bail out now */
>               if (!xenvif_rx_ring_slots_available(vif, max_slots_needed)) {
> +                     /* If the skb needs more than MAX_SKB_FRAGS slots, it
> +                      * can happen that the frontend never gives us enough.
> +                      * To avoid spining on that packet, first we put it back
> +                      * to the top of the queue, but if the next try fail,
> +                      * we drop it.
> +                      */
> +                     if (max_slots_needed > MAX_SKB_FRAGS &&
> +                         vif->rx_last_skb_slots == MAX_SKB_FRAGS) {
> +                             kfree_skb(skb);
> +                             vif->rx_last_skb_slots = 0;
> +                             continue;
> +                     }

A silent discard here doesn't seem right at all.
While it stops the kernel crashing, or the entire interface locking
up; it is likely to leave one connection 'stuck' - a TCP retransmission
is likely to include the same fragments.
From a user point of view this as almost as bad.

        David

>                       skb_queue_head(&vif->rx_queue, skb);
>                       need_to_notify = true;
> -                     vif->rx_last_skb_slots = max_slots_needed;
> +                     /* Cap this otherwise if the guest never gives us
> +                      * enough slot, rx_work_todo will spin
> +                      */
> +                     vif->rx_last_skb_slots =
> +                             max_slots_needed > MAX_SKB_FRAGS ?
> +                             MAX_SKB_FRAGS :
> +                             max_slots_needed;
>                       break;
>               } else
>                       vif->rx_last_skb_slots = 0;
> --
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