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

[Michal Hocko]

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