netdev - Re: [PATCH net-next 1/3] gro: Pull headers into skb head for 1st skb in gro list

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Message-ID: <CALx6S37dwbV=ztLpS6Z+9upXvCkm0Mk3U4HUbsgmxEWOi3dCKw@mail.gmail.com>
Date:	Fri, 3 Jul 2015 11:36:21 -0700
From:	Tom Herbert <tom@...bertland.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 1/3] gro: Pull headers into skb head for 1st skb
 in gro list

On Thu, Jul 2, 2015 at 9:50 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2015-07-02 at 14:04 -0700, Tom Herbert wrote:
>> When setting up the first skb in a gro list we ensure that all the
>> headers up to skb_gro_offset have been pulled into head. In subsequent
>> uses of this skb (e.g. determining same_flow) it is assumed that the
>> headers can be accessed in the skb head.
>>
>> Signed-off-by: Tom Herbert <tom@...bertland.com>
>> ---
>>  net/core/dev.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index 6778a99..05e0e37 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -4228,6 +4228,10 @@ static enum gro_result dev_gro_receive(struct napi_struct *napi, struct sk_buff
>>       } else {
>>               napi->gro_count++;
>>       }
>> +
>> +     /* Ensure all headers are pulled into head for 1st skb */
>> +     skb_gro_header_slow(skb, skb_gro_offset(skb), 0);
>> +
>>       NAPI_GRO_CB(skb)->count = 1;
>>       NAPI_GRO_CB(skb)->age = jiffies;
>>       NAPI_GRO_CB(skb)->last = skb;
>
> I do not see how we can reach this point without having headers already
> pulled.
>
> For example, tcp_gro_receive() has :
>
>         off = skb_gro_offset(skb);
>         hlen = off + sizeof(*th);
>         th = skb_gro_header_fast(skb, off);
>         if (skb_gro_header_hard(skb, hlen)) {
>                 th = skb_gro_header_slow(skb, hlen, off);
>                 if (unlikely(!th))
>                         goto out;
>         }
>
> So by definition, skb is put in gro_list only if it was fully validated as a GRO
> candidate.
>
That code will access the data from fragments if possible, the
skb_gro_header_slow is what pulls the data into head. In
tcp_gro_receive we do tcp_hdr(p) on the 1st skbuf in a gro list so it
is assumed there that the headers have been pulled into head.

> Given the complexity of your 2nd patch, I am stopping the review right
> now.
>
>
>
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