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

[Wei Wang <wei.w.wang@intel.com>]

On 07/25/2017 07:25 PM, Michal Hocko wrote:
> 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.


"pfn walk" - do you mean start from the first pfn, and scan all the pfns 
that the VM has?


> 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?


We don't need to do the pfn walk in the guest kernel. When the API 
reports, for example,
a 2MB free page block, the API caller offers to the hypervisor the base 
address of the page
block, and size=2MB, to the hypervisor.

The hypervisor maintains a bitmap of all the guest physical memory (a 
bit corresponds to
a guest pfn). When migrating memory, only the pfns that are set in the 
bitmap are transferred
to the destination machine. So, when the hypervisor receives a 2MB free 
page block, the
corresponding bits in the bitmap are cleared.

Best,
Wei

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