linux-kernel - Re: [RFC PATCH 1/1] Move two pinned pages to non-movable node in kvm.

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Date:	Thu, 19 Jun 2014 12:20:32 +0300
From:	Gleb Natapov <gleb@...nel.org>
To:	Tang Chen <tangchen@...fujitsu.com>
Cc:	pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, mgorman@...e.de, yinghai@...nel.org,
	isimatu.yasuaki@...fujitsu.com, guz.fnst@...fujitsu.com,
	laijs@...fujitsu.com, kvm@...r.kernel.org, linux-mm@...ck.org,
	x86@...nel.org, linux-kernel@...r.kernel.org, mtosatti@...hat.com
Subject: Re: [RFC PATCH 1/1] Move two pinned pages to non-movable node in kvm.

CCing Marcelo,

On Wed, Jun 18, 2014 at 02:50:44PM +0800, Tang Chen wrote:
> Hi Gleb,
> 
> Thanks for the quick reply. Please see below.
> 
> On 06/18/2014 02:12 PM, Gleb Natapov wrote:
> >On Wed, Jun 18, 2014 at 01:50:00PM +0800, Tang Chen wrote:
> >>[Questions]
> >>And by the way, would you guys please answer the following questions for me ?
> >>
> >>1. What's the ept identity pagetable for ?  Only one page is enough ?
> >>
> >>2. Is the ept identity pagetable only used in realmode ?
> >>    Can we free it once the guest is up (vcpu in protect mode)?
> >>
> >>3. Now, ept identity pagetable is allocated in qemu userspace.
> >>    Can we allocate it in kernel space ?
> >What would be the benefit?
> 
> I think the benefit is we can hot-remove the host memory a kvm guest
> is using.
> 
> For now, only memory in ZONE_MOVABLE can be migrated/hot-removed. And the
> kernel
> will never use ZONE_MOVABLE memory. So if we can allocate these two pages in
> kernel space, we can pin them without any trouble. When doing memory
> hot-remove,
> the kernel will not try to migrate these two pages.
But we can do that by other means, no? The patch you've sent for instance.

> 
> >
> >>
> >>4. If I want to migrate these two pages, what do you think is the best way ?
> >>
> >I answered most of those here: http://www.mail-archive.com/kvm@vger.kernel.org/msg103718.html
> 
> I'm sorry I must missed this email.
> 
> Seeing your advice, we can unpin these two pages and repin them in the next
> EPT violation.
> So about this problem, which solution would you prefer, allocate these two
> pages in kernel
> space, or migrate them before memory hot-remove ?
> 
> I think the first solution is simpler. But I'm not quite sure if there is
> any other pages
> pinned in memory. If we have the same problem with other kvm pages, I think
> it is better to
> solve it in the second way.
> 
> What do you think ?
Remove pinning is preferable. In fact looks like for identity pagetable
it should be trivial, just don't pin. APIC access page is a little bit
more complicated since its physical address needs to be tracked to be
updated in VMCS.

--
			Gleb.
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