netdev - Re: [PATCH net] igb: cope with large MAX_SKB

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Message-ID: <ZrYIlDqUc9obiyt9@calimero.vinschen.de>
Date: Fri, 9 Aug 2024 14:16:20 +0200
From: Corinna Vinschen <vinschen@...hat.com>
To: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc: Tony Nguyen <anthony.l.nguyen@...el.com>, davem@...emloft.net,
	kuba@...nel.org, pabeni@...hat.com, edumazet@...gle.com,
	netdev@...r.kernel.org, Jan Tluka <jtluka@...hat.com>,
	Jirka Hladky <jhladky@...hat.com>,
	Sabrina Dubroca <sd@...asysnail.net>,
	Pucha Himasekhar Reddy <himasekharx.reddy.pucha@...el.com>
Subject: Re: [PATCH net] igb: cope with large MAX_SKB_FRAGS.

On Aug  8 20:20, Maciej Fijalkowski wrote:
> 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:
> > > 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?

Well, IGB_2K_TOO_SMALL_WITH_PADDING is a constant expression evaluated
at build time.  The SKB_HEAD_ALIGN expression is dynamically computed,
basically depending on the MTU.

IGB_2K_TOO_SMALL_WITH_PADDING switches from false to true with
MAX_SKB_FRAGS set to 22 or more.

So with MAX_SKB_FRAGS set to 17, IGB_2K_TOO_SMALL_WITH_PADDING is false,
while the SKB_HEAD_ALIGN expression is true for MTUs >= 1703.

With MAX_SKB_FRAGS set to 45, IGB_2K_TOO_SMALL_WITH_PADDING is true,
the SKB_HEAD_ALIGN expression is true for MTUs >= 1255.

Given that, IGB_2K_TOO_SMALL_WITH_PADDING might be the more careful
check.  Do you want me to send a v2?


Thanks,
Corinna