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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