Subject: Re: [Xen-devel] xen-netfront possibly rides the rocket too often

From: Stefan Bader <[email-masked]>
Date: Fri, 16 May 2014 12:09:50 +0200

On 16.05.2014 11:48, Wei Liu wrote:
> On Thu, May 15, 2014 at 02:14:00PM +0200, Stefan Bader wrote:
> [...]
>>> Wei.
>>>
>> Reading more of the code I would agree. The definition of MAX_SKB_FRAGS (at
>> least now with compound pages) cannot be used in any way to derive the number of
>> 4k slots a transfer will require.
>>
>> Zoltan already commented on worst cases. Not sure it would get as bad as that or
>> "just" 16*4k frags all in the middle of compound pages. That would then end in
>> around 33 or 34 slots, depending on the header.
>>
>> Zoltan wrote:
>>> I think the worst case scenario is when every frag and the linear buffer contains 2 bytes,
>>> which are overlapping a page boundary (that's (17+1)*2=36 so far), plus 15 of
>> them have a 4k
>>> page in the middle of them, so, a 1+4096+1 byte buffer can span over 3 page.
>>> That's 51 individual pages.
>>
>> I cannot claim to really know what to expect worst case. Somewhat I was thinking
>> of a
>> worst case of (16+1)*2, which would be inconvenient enough.
>>
>> So without knowing exactly how to do it, but as Ian said it sounds best to come
>> up with some sort of exception coalescing in cases the slot count goes over 18
>> and we know the data size is below 64K.
>>
> 
> I took a stab at it this morning and came up with this patch. Ran
> redis-benchmark, it seemed to fix that for me -- only saw one "failed to
> linearize skb" during
> 
>   redis-benchmark -h XXX -d 1000 -t lrange
> 
> And before this change, a lot of "rides rocket" were triggered.
> 
> Thought?

It appears at least to me as something that nicely makes use of existing code. I
was wondering about what could or could not be used. Trying to get ones head
around the whole thing is kind of a lot to look at.

The change at least looks straight forward enough.

-Stefan
> 
> ---8<---
> From 743495a2b2d338fc6cfe9bfd4b6e840392b87f4a Mon Sep 17 00:00:00 2001
> From: Wei Liu <[email-masked]>
> Date: Fri, 16 May 2014 10:39:01 +0100
> Subject: [PATCH] xen-netfront: linearize SKB if it occupies too many slots
> 
> Some workload, such as Redis can generate SKBs which make use of compound
> pages. Netfront doesn't quite like that because it doesn't want to send
> exessive slots to the backend as backend might deem it malicious. On the
> flip side these packets are actually legit, the size check at the
> beginning of xennet_start_xmit ensures that packet size is below 64K.
> 
> So we linearize SKB if it occupies too many slots. If the linearization
> fails then the SKB is dropped.
> 
> Signed-off-by: Wei Liu <[email-masked]>
> ---
>  drivers/net/xen-netfront.c |   18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index 895355d..0361fc5 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -573,9 +573,21 @@ 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;
> +		if (skb_linearize(skb)) {
> +			net_alert_ratelimited(
> +				"xennet: failed to linearize skb, skb dropped\n");
> +			goto drop;
> +		}
> +		data = skb->data;
> +		offset = offset_in_page(data);
> +		len = skb_headlen(skb);
> +		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: still too many slots after linerization: %d", slots);
> +			goto drop;
> +		}
>  	}
>  
>  	spin_lock_irqsave(&np->tx_lock, flags);
>