RE: [PATCH net-next] udp: allow forwarding of plain (non-fraglisted) UDP GRO packets

[Alexander Lobakin <alobakin@pm.me>]

From: "'Alexander Lobakin'" <alobakin@pm.me>

From: Dongseok Yi" <dseok.yi@samsung.com>
Date: Wed, 13 Jan 2021 12:59:45 +0900

> On 2021-01-13 12:10, Alexander Duyck wrote:
>> On 1/12/21 1:16 PM, Alexander Lobakin wrote:
>>> Commit 9fd1ff5d2ac7 ("udp: Support UDP fraglist GRO/GSO.") actually
>>> not only added a support for fraglisted UDP GRO, but also tweaked
>>> some logics the way that non-fraglisted UDP GRO started to work for
>>> forwarding too.
>>> Tests showed that currently forwarding and NATing of plain UDP GRO
>>> packets are performed fully correctly, regardless if the target
>>> netdevice has a support for hardware/driver GSO UDP L4 or not.
>>> Add the last element and allow to form plain UDP GRO packets if
>>> there is no socket -> we are on forwarding path.
>>>
>
> Your patch is very similar with the RFC what I submitted but has
> different approach. My concern was NAT forwarding.
> https://lore.kernel.org/patchwork/patch/1362257/

Not really. You tried to forbid forwarding of fraglisted UDP GRO
packets, I allow forwarding of plain UDP GRO packets.
With my patch on, you can switch between plain and fraglisted UDP GRO
with the command:

ethtool -K eth0 rx-gro-list on/off

> Nevertheless, I agreed with your idea that allow fraglisted UDP GRO
> if there is socket.

Recheck my change. There's ||, not &&.

As I said in the commit message, forwarding and NATing of plain
UDP GRO packets are performed correctly, both with and without
driver-side support, so it's safe to enable.

Also, as I said in reply to your RFC, NATing of UDP GRO fraglists
is performed correctly if driver is aware of it and advertises a
support for fraglist GSO.
If not, then there may be problems you described. But the idea to
forbid forwarding and NATing of any UDP GRO skbs is an absolute
overkill.

>>> Plain UDP GRO forwarding even shows better performance than fraglisted
>>> UDP GRO in some cases due to not wasting one skbuff_head per every
>>> segment.
>>>
>>> Signed-off-by: Alexander Lobakin <alobakin@pm.me>
>>> ---
>>>   net/ipv4/udp_offload.c | 5 +++--
>>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
>>> index ff39e94781bf..9d71df3d52ce 100644
>>> --- a/net/ipv4/udp_offload.c
>>> +++ b/net/ipv4/udp_offload.c
>>> @@ -460,12 +460,13 @@ struct sk_buff *udp_gro_receive(struct list_head *head, struct sk_buff *skb,
>>>   	if (skb->dev->features & NETIF_F_GRO_FRAGLIST)
>>>   		NAPI_GRO_CB(skb)->is_flist = sk ? !udp_sk(sk)->gro_enabled: 1;
>
> is_flist can be true even if !sk.
>
>>>
>>> -	if ((sk && udp_sk(sk)->gro_enabled) || NAPI_GRO_CB(skb)->is_flist) {
>>> +	if (!sk || (sk && udp_sk(sk)->gro_enabled) ||
>
> Actually sk would be NULL by udp_encap_needed_key in udp4_gro_receive
> or udp6_gro_receive.
>
>>> +	    NAPI_GRO_CB(skb)->is_flist) {
>>>   		pp = call_gro_receive(udp_gro_receive_segment, head, skb);
>
> udp_gro_receive_segment will check is_flist first and try to do
> fraglisted UDP GRO. Can you check what I'm missing?

I think you miss that is_flist is set to true *only* if the receiving
netdevice has NETIF_F_GRO_FRAGLIST feature on.
If it's not, stack will form a non-fraglisted UDP GRO skb. That was
tested multiple times.

>>>   		return pp;
>>>   	}
>>>
>>
>> The second check for sk in "(sk && udp_sk(sk)->gro_enabled)" is
>> redundant and can be dropped. You already verified it is present when
>> you checked for !sk before the logical OR.

Alex are right, I'll simplify the condition in v2. Thanks!

> Sorry, Alexander Duyck. I believe Alexander Lobakin will answer this.
>
>>> -	if (!sk || NAPI_GRO_CB(skb)->encap_mark ||
>>> +	if (NAPI_GRO_CB(skb)->encap_mark ||
>>>   	    (skb->ip_summed != CHECKSUM_PARTIAL &&
>>>   	     NAPI_GRO_CB(skb)->csum_cnt == 0 &&
>>>   	     !NAPI_GRO_CB(skb)->csum_valid) ||
>>>

Al