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

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Message-ID: <527A109E.5040000@secunet.com>
Date:	Wed, 06 Nov 2013 10:49:18 +0100
From:	Mathias Krause <mathias.krause@...unet.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
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 06.11.2013 10:30, Herbert Xu wrote:
> On Tue, Nov 05, 2013 at 02:54:11PM +0100, Mathias Krause wrote:
>> -	/* If skb is cloned or its head is paged, reallocate
>> -	 * head pulling out all the pages (pages are considered not writable
>> -	 * at the moment even if they are anonymous).
>> +	/* If skb is cloned reallocate head pulling out all the pages (pages are
>> +	 * considered not writable at the moment even if they are anonymous).
>>  	 */
> 
> Hang on, you haven't explained why it is OK to write to pages.

Why wouldn't it if the skb isn't 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?


Regards,
Mathias
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