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Date: Fri, 26 Apr 2024 16:30:43 +0200
From: Corinna Vinschen <vinschen@redhat.com>
To: Eric Dumazet <edumazet@google.com>
Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	intel-wired-lan@lists.osuosl.org,
	Nikolay Aleksandrov <razor@blackwall.org>,
	Jason Xing <kerneljasonxing@gmail.com>,
	Paolo Abeni <pabeni@redhat.com>, Jakub Kicinski <kuba@kernel.org>,
	"David S . Miller" <davem@davemloft.net>,
	Tony Nguyen <anthony.l.nguyen@intel.com>,
	Jesse Brandeburg <jesse.brandeburg@intel.com>
Subject: Re: [PATCH net v2] igb: cope with large MAX_SKB_FRAGS
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	netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	intel-wired-lan@lists.osuosl.org,
	Nikolay Aleksandrov <razor@blackwall.org>,
	Jason Xing <kerneljasonxing@gmail.com>,
	Paolo Abeni <pabeni@redhat.com>, Jakub Kicinski <kuba@kernel.org>,
	"David S . Miller" <davem@davemloft.net>,
	Tony Nguyen <anthony.l.nguyen@intel.com>,
	Jesse Brandeburg <jesse.brandeburg@intel.com>
References: <20240423134731.918157-1-vinschen@redhat.com>
 <CANn89iKv1J3AS3rEmEhFq5McHmM+L=32pWg3Wj4_drsdKUx77A@mail.gmail.com>
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Hi Eric,

On Apr 23 16:10, Eric Dumazet wrote:
> On Tue, Apr 23, 2024 at 3:47 PM Corinna Vinschen <vinschen@redhat.com> wrote:
> >
> > From: Paolo Abeni <pabeni@redhat.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@redhat.com>
> > Reported-by: Jirka Hladky <jhladky@redhat.com>
> > Reported-by: Sabrina Dubroca <sd@queasysnail.net>
> > Tested-by: Sabrina Dubroca <sd@queasysnail.net>
> > Tested-by: Corinna Vinschen <vinschen@redhat.com>
> > Signed-off-by: Paolo Abeni <pabeni@redhat.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.

Erm... did you mean between 1500 and 1580 by any chance?  1200 doesn't
really seem to make sense...

I tested this patch now with mtu 1500, 1540 and 1580 successfully.

Either way, I'm just heading into vacation, so I guess I'll pick this up
again when I'm back, unless Paolo takes another look during my absence.


Thanks,
Corinna