linux-kernel - Re: [PATCH net v2 1/3] net: gro: add {inner_}network_offset to napi_gro

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Date: Tue, 23 Apr 2024 11:27:22 +0200
From: Richard Gobert <richardbgobert@...il.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, davem@...emloft.net,
 edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, dsahern@...nel.org,
 alexander.duyck@...il.com, aleksander.lobakin@...el.com,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v2 1/3] net: gro: add {inner_}network_offset to
 napi_gro_cb

Willem de Bruijn wrote:
> Richard Gobert wrote:
>> Willem de Bruijn wrote:
>>> Richard Gobert wrote:
>>>> This patch adds network_offset and inner_network_offset to napi_gro_cb, and
>>>> makes sure both are set correctly. In the common path there's only one
>>>> write (skb_gro_reset_offset, which replaces skb_set_network_header).
>>>>
>>>> Signed-off-by: Richard Gobert <richardbgobert@...il.com>
>>>> ---
>>>>  drivers/net/geneve.c           |  1 +
>>>>  drivers/net/vxlan/vxlan_core.c |  1 +
>>>>  include/net/gro.h              | 18 ++++++++++++++++--
>>>>  net/8021q/vlan_core.c          |  2 ++
>>>>  net/core/gro.c                 |  1 +
>>>>  net/ethernet/eth.c             |  1 +
>>>>  net/ipv4/af_inet.c             |  5 +----
>>>>  net/ipv4/gre_offload.c         |  1 +
>>>>  net/ipv6/ip6_offload.c         |  8 ++++----
>>>>  9 files changed, 28 insertions(+), 10 deletions(-)
>>>>
>>>
>>>> +static inline int skb_gro_network_offset(const struct sk_buff *skb)
>>>> +{
>>>> +	return NAPI_GRO_CB(skb)->network_offsets[NAPI_GRO_CB(skb)->encap_mark];
>>>> +}
>>>> +
>>>
>>>
>>>> @@ -236,8 +236,6 @@ INDIRECT_CALLABLE_SCOPE struct sk_buff *ipv6_gro_receive(struct list_head *head,
>>>>  	if (unlikely(!iph))
>>>>  		goto out;
>>>>  
>>>> -	skb_set_network_header(skb, off);
>>>> -
>>>
>>> Especially for net, this is still a large patch.
>>>
>>> Can we avoid touching all those tunnel callbacks and just set the
>>> offsets in inet_gro_receive and ipv6_gro_receive themselves, just
>>> as skb_set_network_header now:
>>>
>>> @@ -236,7 +236,7 @@ INDIRECT_CALLABLE_SCOPE struct sk_buff *ipv6_gro_receive(struct list_head *head,
>>>         if (unlikely(!iph))
>>>                 goto out;
>>>  
>>> -       skb_set_network_header(skb, off);
>>> +       NAPI_GRO_CB(skb)->network_offsets[NAPI_GRO_CB(skb)->encap_mark] = off;
>>>
>>
>> Thanks for the reply!
>>
>> Setting network_offset on dev_gro_receive and inner_network_offset only
>> in the tunnel callbacks is the best option IMO. I agree that
>> we want a small patch to net that solves the problem, although I 
>> think always using ->encap_mark in the common path is not ideal. 
>>
>> We can avoid changing all the tunnel callbacks by always setting
>> inner_network_offset in {ipv6,inet}_gro_receive and initialize
>> network_offset to 0 in dev_gro_receive. It will result in a small
>> change, without using ->encap_mark.
>>
>> What are your thoughts?
> 
> That works. It's a bit ugly that inner_network_offset will always be
> set, even if a packet only traverses inet_gro_receive once. What is
> your concern with testing encap_mark?
> 
> How do you want to detect in udp[46]_lib_lookup_skb which of the two
> offsets to use? That would still be encap_mark based?
> 

I'd like to minimize any potential overhead, even a small one, and this way
we do not need to access encap_mark at all in the common path.

NAPI_GRO_CB(skb)->network_offsets[NAPI_GRO_CB(skb)->encap_mark] = off;

compiles to:

	movzx   eax, byte ptr [rbx+46h]
	shr     al, 1
	and     eax, 1
	mov     [rbx+rax*2+4Ch], r14w

while

NAPI_GRO_CB(skb)->inner_network_offset = off;

compiles to:

	mov     [rbx+4Eh], r14w

I do plan to add a patch to net-next after this to remove the access
entirely from inet gro callbacks, in the meantime, it looks to me like a
reasonable patch and small enough to not raise concerns.

For udp_lib_lookup I don't see a way around it so yes, it would still be
dependent on encap_mark. Since this runs in the complete phase it's less
concerning.

Let me know that you're ok with that and I'll post a v3.