netdev - Re: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in netdevice

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Message-ID: <1305734543.32080.50.camel@localhost.localdomain>
Date:	Wed, 18 May 2011 09:02:23 -0700
From:	Shirley Ma <mashirle@...ibm.com>
To:	Michał Mirosław <mirqus@...il.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Ben Hutchings <bhutchings@...arflare.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Avi Kivity <avi@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	netdev@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in
 netdevice

On Wed, 2011-05-18 at 07:38 -0700, Shirley Ma wrote:
> On Wed, 2011-05-18 at 13:40 +0200, Michał Mirosław wrote:
> > >> >> Not more other restrictions, skb clone is OK.
> pskb_expand_head()
> > looks
> > >> >> OK to me from code review.
> > >> > Hmm. pskb_expand_head calls skb_release_data while keeping
> > >> > references to pages. How is that ok? What do I miss?
> > >> It's making copy of the skb_shinfo earlier, so the pages refcount
> > >> stays the same.
> > > Exactly. But the callback is invoked so the guest thinks it's ok
> to
> > > change this memory. If it does a corrupted packet will be sent
> out.
> > 
> > Hmm. I tool a quick look at skb_clone(), and it looks like this
> > sequence will break this scheme:
> > 
> > skb2 = skb_clone(skb...);
> > kfree_skb(skb) or pskb_expand_head(skb);  /* callback called */
> > [use skb2, pages still referenced]
> > kfree_skb(skb); /* callback called again */
> > 
> > This sequence is common in bridge, might be in other places.
> > 
> > Maybe this ubuf thing should just track clones? This will make it
> work
> > on all devices then.
> 
> The callback was only invoked when last reference of skb was gone.
> skb_clone does increase skb refcnt. I tested tcpdump on lower device,
> it
> worked.
> 
> For the sequence of:
> 
> skb_clone  -> last refcnt + 1
> kfree_skb() or pskb_expand_head -> callback not called
> kfree_skb() -> callback called
> 
> I will check page refcount to see whether it's balanced. 

The page refcounts are balanced too. 

In macvtap/vhost Real NIC zerocopy case, it always goes to fastpath in
pskb_expand_head, so I didn't hit any issue.

But rethinking about pskb_expand_head(), it calls skb_release_data() to
free old skb head when it's not in the fastpath (pskb_expand_head is not
the last reference of this skb); And it's impossible to track which skb
head (old one or new one) will be the last one to free. So better to
return error for zero-copy skbs when not using fastpath. Does it make
sense? 

Besides this, any other issue?


Thanks
Shirley

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