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Date: Mon, 19 Mar 2012 17:34:49 +0800
From: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
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To: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
CC: avi@redhat.com, mtosatti@redhat.com, kvm@vger.kernel.org,
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Subject: Re: [PATCH 3/4] KVM: Switch to srcu-less get_dirty_log()
References: <20120301193007.04b2db8e.yoshikawa.takuya@oss.ntt.co.jp> <20120301193316.96682d60.yoshikawa.takuya@oss.ntt.co.jp> <4F62C9B4.7060804@linux.vnet.ibm.com> <20120316155511.70455b4a.yoshikawa.takuya@oss.ntt.co.jp> <4F62EC25.5060306@linux.vnet.ibm.com> <20120316165547.4df2abe4.yoshikawa.takuya@oss.ntt.co.jp> <4F62F9C8.6090803@linux.vnet.ibm.com> <20120316184459.3c64c8cf.yoshikawa.takuya@oss.ntt.co.jp>
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On 03/16/2012 05:44 PM, Takuya Yoshikawa wrote:

> On Fri, 16 Mar 2012 16:28:56 +0800
> Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com> wrote:
> 
>> Thanks for your explanation, maybe you are right, i do not know migration
>> much.
>>
>> What i worried about is, you have changed the behaviour of GET_DIRTY_LOG,
>> in the current one, it can get all the dirty pages when it is called; after
>> your change, GET_DIRTY_LOG can get a empty dirty bitmap but dirty page exists.
> 
> The current code also see the same situation because nothing prevents the
> guest from writing to pages before GET_DIRTY_LOG returns and the userspace
> checks the bitmap.  Everything is running.
> 


The current code is under the protection of s-rcu:
IIRC, it always holds s-rcu when write guest page and set dirty bit,
that mean the dirty page is logged either in the old dirty_bitmap or in the
current memslot->dirty_bitmap. Yes?