netdev - Re: [PATCH RFC 1/6] skbuff: support per-page destructors in copy

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Message-Id: <20120511.171202.1845086514372654549.davem@davemloft.net>
Date:	Fri, 11 May 2012 17:12:02 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	mst@...hat.com
Cc:	Ian.Campbell@...rix.com, netdev@...r.kernel.org,
	eric.dumazet@...il.com
Subject: Re: [PATCH RFC 1/6] skbuff: support per-page destructors in
 copy_ubufs

From: "Michael S. Tsirkin" <mst@...hat.com>
Date: Fri, 11 May 2012 15:08:37 +0300

> On Fri, May 11, 2012 at 11:58:12AM +0100, Ian Campbell wrote:
>> On Fri, 2012-05-11 at 10:00 +0100, Ian Campbell wrote:
>> > I'm seeing copy_ubufs called in my remote NFS test, which I don't
>> > think I expected -- I'll investigate why this is happening today. 
>> 
>> It's tcp_transmit_skb which can (conditionally) call skb_clone
>> (backtrace below)
> 
> Interesting. I didn't realise we clone skbs on data path:
> tcp_write_xmit calls tcp_transmit_skb with clone_it flag.
> Could someone comment on why we need to clone on good path
> like this?

We can't send the original SKB that's linked into the retransmit
queue.  It's linkage must stay secure in that queue.

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