From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:48867) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dXncw-0003zM-J4 for qemu-devel@nongnu.org; Wed, 19 Jul 2017 07:58:51 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dXncr-000687-Kg for qemu-devel@nongnu.org; Wed, 19 Jul 2017 07:58:50 -0400 Received: from mga11.intel.com ([192.55.52.93]:60478) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dXncr-00067h-A4 for qemu-devel@nongnu.org; Wed, 19 Jul 2017 07:58:45 -0400 Message-ID: <596F4A0E.4010507@intel.com> Date: Wed, 19 Jul 2017 20:01:18 +0800 From: Wei Wang MIME-Version: 1.0 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> In-Reply-To: <20170719081311.GC26779@dhcp22.suse.cz> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v12 6/8] mm: support reporting free page blocks List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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 07/19/2017 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