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

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Message-ID: <1336802484.3891.24.camel@dagon.hellion.org.uk>
Date:	Sat, 12 May 2012 07:01:24 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"eric.dumazet@...il.com" <eric.dumazet@...il.com>
Subject: Re: [PATCH RFC 1/6] skbuff: support per-page destructors in
 copy_ubufs

On Fri, 2012-05-11 at 17:30 +0100, Michael S. Tsirkin wrote:
> On Fri, May 11, 2012 at 03:08:36PM +0300, Michael S. Tsirkin wrote:
> > 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?
> 
> Hmm, it's in case we need to retransmit it later.

I wonder if we could avoid the copy_ubuf in this particular clone path
and have any subsequent calls to copy_ubufs use skb->fclone to determine
if it can safely replace the frags?

If it cannot then could it do a full copy of the skb (including new
shinfo, new frag pages etc) as a fallback?

Ian.


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