netdev - Re: [PATCH v2 net-next 2/3] net/mlx4: MLX4_TX_BOUNCE_BUFFER_SIZE depends on MAX_SKB

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Date:   Wed, 7 Dec 2022 20:53:51 +0200
From:   Tariq Toukan <ttoukan.linux@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     Tariq Toukan <tariqt@...dia.com>,
        Leon Romanovsky <leonro@...dia.com>, netdev@...r.kernel.org,
        eric.dumazet@...il.com, Wei Wang <weiwan@...gle.com>
Subject: Re: [PATCH v2 net-next 2/3] net/mlx4: MLX4_TX_BOUNCE_BUFFER_SIZE
 depends on MAX_SKB_FRAGS



On 12/7/2022 4:12 PM, Eric Dumazet wrote:
> Google production kernel has increased MAX_SKB_FRAGS to 45
> for BIG-TCP rollout.
> 
> Unfortunately mlx4 TX bounce buffer is not big enough whenever
> an skb has up to 45 page fragments.
> 
> This can happen often with TCP TX zero copy, as one frag usually
> holds 4096 bytes of payload (order-0 page).
> 
> Tested:
>   Kernel built with MAX_SKB_FRAGS=45
>   ip link set dev eth0 gso_max_size 185000
>   netperf -t TCP_SENDFILE
> 
> I made sure that "ethtool -G eth0 tx 64" was properly working,
> ring->full_size being set to 15.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: Wei Wang <weiwan@...gle.com>
> Cc: Tariq Toukan <tariqt@...dia.com>

Reviewed-by: Tariq Toukan <tariqt@...dia.com>