From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f72.google.com (mail-pa0-f72.google.com [209.85.220.72]) by kanga.kvack.org (Postfix) with ESMTP id 375946B007E for ; Sun, 24 Apr 2016 23:11:09 -0400 (EDT) Received: by mail-pa0-f72.google.com with SMTP id dx6so253048973pad.0 for ; Sun, 24 Apr 2016 20:11:09 -0700 (PDT) Received: from mga01.intel.com (mga01.intel.com. [192.55.52.88]) by mx.google.com with ESMTP id r123si9309735pfr.154.2016.04.24.20.11.08 for ; Sun, 24 Apr 2016 20:11:08 -0700 (PDT) From: "Li, Liang Z" Subject: RE: [PATCH kernel 1/2] mm: add the related functions to build the free page bitmap Date: Mon, 25 Apr 2016 03:11:05 +0000 Message-ID: References: <1461076474-3864-1-git-send-email-liang.z.li@intel.com> <1461076474-3864-2-git-send-email-liang.z.li@intel.com> <1461077659.3200.8.camel@redhat.com> <20160419191111-mutt-send-email-mst@redhat.com> <20160422094837.GC2239@work-vm> <20160422164936-mutt-send-email-mst@redhat.com> In-Reply-To: <20160422164936-mutt-send-email-mst@redhat.com> Content-Language: en-US Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Sender: owner-linux-mm@kvack.org List-ID: To: "Michael S. Tsirkin" , "Dr. David Alan Gilbert" Cc: Rik van Riel , "viro@zeniv.linux.org.uk" , "linux-kernel@vger.kernel.org" , "quintela@redhat.com" , "amit.shah@redhat.com" , "pbonzini@redhat.com" , "linux-mm@kvack.org" , "kvm@vger.kernel.org" , "qemu-devel@nongnu.org" , "agraf@suse.de" , "borntraeger@de.ibm.com" > On Fri, Apr 22, 2016 at 10:48:38AM +0100, Dr. David Alan Gilbert wrote: > > * Michael S. Tsirkin (mst@redhat.com) wrote: > > > On Tue, Apr 19, 2016 at 03:02:09PM +0000, Li, Liang Z wrote: > > > > > On Tue, 2016-04-19 at 22:34 +0800, Liang Li wrote: > > > > > > The free page bitmap will be sent to QEMU through virtio > > > > > > interface and used for live migration optimization. > > > > > > Drop the cache before building the free page bitmap can get > > > > > > more free pages. Whether dropping the cache is decided by user. > > > > > > > > > > > > > > > > How do you prevent the guest from using those recently-freed > > > > > pages for something else, between when you build the bitmap and > > > > > the live migration completes? > > > > > > > > Because the dirty page logging is enabled before building the > > > > bitmap, there is no need to prevent the guest from using the recent= ly- > freed pages ... > > > > > > > > Liang > > > > > > Well one point of telling host that page is free is so that it can > > > mark it clean even if it was dirty previously. > > > So I think you must pass the pages to guest under the lock. > > > This will allow host optimizations such as marking these pages > > > MADV_DONTNEED or MADV_FREE. > > > Otherwise it's all too tied up to a specific usecase - you aren't > > > telling host that a page is free, you are telling it that a page was > > > free in the past. > > > > But doing it under lock sounds pretty expensive, especially given how > > long the userspace side is going to take to work through the bitmap > > and device what to do. > > > > Dave >=20 > We need to make it as fast as we can since the VCPU is stopped on exit > anyway. This just means e.g. sizing the bitmap reasonably - don't always = try > to fit all memory in a single bitmap. Then we should pause the whole VM when using the bitmap, too expensive? > Really, if the page can in fact be in use when you tell host it's free, t= hen it's > rather hard to explain what does it mean from host/guest interface point = of > view. >=20 How about rename the interface to a more appropriate name other than 'free = page' ? Liang. > It probably can be defined but the interface seems very complex. >=20 > Let's start with a simple thing instead unless it can be shown that there= 's a > performance problem. >=20 >=20 > > > > > > -- > > > MST > > -- > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK -- 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