netdev - Re: [PATCH RFC 4/5] tun: vringfd xmit support.

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Message-Id: <200804071724.51781.rusty@rustcorp.com.au>
Date:	Mon, 7 Apr 2008 17:24:51 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, maxk@...lcomm.com
Subject: Re: [PATCH RFC 4/5] tun: vringfd xmit support.

On Monday 07 April 2008 15:13:44 Herbert Xu wrote:
> Rusty Russell <rusty@...tcorp.com.au> wrote:
> > +/* We are done with this skb: put it in the used pile. */
> > +static void skb_finished(struct skb_shared_info *sinfo)
> > +{
> > +       struct skb_shinfo_tun *sht = (void *)(sinfo + 1);
> > +
> > +       /* FIXME: Race prevention */
> > +       vring_used_buffer_atomic(sht->tun->outring, sht->id, sht->len);
> > +       vring_wake(sht->tun->outring);
> > +
> > +       /* Release device. */
> > +       dev_put(sht->tun->dev);
> > +}
>
> On second thought, this is not going to work.  The network stack
> can clone individual pages out of this skb and put it into a new
> skb.  Therefore whatever scheme we come up with will either need
> to be page-based, or add a flag to tell the network stack that it
> can't clone those pages.

Erk... I'll put in the latter for now.   A page-level solution is not really 
an option: if userspace hands us mmaped pages for example.

Thanks,
Rusty.
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