netdev - Re: [PATCH net v2] net/mlx5: Correctly set gso

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Message-ID: <d1ab8d54-e524-4b83-bc1d-3f89e47b6237@nvidia.com>
Date: Wed, 16 Jul 2025 10:38:45 +0300
From: Gal Pressman <gal@...dia.com>
To: cpaasch@...nai.com, Saeed Mahameed <saeedm@...dia.com>,
 Tariq Toukan <tariqt@...dia.com>, Mark Bloch <mbloch@...dia.com>,
 Leon Romanovsky <leon@...nel.org>, Andrew Lunn <andrew+netdev@...n.ch>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, linux-rdma@...r.kernel.org
Subject: Re: [PATCH net v2] net/mlx5: Correctly set gso_size when LRO is used

On 15/07/2025 23:20, Christoph Paasch via B4 Relay wrote:
> From: Christoph Paasch <cpaasch@...nai.com>
> 
> gso_size is expected by the networking stack to be the size of the
> payload (thus, not including ethernet/IP/TCP-headers). However, cqe_bcnt
> is the full sized frame (including the headers). Dividing cqe_bcnt by
> lro_num_seg will then give incorrect results.
> 
> For example, running a bpftrace higher up in the TCP-stack
> (tcp_event_data_recv), we commonly have gso_size set to 1450 or 1451 even
> though in reality the payload was only 1448 bytes.
> 
> This can have unintended consequences:
> - In tcp_measure_rcv_mss() len will be for example 1450, but. rcv_mss
> will be 1448 (because tp->advmss is 1448). Thus, we will always
> recompute scaling_ratio each time an LRO-packet is received.
> - In tcp_gro_receive(), it will interfere with the decision whether or
> not to flush and thus potentially result in less gro'ed packets.
> 
> So, we need to discount the protocol headers from cqe_bcnt so we can
> actually divide the payload by lro_num_seg to get the real gso_size.
> 
> v2:
>  - Use "(unsigned char *)tcp + tcp->doff * 4 - skb->data)" to compute header-len
>    (Tariq Toukan <tariqt@...dia.com>)
>  - Improve commit-message (Gal Pressman <gal@...dia.com>)

This usually comes after the --- so it's not part of the commit message.

> 
> Fixes: e586b3b0baee ("net/mlx5: Ethernet Datapath files")
> Signed-off-by: Christoph Paasch <cpaasch@...nai.com>

Reviewed-by: Gal Pressman <gal@...dia.com>