netdev - Re: [PATCH net-next 3/3] net: allow to leave the buffer fragmented in skb_cow

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Message-ID: <20131106095217.GA18851@gondor.apana.org.au>
Date:	Wed, 6 Nov 2013 17:52:17 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Mathias Krause <mathias.krause@...unet.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Steffen Klassert <steffen.klassert@...unet.com>,
	Dmitry Tarnyagin <dmitry.tarnyagin@...kless.no>,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next 3/3] net: allow to leave the buffer fragmented
 in skb_cow_data()

On Wed, Nov 06, 2013 at 10:49:18AM +0100, Mathias Krause wrote:
> On 06.11.2013 10:30, Herbert Xu wrote:
>
> > Hang on, you haven't explained why it is OK to write to pages.
> 
> Why wouldn't it if the skb isn't cloned?

Because if it's owned by an entity outside our stack (e.g., virt
host or app) then the skb itself won't be cloned.

> > What if said page is owned by the virt host or some app?
> 
> How would one detect such a case. I could image not by testing
> skb_shinfo(skb)->nr_frags as it is right now?

You can't.  That's why we always copy.  If you want to do this
properly then we'll need to add at least a bit to indicate whether
the page originated from within the network stack and we have
full ownership.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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