Re: [Xen-devel] [PATCH net-next RESEND] xen-netfront: avoid packet loss when ethernet header crosses page boundary
David Vrabel writes:
> On 19/09/16 11:22, Vitaly Kuznetsov wrote:
>> David Miller writes:
>>
>>> From: Vitaly Kuznetsov
>>> Date: Fri, 16 Sep 2016 12:59:14 +0200
>>>
@@ -595,6 +596,19 @@ static int xennet_start_xmit(struct sk_buff *skb,
struct net_device *dev)
offset = offset_in_page(skb->data);
len = skb_headlen(skb);
+ /* The first req should be at least ETH_HLEN size or the packet will be
+ * dropped by netback.
+ */
+ if (unlikely(PAGE_SIZE - offset < ETH_HLEN)) {
+ nskb = skb_copy(skb, GFP_ATOMIC);
+ if (!nskb)
+ goto drop;
+ dev_kfree_skb_any(skb);
+ skb = nskb;
+ page = virt_to_page(skb->data);
+ offset = offset_in_page(skb->data);
+ }
+
spin_lock_irqsave(&queue->tx_lock, flags);
>>>
>>> I think you also have to recalculate 'len' in this case too, as
>>> skb_headlen() will definitely be different for nskb.
>>>
>>> In fact, I can't see how this code can work properly without that fix.
>>
>> Thank you for your feedback David,
>>
>> in my testing (even when I tried doing skb_copy() for all skbs
>> unconditionally) skb_headlen(nskb) always equals 'len' so I was under an
>> impression that both 'skb->len' and 'skb->data_len' remain the same when
>> we do skb_copy(). However, in case you think there are cases when
>> headlen changes, I see no problem with re-calculating 'len' as it won't
>> bring any significant performace penalty compared to the already added
>> skb_copy().
>
> I think you can move the len = skb_headlen(skb) after the if, no need to
> recalculate it.
Sorry, I was too fast with sending 'v2' and did the other way
around. I'll do v3.
--
Vitaly
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Re: [Xen-devel] [PATCH net-next RESEND] xen-netfront: avoid packet loss when ethernet header crosses page boundary
On 19/09/16 11:22, Vitaly Kuznetsov wrote:
> David Miller writes:
>
>> From: Vitaly Kuznetsov
>> Date: Fri, 16 Sep 2016 12:59:14 +0200
>>
>>> @@ -595,6 +596,19 @@ static int xennet_start_xmit(struct sk_buff *skb,
>>> struct net_device *dev)
>>> offset = offset_in_page(skb->data);
>>> len = skb_headlen(skb);
>>>
>>> + /* The first req should be at least ETH_HLEN size or the packet will be
>>> +* dropped by netback.
>>> +*/
>>> + if (unlikely(PAGE_SIZE - offset < ETH_HLEN)) {
>>> + nskb = skb_copy(skb, GFP_ATOMIC);
>>> + if (!nskb)
>>> + goto drop;
>>> + dev_kfree_skb_any(skb);
>>> + skb = nskb;
>>> + page = virt_to_page(skb->data);
>>> + offset = offset_in_page(skb->data);
>>> + }
>>> +
>>> spin_lock_irqsave(&queue->tx_lock, flags);
>>
>> I think you also have to recalculate 'len' in this case too, as
>> skb_headlen() will definitely be different for nskb.
>>
>> In fact, I can't see how this code can work properly without that fix.
>
> Thank you for your feedback David,
>
> in my testing (even when I tried doing skb_copy() for all skbs
> unconditionally) skb_headlen(nskb) always equals 'len' so I was under an
> impression that both 'skb->len' and 'skb->data_len' remain the same when
> we do skb_copy(). However, in case you think there are cases when
> headlen changes, I see no problem with re-calculating 'len' as it won't
> bring any significant performace penalty compared to the already added
> skb_copy().
I think you can move the len = skb_headlen(skb) after the if, no need to
recalculate it.
David
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Re: [Xen-devel] [PATCH net-next RESEND] xen-netfront: avoid packet loss when ethernet header crosses page boundary
David Miller writes:
> From: Vitaly Kuznetsov
> Date: Fri, 16 Sep 2016 12:59:14 +0200
>
>> @@ -595,6 +596,19 @@ static int xennet_start_xmit(struct sk_buff *skb,
>> struct net_device *dev)
>> offset = offset_in_page(skb->data);
>> len = skb_headlen(skb);
>>
>> +/* The first req should be at least ETH_HLEN size or the packet will be
>> + * dropped by netback.
>> + */
>> +if (unlikely(PAGE_SIZE - offset < ETH_HLEN)) {
>> +nskb = skb_copy(skb, GFP_ATOMIC);
>> +if (!nskb)
>> +goto drop;
>> +dev_kfree_skb_any(skb);
>> +skb = nskb;
>> +page = virt_to_page(skb->data);
>> +offset = offset_in_page(skb->data);
>> +}
>> +
>> spin_lock_irqsave(&queue->tx_lock, flags);
>
> I think you also have to recalculate 'len' in this case too, as
> skb_headlen() will definitely be different for nskb.
>
> In fact, I can't see how this code can work properly without that fix.
Thank you for your feedback David,
in my testing (even when I tried doing skb_copy() for all skbs
unconditionally) skb_headlen(nskb) always equals 'len' so I was under an
impression that both 'skb->len' and 'skb->data_len' remain the same when
we do skb_copy(). However, in case you think there are cases when
headlen changes, I see no problem with re-calculating 'len' as it won't
bring any significant performace penalty compared to the already added
skb_copy().
I'll send 'v2'.
--
Vitaly
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Re: [Xen-devel] [PATCH net-next RESEND] xen-netfront: avoid packet loss when ethernet header crosses page boundary
From: Vitaly Kuznetsov
Date: Fri, 16 Sep 2016 12:59:14 +0200
> @@ -595,6 +596,19 @@ static int xennet_start_xmit(struct sk_buff *skb, struct
> net_device *dev)
> offset = offset_in_page(skb->data);
> len = skb_headlen(skb);
>
> + /* The first req should be at least ETH_HLEN size or the packet will be
> + * dropped by netback.
> + */
> + if (unlikely(PAGE_SIZE - offset < ETH_HLEN)) {
> + nskb = skb_copy(skb, GFP_ATOMIC);
> + if (!nskb)
> + goto drop;
> + dev_kfree_skb_any(skb);
> + skb = nskb;
> + page = virt_to_page(skb->data);
> + offset = offset_in_page(skb->data);
> + }
> +
> spin_lock_irqsave(&queue->tx_lock, flags);
I think you also have to recalculate 'len' in this case too, as
skb_headlen() will definitely be different for nskb.
In fact, I can't see how this code can work properly without that fix.
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