Linux Virtualization - Jul 2017 - [PATCH v12 6/8] mm: support reporting free page blocks

On Tue 18-07-17 10:12:14, Wei Wang wrote:
[...]
> Probably I should have included the introduction of the usages in
> the log. Hope it is not too later to explain here:
Yes this should have been described in the cover.
 
> Live migration needs to transfer the VM's memory from the source
> machine to the destination round by round. For the 1st round, all the
VM's
> memory is transferred. From the 2nd round, only the pieces of memory
> that were written by the guest (after the 1st round) are transferred. One
> method that is popularly used by the hypervisor to track which part of
> memory is written is to write-protect all the guest memory.
> 
> This patch enables the optimization of the 1st round memory transfer -
> the hypervisor can skip the transfer of guest unused pages in the 1st
round.
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.
-- 
Michal Hocko
SUSE Labs

On 07/19/2017 04:13 PM, Michal Hocko wrote:
> On Tue 18-07-17 10:12:14, Wei Wang wrote:
> [...]
>> Probably I should have included the introduction of the usages in
>> the log. Hope it is not too later to explain here:
> Yes this should have been described in the cover.

OK, I will do it in the next version.

>   
>> Live migration needs to transfer the VM's memory from the source
>> machine to the destination round by round. For the 1st round, all the
VM's
>> memory is transferred. From the 2nd round, only the pieces of memory
>> that were written by the guest (after the 1st round) are transferred.
One
>> method that is popularly used by the hypervisor to track which part of
>> memory is written is to write-protect all the guest memory.
>>
>> This patch enables the optimization of the 1st round memory transfer -
>> the hypervisor can skip the transfer of guest unused pages in the 1st
round.
> 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.


Best,
Wei

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