netdev - RE: [PATCH net] xen-netback: Fix slot estimation

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Date:	Tue, 3 Jun 2014 13:52:32 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Zoltan Kiss' <zoltan.kiss@...rix.com>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	"ian.campbell@...rix.com" <ian.campbell@...rix.com>,
	"wei.liu2@...rix.com" <wei.liu2@...rix.com>,
	"paul.durrant@...rix.com" <paul.durrant@...rix.com>,
	"linux@...elenboom.it" <linux@...elenboom.it>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"david.vrabel@...rix.com" <david.vrabel@...rix.com>,
	"davem@...emloft.net" <davem@...emloft.net>
Subject: RE: [PATCH net] xen-netback: Fix slot estimation

From: netdev-owner@...r.kernel.org
> 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@...rix.com>
> Cc: Paul Durrant <paul.durrant@...rix.com>
> Cc: Wei Liu <wei.liu2@...rix.com>
> Cc: Ian Campbell <ian.campbell@...rix.com>
> Cc: David Vrabel <david.vrabel@...rix.com>
> 
> 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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