netdev - Re: [PATCH v3] net: af_packet: don't call tpacket_destruct

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Date:	Mon, 27 Sep 2010 07:40:01 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	jarkao2@...il.com, xiaosuo@...il.com, socketcan@...tkopp.net,
	mst@...hat.com, netdev@...r.kernel.org
Subject: Re: [PATCH v3] net: af_packet: don't call tpacket_destruct_skb()
 until the skb is sent out

Le dimanche 26 septembre 2010 à 18:25 -0700, David Miller a écrit :
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Fri, 24 Sep 2010 09:01:00 +0200
> 
> > af_packet (tx with mmap) is broken. I wonder who really uses it ?
> 
> I suspect now that af_packet supports VNET headers on transmit,
> there are some things using this tx+mmap thing for sure.
> 
> > To properly cope with paged skbs, it should not try to fit several
> > packets per page.
> > 
> > The mmap api should change so that one mmaped page belongs to at most
> > one skb, or else we need invasive changes in net/core
> > 
> > This probably makes this stuff less interesting, unless the need is to
> > send big packets. In this case, why splice was not used instead of
> > custom mmap ?
> 
> I don't really see what the big issue is.
> 
> When the data destructor runs it means that packet's part of the pages
> are available for reuse for the tx mmap client.  And if I read it
> correctly, that's exactly what tpacket_destruct_skb() is in fact doing.
> 
> There seems to be no conflict with that rule and reusing a page for
> multiple packets.

I was wondering if somewhere we transfert a frag from one skb1 to
another skb2, and eventually free skb1.

I just check tcp collapse and found it was not coping with frags, yet.



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