netdev - Re: BUG:af_packet fails to TX TSO frames

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Message-ID: <CAF=yD-+gc5oQ=XT0+yw4Wt_h3rObETti3FvXm+V-nu=87zaMyA@mail.gmail.com>
Date:   Wed, 11 Oct 2017 17:05:19 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Anton Ivanov <anton.ivanov@...-begemot.co.uk>
Cc:     Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>
Subject: Re: BUG:af_packet fails to TX TSO frames

On Wed, Oct 11, 2017 at 3:39 PM, Anton Ivanov
<anton.ivanov@...-begemot.co.uk> wrote:
> On 11/10/17 19:57, Willem de Bruijn wrote:
>> On Wed, Oct 11, 2017 at 2:39 PM, Anton Ivanov
>> <anton.ivanov@...-begemot.co.uk> wrote:
>>> The check as now insists that the actual driver supports GSO_ROBUST, because
>>> we have marked the skb dodgy.
>>>
>>> The specific bit which does this check is in net_gso_ok()
>>>
>>> Now, lets's see how many Ethernet drivers set GSO_ROBUST.
>>>
>>> find drivers/net/ethernet -type f -name "*.[c,h]" -exec grep -H GSO_ROBUST
>>> {} \;
>>>
>>> That returns nothing in 4.x
>>>
>>> IMHO - af_packet allocates the skb, does all checks (and extra may be added)
>>> on the gso, why is this set dodgy in the first place?
>> It is set when the header has to be validated.
>>
>> The segmentation logic will validate and fixup gso_segs. See for
>> instance tcp_gso_segment:
>>
>>         if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) {
>>                 /* Packet is from an untrusted source, reset gso_segs. */
>>
>>                 skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len, mss);
>>
>>                 segs = NULL;
>>                 goto out;
>>         }
>>
>> If the device would have the robust bit set and otherwise supports the
>> required features, fix up gso_segs and pass the large packet to the
>> device.
>>
>> Else it continues to the software gso path.
>>
>> Large packets generated with psock_txring_vnet.c pass this test. I
>
> That test is indeed a different path

The test can be run both with and without ring:

  psock_txring_vnet -l 8000 -s $src_ip -d $dst_ip -v
  psock_txring_vnet -l 8000 -s $src_ip -d $dst_ip -v -N

both with and without qdisc bypass ('-q').

>  - this goes via the tpacket_snd
> which allocs via sock_alloc_send_skb. That results in a non-fragged skb
> as it calls pskb after that with data_len = 0 asking for a contiguous one.

but attached the ring slot as fragments in tpacket_fill_skb.

> My stuff is using sendmmsg which ends up via packet_snd which allocs
> via  sock_alloc_send_pskb which is invoked in a way which always creates
> 2 segments - one for the linear section and one for the rest (and more
> if needed). It is faster than tpacket by the way (several times).
>
> As a comparison tap and other virtual drivers use sock_alloc_send_pskb
> with non-zero data length which results in multiple frags. The code in
> packet_snd is in fact identical with tap (+/- some cosmetic differences).
>
> That is the difference between the tests and that is why your test works
> and mine fails.

All the above test cases work for me, including those that build skbs
with fragments. Could you try those.