netdev - Re: skb_try_coalesce bug?

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Message-ID: <5357FE65.1010400@ericsson.com>
Date:	Wed, 23 Apr 2014 13:54:45 -0400
From:	Jon Maloy <jon.maloy@...csson.com>
To:	David Miller <davem@...emloft.net>
CC:	<eric.dumazet@...il.com>, <erik.hugne@...csson.com>,
	<netdev@...r.kernel.org>
Subject: Re: skb_try_coalesce bug?

On 04/23/2014 01:33 PM, David Miller wrote:
> 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.
> 

Thank you for the clarification. We'll have to fix this. I am just puzzled that
our defragmentation algorithm has worked as well as it has until now.

///jon
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