linux-kernel - Re: [PATCH net-next v4 0/8] bpf: cpumap: enable GRO for XDP

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Message-ID: <1dd14ece-578b-4fe6-8ef1-557b0f5d3144@intel.com>
Date: Wed, 12 Feb 2025 16:55:52 +0100
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Jakub Kicinski <kuba@...nel.org>
CC: Eric Dumazet <edumazet@...gle.com>, Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>, Paolo Abeni <pabeni@...hat.com>,
	Lorenzo Bianconi <lorenzo@...nel.org>, Daniel Xu <dxu@...uu.xyz>, "Alexei
 Starovoitov" <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
	"Andrii Nakryiko" <andrii@...nel.org>, John Fastabend
	<john.fastabend@...il.com>, Toke Høiland-Jørgensen
	<toke@...nel.org>, "Jesper Dangaard Brouer" <hawk@...nel.org>, Martin KaFai
 Lau <martin.lau@...ux.dev>, <netdev@...r.kernel.org>, <bpf@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next v4 0/8] bpf: cpumap: enable GRO for XDP_PASS
 frames

From: Alexander Lobakin <aleksander.lobakin@...el.com>
Date: Wed, 12 Feb 2025 15:22:55 +0100

> From: Jakub Kicinski <kuba@...nel.org>
> Date: Mon, 10 Feb 2025 16:35:29 -0800
> 
>> On Mon, 10 Feb 2025 16:31:52 +0100 Alexander Lobakin wrote:
>>> This was rejected by Kuba in v2.
>>> He didn't like to have napi_id two times within napi_struct (one inside
>>> gro_node, one outside).
>>
>> Do you mean:
>>
>>   the napi_id in gro sticks out..
>>
>> https://lore.kernel.org/netdev/20250113130104.5c2c02e0@kernel.org/ ?
>>
>> That's more of a nudge to try harder than a "no". We explored 
>> the alternatives, there's no perfect way to layer this. I think 
>> Eric's suggestion is probably as clean as we can get.
> 
> You mean to cache napi_id in gro_node?
> 
> Then we get +8 bytes to sizeof(napi_struct) for little reason... Dunno,
> if you really prefer, I can do it that way.

Alternative to avoid +8 bytes:

struct napi_struct {
	...

	union {
		struct gro_node	gro;
		struct {
			u8 pad[offsetof(struct gro_node, napi_id)];
			u32 napi_id;
		};
	};

This is effectively the same what struct_group() does, just more ugly.
But allows to declare gro_node separately.

> 
> OTOH you gave Acked-by on struct_group(), then Eric appeared and you
> took your ack back :D
Thanks,
Olek