linux-kernel - Re: [PATCH V3 2/8] Add a new zerocopy device flag

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Date:	Tue, 03 May 2011 13:11:21 -0700
From:	Shirley Ma <mashirle@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	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 V3 2/8] Add a new zerocopy device flag

On Tue, 2011-05-03 at 10:42 -0700, Shirley Ma wrote:
> Better to prevent this kind of skbs to be used in skb_clone, expand
> head
> for now.

I looked at the code, skb_clone shouldn't have any problem since ubuf
callback is only called after the lower device DMA has done. I can
modify the zerocopy len to 256 bytes so expand head should be OK as
well. So we only need to prevent recycle skb. I also checked the device
drivers, only a few device do RX buffers recycle. So there shouldn't be
any problem.

I will add more comments here to make sure when ZEROCOPY flag is set,
the ubuf callback should only be called when last reference to this skb
is gone.

Thanks
Shirley

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