linux-kernel - Re: [PATCH 3/4] KVM: Switch to srcu-less get_dirty

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Date:	Mon, 19 Mar 2012 19:15:44 +0900
From:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	avi@...hat.com, mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] KVM: Switch to srcu-less get_dirty_log()

On Mon, 19 Mar 2012 17:34:49 +0800
Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com> wrote:

> 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?

Yes.

I just wanted to explain that getting clear dirty bitmap by GET_DIRTY_LOG
does not mean there is no dirty page: it just means that there was nothing
logged when we updated the bitmap in get_dirty_log().

We cannot know if anything happend between the bitmap update and result
check in the userspace.

So even when we get a clear bitmap, we need to stop the VCPU threads and
then do GET_DIRTY_LOG once more for live migration.

The important thing is that every bit set by mark_page_dirty() can be
found at some time in the future, including the final GET_DIRTY_LOG.

	Takuya
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