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    SubjectRe: [PATCH v12 6/8] mm: support reporting free page blocks
    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

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