linux-kernel - Re: [PATCH v12 6/8] mm: support reporting free page blocks

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Date:   Tue, 25 Jul 2017 13:25:13 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        linux-kernel@...r.kernel.org, qemu-devel@...gnu.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, david@...hat.com, cornelia.huck@...ibm.com,
        akpm@...ux-foundation.org, mgorman@...hsingularity.net,
        aarcange@...hat.com, amit.shah@...hat.com, pbonzini@...hat.com,
        liliang.opensource@...il.com, virtio-dev@...ts.oasis-open.org,
        yang.zhang.wz@...il.com, quan.xu@...yun.com
Subject: Re: [PATCH v12 6/8] mm: support reporting free page blocks

On Tue 25-07-17 17:32:00, Wei Wang wrote:
> On 07/24/2017 05:00 PM, Michal Hocko wrote:
> >On Wed 19-07-17 20:01:18, Wei Wang wrote:
> >>On 07/19/2017 04:13 PM, Michal Hocko wrote:
> >[...
> >>>All you should need is the check for the page reference count, no?  I
> >>>assume you do some sort of pfn walk and so you should be able to get an
> >>>access to the struct page.
> >>Not necessarily - the guest struct page is not seen by the hypervisor. The
> >>hypervisor only gets those guest pfns which are hinted as unused. From the
> >>hypervisor (host) point of view, a guest physical address corresponds to a
> >>virtual address of a host process. So, once the hypervisor knows a guest
> >>physical page is unsued, it knows that the corresponding virtual memory of
> >>the process doesn't need to be transferred in the 1st round.
> >I am sorry, but I do not understand. Why cannot _guest_ simply check the
> >struct page ref count and send them to the hypervisor?
> 
> Were you suggesting the following?
> 1) get a free page block from the page list using the API;

No. Use a pfn walk, check the reference count and skip those pages which
have 0 ref count. I suspected that you need to do some sort of the pfn
walk anyway because you somehow have to evaluate a memory to migrate,
right?

> 2) if page->ref_count == 0, send it to the hypervisor

yes

> Btw, ref_count may also change at any time.
> 
> >Is there any
> >documentation which describes the workflow or code which would use your
> >new API?
> >
> 
> It's used in the balloon driver (patch 8). We don't have any docs yet, but
> I think the high level workflow is the two steps above.

I will have a look.
-- 
Michal Hocko
SUSE Labs