Subject: Re: [PATCH net] igb: cope with large MAX_SKB_FRAGS.

From: Maciej Fijalkowski <[email-masked]>
Date: Wed, 7 Aug 2024 18:09:20 +0200

On Tue, Aug 06, 2024 at 03:15:31PM -0700, Tony Nguyen wrote:
> From: Paolo Abeni <[email-masked]>
> 
> 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.

any splat?

> 
> 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_FRAGS")
> Reported-by: Jan Tluka <[email-masked]>
> Reported-by: Jirka Hladky <[email-masked]>
> Reported-by: Sabrina Dubroca <[email-masked]>

Where was this reported?

> Tested-by: Sabrina Dubroca <[email-masked]>
> Tested-by: Corinna Vinschen <[email-masked]>
> Signed-off-by: Paolo Abeni <[email-masked]>
> Reviewed-by: Eric Dumazet <[email-masked]>
> Tested-by: Pucha Himasekhar Reddy <[email-masked]> (A Contingent worker at Intel)
> Signed-off-by: Tony Nguyen <[email-masked]>
> ---
> iwl-net: https://lore.kernel.org/intel-wired-lan/[email-masked]/
> 
>  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 11be39f435f3..232d6cb836a9 100644
> --- a/drivers/net/ethernet/intel/igb/igb_main.c
> +++ b/drivers/net/ethernet/intel/igb/igb_main.c
> @@ -4808,6 +4808,7 @@ 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) ||

We should address IGB_2K_TOO_SMALL_WITH_PADDING for this case. I'll think
about it tomorrow.

>  	    rd32(E1000_RCTL) & E1000_RCTL_SBP)
>  		set_ring_uses_large_buffer(rx_ring);
>  #endif
> -- 
> 2.42.0
> 
>