Subject: Re: [Patch net] igb: pass the correct maxlen for eth_get_headlen()

From: Alexander Duyck <[email-masked]>
Date: Wed, 22 Apr 2015 13:21:46 -0700

On 04/22/2015 01:14 PM, Cong Wang wrote:
> On Wed, Apr 22, 2015 at 12:43 PM, Alexander Duyck
> <[email-masked]> wrote:
>> On 04/22/2015 10:45 AM, Cong Wang wrote:
>>> The second parameter of eth_get_headlen() is the length of
>>> the frame buffer, not the header length of skb.
>>>
>>> Cc: Jeff Kirsher <[email-masked]>
>>> Signed-off-by: Cong Wang <[email-masked]>
>>> ---
>>>  drivers/net/ethernet/intel/igb/igb_main.c | 4 +++-
>>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
>>> index a0a9b1f..7b3a370 100644
>>> --- a/drivers/net/ethernet/intel/igb/igb_main.c
>>> +++ b/drivers/net/ethernet/intel/igb/igb_main.c
>>> @@ -6852,7 +6852,9 @@ static void igb_pull_tail(struct igb_ring *rx_ring,
>>>       /* we need the header to contain the greater of either ETH_HLEN or
>>>        * 60 bytes if the skb->len is less than 60 for skb_pad.
>>>        */
>>> -     pull_len = eth_get_headlen(va, IGB_RX_HDR_LEN);
>>> +     pull_len = eth_get_headlen(va, skb_frag_size(frag));
>>> +     if (unlikely(pull_len > IGB_RX_HDR_LEN))
>>> +             pull_len = IGB_RX_HDR_LEN;
>>>
>>>       /* align pull length to size of long to optimize memcpy performance */
>>>       skb_copy_to_linear_data(skb, va, ALIGN(pull_len, sizeof(long)));
>> You have this part right.  The length represents the maximum length we
>> are willing to traverse in the buffer.  So if we 100% want to get the
>> entire header regardless of what we can copy into then we could follow
>> your approach.  However, since the allocated space in the skb is only
>> IGB_RX_HDR_LEN we only really want to traverse up to that length.  Then
>> that is all we copy out of the header.
> But the frag size could be smaller than IGB_RX_HDR_LEN which is
> the case this patch tries to fix.

No it can't.  We only add frags if the first frag is larger than
IGB_RX_HDR_LEN.  Go look at the code where this driver calls
add_rx_frag.  You should find there is copybreak code in there that
kicks in for anything less than or equal to IGB_RX_HDR_LEN in size on
the first frag.  It is there to allow us to avoid having to perform an
atomic inc/dec on the page.

>> As a result we don't need the extra code for putting the upper limit on
>> pull_len since that is factored in by passing IGB_RX_HDR_LEN as the
>> maximum traversal length.
> That is why I put unlikely() there, even with tunnel headers we unlikely
> pull header longer than IGB_RX_HDR_LEN, the code is just for
> completeness.

The code is fine as is.  I suggest you go look at the code where frags
are attached to the sk_buff.

- Alex