From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751823AbdGYLxs (ORCPT ); Tue, 25 Jul 2017 07:53:48 -0400 Received: from mga07.intel.com ([134.134.136.100]:13879 "EHLO mga07.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751676AbdGYLxr (ORCPT ); Tue, 25 Jul 2017 07:53:47 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.40,411,1496127600"; d="scan'208";a="130943821" Message-ID: <597731E8.9040803@intel.com> Date: Tue, 25 Jul 2017 19:56:24 +0800 From: Wei Wang User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: Michal Hocko CC: "Michael S. Tsirkin" , linux-kernel@vger.kernel.org, qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, linux-mm@kvack.org, david@redhat.com, cornelia.huck@de.ibm.com, akpm@linux-foundation.org, mgorman@techsingularity.net, aarcange@redhat.com, amit.shah@redhat.com, pbonzini@redhat.com, liliang.opensource@gmail.com, virtio-dev@lists.oasis-open.org, yang.zhang.wz@gmail.com, quan.xu@aliyun.com Subject: Re: [PATCH v12 6/8] mm: support reporting free page blocks References: <1499863221-16206-1-git-send-email-wei.w.wang@intel.com> <1499863221-16206-7-git-send-email-wei.w.wang@intel.com> <20170714123023.GA2624@dhcp22.suse.cz> <20170714181523-mutt-send-email-mst@kernel.org> <20170717152448.GN12888@dhcp22.suse.cz> <596D6E7E.4070700@intel.com> <20170719081311.GC26779@dhcp22.suse.cz> <596F4A0E.4010507@intel.com> <20170724090042.GF25221@dhcp22.suse.cz> <59771010.6080108@intel.com> <20170725112513.GD26723@dhcp22.suse.cz> In-Reply-To: <20170725112513.GD26723@dhcp22.suse.cz> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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