From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f70.google.com (mail-pg0-f70.google.com [74.125.83.70]) by kanga.kvack.org (Postfix) with ESMTP id 5D91D6B0292 for ; Tue, 25 Jul 2017 07:53:48 -0400 (EDT) Received: by mail-pg0-f70.google.com with SMTP id 83so99262817pgb.14 for ; Tue, 25 Jul 2017 04:53:48 -0700 (PDT) Received: from mga06.intel.com (mga06.intel.com. [134.134.136.31]) by mx.google.com with ESMTPS id a186si7995756pfb.402.2017.07.25.04.53.46 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 25 Jul 2017 04:53:47 -0700 (PDT) Message-ID: <597731E8.9040803@intel.com> Date: Tue, 25 Jul 2017 19:56:24 +0800 From: Wei Wang MIME-Version: 1.0 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: owner-linux-mm@kvack.org List-ID: 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 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org