netdev - RE: [PATCH net-next] tcp: add tcp_add

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Date:   Fri, 23 Sep 2016 14:43:56 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Eric Dumazet' <eric.dumazet@...il.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
CC:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        "Neal Cardwell" <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: RE: [PATCH net-next] tcp: add tcp_add_backlog()

From: Eric Dumazet
> Sent: 23 September 2016 15:37
> On Fri, 2016-09-23 at 11:09 -0300, Marcelo Ricardo Leitner wrote:
> > On Fri, Sep 23, 2016 at 06:42:51AM -0700, Eric Dumazet wrote:
> > > On Fri, 2016-09-23 at 09:45 -0300, Marcelo Ricardo Leitner wrote:
> > >
> > > > Aye. In that case, what about using tail instead of end?
> > >
> > >
> > > What do you mean exactly ?
> >
> > Something like:
> > -skb->truesize = SKB_TRUESIZE(skb_end_offset(skb));
> > +skb->truesize = SKB_TRUESIZE(skb_tail_offset(skb));
> 
> Certainly not ;)
> 
> This would be lying.
> We really want a precise memory accounting to avoid OOM.
> 
> Some USB drivers use 8KB for their skb->head, you do not want to pretend
> its 66+NET_SKB_PAF=F bytes just because there is no payload in the
> packet.

Last I look some of the USBnet drivers used 64k+ skb (and then
lied about truesize).

That whole usbnet stuff needs a rework for usb3 (xhci) so that once
the rings (etc) are all setup packet transfer looks much more like
a normal ethernet device.
I think it needs to stop trying to receive into skb, instead just
receive into usb sized buffers - and then generate skb that contain
the ethernet frame data.
Not that I'm volunteering :-)

	David