Subject: Re: [Xen-devel] [PATCH] xen-netfront: Fix handling packets on compound pages with skb_linearize

From: Stefan Bader <[email-masked]>
Date: Mon, 01 Dec 2014 09:55:24 +0100

On 11.08.2014 19:32, Zoltan Kiss wrote:
> There is a long known problem with the netfront/netback interface: if the guest
> tries to send a packet which constitues more than MAX_SKB_FRAGS + 1 ring slots,
> it gets dropped. The reason is that netback maps these slots to a frag in the
> frags array, which is limited by size. Having so many slots can occur since
> compound pages were introduced, as the ring protocol slice them up into
> individual (non-compound) page aligned slots. The theoretical worst case
> scenario looks like this (note, skbs are limited to 64 Kb here):
> linear buffer: at most PAGE_SIZE - 17 * 2 bytes, overlapping page boundary,
> using 2 slots
> first 15 frags: 1 + PAGE_SIZE + 1 bytes long, first and last bytes are at the
> end and the beginning of a page, therefore they use 3 * 15 = 45 slots
> last 2 frags: 1 + 1 bytes, overlapping page boundary, 2 * 2 = 4 slots
> Although I don't think this 51 slots skb can really happen, we need a solution
> which can deal with every scenario. In real life there is only a few slots
> overdue, but usually it causes the TCP stream to be blocked, as the retry will
> most likely have the same buffer layout.
> This patch solves this problem by linearizing the packet. This is not the
> fastest way, and it can fail much easier as it tries to allocate a big linear
> area for the whole packet, but probably easier by an order of magnitude than
> anything else. Probably this code path is not touched very frequently anyway.
> 
> Signed-off-by: Zoltan Kiss <[email-masked]>
> Cc: Wei Liu <[email-masked]>
> Cc: Ian Campbell <[email-masked]>
> Cc: Paul Durrant <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> Cc: [email-masked]

This does not seem to be marked explicitly as stable. Has someone already asked
David Miller to put it on his stable queue? IMO it qualifies quite well and the
actual change should be simple to pick/backport.

-Stefan

> 
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index 055222b..23359ae 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -628,9 +628,10 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  	slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) +
>  		xennet_count_skb_frag_slots(skb);
>  	if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
> -		net_alert_ratelimited(
> -			"xennet: skb rides the rocket: %d slots\n", slots);
> -		goto drop;
> +		net_dbg_ratelimited("xennet: skb rides the rocket: %d slots, %d bytes\n",
> +				    slots, skb->len);
> +		if (skb_linearize(skb))
> +			goto drop;
>  	}
>  
>  	spin_lock_irqsave(&queue->tx_lock, flags);
> 
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