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From: | Wei Wang |
Subject: | Re: [Qemu-devel] [PATCH v12 6/8] mm: support reporting free page blocks |
Date: | Tue, 25 Jul 2017 19:56:24 +0800 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 |
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
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