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
