Re: [PATCH v12 6/8] mm: support reporting free page blocks
From: Wei Wang
Date: Tue Jul 25 2017 - 07:53:52 EST
On 07/25/2017 07:25 PM, Michal Hocko wrote:
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.
"pfn walk" - do you mean start from the first pfn, and scan all the pfns
that the VM has?
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?
We don't need to do the pfn walk in the guest kernel. When the API
reports, for example,
a 2MB free page block, the API caller offers to the hypervisor the base
address of the page
block, and size=2MB, to the hypervisor.
The hypervisor maintains a bitmap of all the guest physical memory (a
bit corresponds to
a guest pfn). When migrating memory, only the pfns that are set in the
bitmap are transferred
to the destination machine. So, when the hypervisor receives a 2MB free
page block, the
corresponding bits in the bitmap are cleared.
Best,
Wei