netdev - Re: [Patch net] igb: pass the correct maxlen for eth_get

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Message-ID: <CAHA+R7PtzNd2asXFtiqsWyzCgeE1k_iF84358cvKg6ZCU-yVkQ@mail.gmail.com>
Date:	Wed, 22 Apr 2015 13:33:09 -0700
From:	Cong Wang <cwang@...pensource.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Cong Wang <xiyou.wangcong@...il.com>,
	netdev <netdev@...r.kernel.org>,
	intel-wired-lan@...ts.osuosl.org,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Subject: Re: [Patch net] igb: pass the correct maxlen for eth_get_headlen()

On Wed, Apr 22, 2015 at 1:21 PM, Alexander Duyck
<alexander.duyck@...il.com> wrote:
> On 04/22/2015 01:14 PM, Cong Wang wrote:
>> On Wed, Apr 22, 2015 at 12:43 PM, Alexander Duyck
>> <alexander.duyck@...il.com> 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 <jeffrey.t.kirsher@...el.com>
>>>> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
>>>> ---
>>>>  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.
>

First, make sure you don't miss the TSIP case right above:

The frag starting pointer and its size are advanced by:

skb_frag_size_sub(frag, IGB_TS_HDR_LEN);
...
va += IGB_TS_HDR_LEN;

even though we unlikely pull header longer than
IGB_RX_HDR_LEN - IGB_TS_HDR_LEN either.


Second, the check you mentioned above is:

if ((size <= IGB_RX_HDR_LEN) && !skb_is_nonlinear(skb))

skb is nonlinear _after_ the first igb_add_rx_frag(), a second
igb_add_rx_frag() is possible since igb_is_non_eop() could
return true.
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