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

[Wei Wang]

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