netdev - Re: skb_try_coalesce bug?

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Message-Id: <20140423.133349.1007553448019102626.davem@davemloft.net>
Date:	Wed, 23 Apr 2014 13:33:49 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	jon.maloy@...csson.com
Cc:	eric.dumazet@...il.com, erik.hugne@...csson.com,
	netdev@...r.kernel.org
Subject: Re: skb_try_coalesce bug?

From: Jon Maloy <jon.maloy@...csson.com>
Date: Wed, 23 Apr 2014 12:56:20 -0400

> On 04/22/2014 05:37 PM, Eric Dumazet wrote:
>> On Tue, 2014-04-22 at 14:29 -0700, Eric Dumazet wrote:
>> 
>>> Then if you do that, you also need to change head->data_len !
>> 
>> Untested patch would be :
>> 
>> diff --git a/net/tipc/link.c b/net/tipc/link.c
>> index c5190ab75290..85077dd7c63e 100644
>> --- a/net/tipc/link.c
>> +++ b/net/tipc/link.c
>> @@ -2349,6 +2349,7 @@ int tipc_link_frag_rcv(struct sk_buff **head, struct sk_buff **tail,
>>  			(*tail)->next = frag;
>>  		*tail = frag;
>>  		(*head)->truesize += frag->truesize;
>> +		(*head)->data_len += frag->len;
> 
> Just to confirm, does this mean that head's own (linear) data is not
> included in data_len? 

For a given SKB, skb->len is the entire length of the packet, fragments and
all.

skb->data_len counts the sum of all of the page and SKB based fragments, ie.
all bytes which are not in the top-level SKBs linear area.

So the linear length is always "skb->len - skb->data_len", and this is exactly
what skb_headlen() does.
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