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

From: Maciej Fijalkowski <[email-masked]>
Date: Thu, 8 Aug 2024 20:20:23 +0200

On Wed, Aug 07, 2024 at 06:09:20PM +0200, Maciej Fijalkowski wrote:
> 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.

Actually from what I currently understand IGB_2K_TOO_SMALL_WITH_PADDING
will give us 'true' for case you are addressing so we could reuse it here?

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