Linux Virtualization - Jul 2017 - [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

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

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.


Best,
Wei

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