netdev - Re: Is it a possible bug in dev_gro

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Date:	Tue, 10 Aug 2010 08:34:26 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	"Xin, Xiaohui" <xiaohui.xin@...el.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
	"davem@...emloft.net" <davem@...emloft.net>
Subject: Re: Is it a possible bug in dev_gro_receive()?

On Tue, Aug 10, 2010 at 04:11:54PM +0800, Xin, Xiaohui wrote:
> Jarek,
> Seems community agree with your patch more.
> So may you send out your patch then? Thanks!
> Some of my related patches still need this fix.

Hmm... But there was no my patch. Only a tiny, cosmetical suggestion
to your patch. I'd be glad if you add some credit or my "Acked-by",
of course. But if you really have a big problem, e.g. you don't like
my suggestion, please confirm.

Thanks,
Jarek P.

> 
> Thanks
> Xiaohui
> 
> >-----Original Message-----
> >From: Jarek Poplawski [mailto:jarkao2@...il.com]
> >Sent: Tuesday, August 03, 2010 2:45 PM
> >To: Xin, Xiaohui
> >Cc: netdev@...r.kernel.org; herbert@...dor.apana.org.au; davem@...emloft.net
> >Subject: Re: Is it a possible bug in dev_gro_receive()?
> >
> >On Tue, Aug 03, 2010 at 10:33:24AM +0800, Xin, Xiaohui wrote:
> >> >-----Original Message-----
> >> >From: Jarek Poplawski [mailto:jarkao2@...il.com]
> >> >Sent: Monday, August 02, 2010 6:29 PM
> >> >To: Xin, Xiaohui
> >> >Cc: netdev@...r.kernel.org; herbert@...dor.apana.org.au; davem@...emloft.net
> >> >Subject: Re: Is it a possible bug in dev_gro_receive()?
> >> >
> >> >Xin Xiaohui wrote:
> >> >> I looked into the code dev_gro_receive(), found the code here:
> >> >> if the frags[0] is pulled to 0, then the page will be released,
> >> >> and memmove() frags left.
> >> >> Is that right? I'm not sure if memmove do right or not, but
> >> >> frags[0].size is never set after memove at least. what I think
> >> >> a simple way is not to do anything if we found frags[0].size == 0.
> >> >> The patch is as followed.
> >> >>
> >> >> Or am I missing something here?
> >> >
> >> >I think, you're right, but fixing memmove looks nicer to me:
> >> >
> >> > -	--skb_shinfo(skb)->nr_frags);
> >> > +	--skb_shinfo(skb)->nr_frags * sizeof(skb_frag_t));
> >> >
> >> >Jarek P.
> >>
> >> Is there a little hurt of performance to do memmove() if skb_shinfo(skb)->nr_frags is
> >large?
> >> We're now working on the zero-copy patches based on napi_gro_frags() interface, and in
> >> this case, we have found a lot of skbs which frags[0] is pulled to 0. And after the memmove
> >is
> >> fixed, each frags[x].size is needed to modify too.
> >> So I think don't do anything is better. Or is there any side effect with a null page in the
> >stack?
> >
> >Even if it's better, generally you should separate fixes from
> >optimizations. On the other hand, it was expected to be "unlikely" by
> >design, so you should probably explain more why it has to be changed
> >here too.
> >
> >Thanks,
> >Jarek P.
> >
> >>
> >> Thanks
> >> Xiaohui
> >> >
> >> >>
> >> >> ---
> >> >>  net/core/dev.c |    7 -------
> >> >>  1 files changed, 0 insertions(+), 7 deletions(-)
> >> >>
> >> >> diff --git a/net/core/dev.c b/net/core/dev.c
> >> >> index 264137f..28cdbbf 100644
> >> >> --- a/net/core/dev.c
> >> >> +++ b/net/core/dev.c
> >> >> @@ -2730,13 +2730,6 @@ pull:
> >> >>
> >> >>  		skb_shinfo(skb)->frags[0].page_offset += grow;
> >> >>  		skb_shinfo(skb)->frags[0].size -= grow;
> >> >> -
> >> >> -		if (unlikely(!skb_shinfo(skb)->frags[0].size)) {
> >> >> -			put_page(skb_shinfo(skb)->frags[0].page);
> >> >> -			memmove(skb_shinfo(skb)->frags,
> >> >> -				skb_shinfo(skb)->frags + 1,
> >> >> -				--skb_shinfo(skb)->nr_frags);
> >> >> -		}
> >> >>  	}
> >> >>
> >> >>  ok:
> >> >
> >> >
> >>
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