linux-kernel - Re: [PATCH net-next v3 6/9] xen-netback: Handle guests with too many frags

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Message-ID: <52CD5CF8.4000004@citrix.com>
Date:	Wed, 8 Jan 2014 14:13:12 +0000
From:	Zoltan Kiss <zoltan.kiss@...rix.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	<ian.campbell@...rix.com>, <wei.liu2@...rix.com>,
	<xen-devel@...ts.xenproject.org>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <jonathan.davies@...rix.com>
Subject: Re: [PATCH net-next v3 6/9] xen-netback: Handle guests with too many
 frags

On 08/01/14 13:54, Eric Dumazet wrote:
> On Wed, 2014-01-08 at 13:49 +0000, Zoltan Kiss wrote:
>> On 08/01/14 02:12, Eric Dumazet wrote:
>>> On Wed, 2014-01-08 at 00:10 +0000, Zoltan Kiss wrote:
>>>
>>>>
>>>> +		if (skb_shinfo(skb)->frag_list) {
>>>> +			nskb = skb_shinfo(skb)->frag_list;
>>>> +			xenvif_fill_frags(vif, nskb, INVALID_PENDING_IDX);
>>>> +			skb->len += nskb->len;
>>>> +			skb->data_len += nskb->len;
>>>> +			skb->truesize += nskb->truesize;
>>>> +			skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
>>>> +			skb_shinfo(nskb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
>>>> +			vif->tx_zerocopy_sent += 2;
>>>> +			nskb = skb;
>>>> +
>>>> +			skb = skb_copy_expand(skb,
>>>> +					0,
>>>> +					0,
>>>> +					GFP_ATOMIC | __GFP_NOWARN);
>>>
>>> skb can be NULL here
>>
>> Thanks, fixed that.
>
> BTW, I am not sure why you copy the skb.
>
> Is it to get rid of frag_list, and why ?

Yes, it is to get rid of the frag_list, just to be on the safe side. I'm 
not sure if it is normal to send a big skb with MAX_SKB_FRAGS frags plus 
an empty skb on the frag_list with one frag, so I just consolidate them 
here. This scenario shouldn't happen very often anyway, even guests 
which can send more than MAX_SKB_FRAGS slots tends to do it rarely.

Zoli

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