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

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Message-ID: <20170724090042.GF25221@dhcp22.suse.cz>
Date:   Mon, 24 Jul 2017 11:00:43 +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 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? Is there any
documentation which describes the workflow or code which would use your
new API?

-- 
Michal Hocko
SUSE Labs