netdev - Re: [PATCH 7/9] net: add skb_orphan_frags to copy aside frags with destructors

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Date:	Fri, 4 May 2012 00:10:24 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	David Miller <davem@...emloft.net>
Cc:	ian.campbell@...rix.com, netdev@...r.kernel.org,
	eric.dumazet@...il.com
Subject: Re: [PATCH 7/9] net: add skb_orphan_frags to copy aside frags with
 destructors

On Thu, May 03, 2012 at 01:55:32PM -0400, David Miller wrote:
> From: "Michael S. Tsirkin" <mst@...hat.com>
> Date: Thu, 3 May 2012 18:41:43 +0300
> 
> > On Thu, May 03, 2012 at 03:56:09PM +0100, Ian Campbell wrote:
> >> This should be used by drivers which need to hold on to an skb for an extended
> >> (perhaps unbounded) period of time. e.g. the tun driver which relies on
> >> userspace consuming the skb.
> >> 
> >> Signed-off-by: Ian Campbell <ian.campbell@...rix.com>
> >> Cc: mst@...hat.com
> > 
> > 
> > Right. But local sockets queue at socket forever as well.
> > I think this should be called in skb_set_owner_r?
> > 
> > This might somewhat penalize speed for local clients in the name
> > of correctness but these are rare so being correct is
> > more important I think.
> 
> But, on the other hand, putting the check into skb_set_owner_r() is a
> not so nice test to have in the fast path of every socket receive.

True.  Hmm we orphan skbs when we loop them back
so how about reusing the skb->destructor for this?

We could teach pskb_copy pskb_expand_head etc that
when skb with this flag is cloned (expand head etc)
destructor would be set to a function that copies frags.
(clone is less of a fast path so I think adding
 a branch there is less of an issue).

Of course destructor is also called from kfree_skb
but we could clear this flag before the call
in kfree_skb so that destructor can distinguish.

-- 
MST
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