linux-kernel - Re: [PATCH net v2] igb: cope with large MAX_SKB

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Date: Tue, 23 Apr 2024 16:10:05 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Corinna Vinschen <vinschen@...hat.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	intel-wired-lan@...ts.osuosl.org, Nikolay Aleksandrov <razor@...ckwall.org>, 
	Jason Xing <kerneljasonxing@...il.com>, Paolo Abeni <pabeni@...hat.com>, 
	Jakub Kicinski <kuba@...nel.org>, "David S . Miller" <davem@...emloft.net>, 
	Tony Nguyen <anthony.l.nguyen@...el.com>, Jesse Brandeburg <jesse.brandeburg@...el.com>
Subject: Re: [PATCH net v2] igb: cope with large MAX_SKB_FRAGS

On Tue, Apr 23, 2024 at 3:47 PM Corinna Vinschen <vinschen@...hat.com> wrote:
>
> From: Paolo Abeni <pabeni@...hat.com>
>
> Sabrina reports that the igb driver does not cope well with large
> MAX_SKB_FRAG values: setting MAX_SKB_FRAG to 45 causes payload
> corruption on TX.
>
> An easy reproducer is to run ssh to connect to the machine.  With
> MAX_SKB_FRAGS=17 it works, with MAX_SKB_FRAGS=45 it fails.
>
> The root cause of the issue is that the driver does not take into
> account properly the (possibly large) shared info size when selecting
> the ring layout, and will try to fit two packets inside the same 4K
> page even when the 1st fraglist will trump over the 2nd head.
>
> Address the issue forcing the driver to fit a single packet per page,
> leaving there enough room to store the (currently) largest possible
> skb_shared_info.
>
> Fixes: 3948b05950fd ("net: introduce a config option to tweak MAX_SKB_FRAG")
> Reported-by: Jan Tluka <jtluka@...hat.com>
> Reported-by: Jirka Hladky <jhladky@...hat.com>
> Reported-by: Sabrina Dubroca <sd@...asysnail.net>
> Tested-by: Sabrina Dubroca <sd@...asysnail.net>
> Tested-by: Corinna Vinschen <vinschen@...hat.com>
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> ---
> v2: fix subject, add a simple reproducer
>
>  drivers/net/ethernet/intel/igb/igb_main.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
> index a3f100769e39..22fb2c322bca 100644
> --- a/drivers/net/ethernet/intel/igb/igb_main.c
> +++ b/drivers/net/ethernet/intel/igb/igb_main.c
> @@ -4833,6 +4833,7 @@ static void igb_set_rx_buffer_len(struct igb_adapter *adapter,
>
>  #if (PAGE_SIZE < 8192)
>         if (adapter->max_frame_size > IGB_MAX_FRAME_BUILD_SKB ||
> +           SKB_HEAD_ALIGN(adapter->max_frame_size) > (PAGE_SIZE / 2) ||

I wonder if adding IGB_SKB_PAD would be needed ?

adapter->max_frame_size does not seem to include it.

I would try using all mtus between 1200 and 1280 to make sure this works.

Thanks !

>             rd32(E1000_RCTL) & E1000_RCTL_SBP)
>                 set_ring_uses_large_buffer(rx_ring);
>  #endif
> --
> 2.44.0
>