Subject: Re: [RFC PATCH v8 00/16] Provide a zero-copy method on KVM
 virtio-net.
From: Shirley Ma <mashirle@us.ibm.com>
To: Avi Kivity <avi@redhat.com>
Cc: xiaohui.xin@intel.com, netdev@vger.kernel.org, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, mst@redhat.com, mingo@elte.hu,
        davem@davemloft.net, herbert@gondor.hengli.com.au,
        jdike@linux.intel.com
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Date: Mon, 02 Aug 2010 09:01:58 -0700
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Hello Avi,

On Sun, 2010-08-01 at 11:18 +0300, Avi Kivity wrote:
> I don't understand.  Under what conditions do you use
> get_user_pages() 
> instead of get_user_pages_fast()?  Why?

The code always calls get_user_pages_fast, however, the page will be
unpinned in skb_free if the same page is not used again for a new
buffer. The reason for unpin the page is we don't want to pin all of the
guest kernel memory(memory over commit). So get_user_pages_fast will
call slow path get_user_pages. 

Your previous comment is suggesting to keep the page pinned for
get_user_pages_fast fast path?

Thanks
Shirley

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