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

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Date:	Fri, 24 Sep 2010 09:01:00 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jarek Poplawski <jarkao2@...il.com>
Cc:	Changli Gao <xiaosuo@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Oliver Hartkopp <socketcan@...tkopp.net>,
	"Michael S. Tsirkin" <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 vendredi 24 septembre 2010 à 06:36 +0000, Jarek Poplawski a écrit :
> On 2010-09-23 12:15, Changli Gao wrote:
> > Since skb->destructor() is used to account socket memory, and maybe called
> > before the skb is sent out, a corrupt skb maybe sent out finally.
> > 
> > A new destructor is added into structure skb_shared_info(), and it won't
> > be called until the last reference to the data of an skb is put. af_packet
> > uses this destructor instead.
> 
> IMHO, we shouldn't allow for fixing the bad design of one protocol at
> the expense of others by adding more and more conditionals. The proper
> way of handling paged skbs (splice compatible) exists. And the current
> patch doesn't even fix the problem completely against things like
> pskb_expand_head or pskb_copy.
> 
> af_packet could check some flag which guarantees the queued dev can do
> skb_orphan after the real xmit and copy buffers otherwise.

Agreed.

af_packet (tx with mmap) is broken. I wonder who really uses it ?

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 ?


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