From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx147.postini.com [74.125.245.147]) by kanga.kvack.org (Postfix) with SMTP id 721506B0037 for ; Wed, 19 Jun 2013 11:18:57 -0400 (EDT) Message-ID: <1371655111.22206.29.camel@misato.fc.hp.com> Subject: Re: [Part3 PATCH v2 0/4] Support hot-remove local pagetable pages. From: Toshi Kani Date: Wed, 19 Jun 2013 09:18:31 -0600 In-Reply-To: <51C15DC2.3030501@cn.fujitsu.com> References: <1371128636-9027-1-git-send-email-tangchen@cn.fujitsu.com> <20130618170515.GC4553@dhcp-192-168-178-175.profitbricks.localdomain> <51C15DC2.3030501@cn.fujitsu.com> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Tang Chen Cc: Vasilis Liaskovitis , yinghai@kernel.org, tglx@linutronix.de, mingo@elte.hu, hpa@zytor.com, akpm@linux-foundation.org, tj@kernel.org, trenn@suse.de, jiang.liu@huawei.com, wency@cn.fujitsu.com, laijs@cn.fujitsu.com, isimatu.yasuaki@jp.fujitsu.com, mgorman@suse.de, minchan@kernel.org, mina86@mina86.com, gong.chen@linux.intel.com, lwoodman@redhat.com, riel@redhat.com, jweiner@redhat.com, prarit@redhat.com, x86@kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org On Wed, 2013-06-19 at 15:29 +0800, Tang Chen wrote: > Hi Vasilis, Yinghai, > > On 06/19/2013 01:05 AM, Vasilis Liaskovitis wrote: > ...... > > > > This could be a design problem of part3: if we allow local pagetable memory > > to not be offlined but allow the offlining to return successfully, then > > hot-remove is going to succeed. But the direct mapped pagetable pages are still > > mapped in the kernel. The hot-removed memblocks will suddenly disappear (think > > physical DIMMs getting disabled in real hardware, or in a VM case the > > corresponding guest memory getting freed from the emulator e.g. qemu/kvm). The > > system can crash as a result. > > > > Yes. Since the pagetable pages is only allocated to local node, a node may > have more than one device, hot-remove only one memory device could be > problematic. > > But I think it will work if we hot-remove a whole node. I should have > mentioned it. And sorry for the not fully test. > > I think allocating pagetable pages to local device will resolve this problem. > And need to restructure this patch-set. > > > I think these local pagetables do need to be unmapped from kernel, offlined and > > removed somehow - otherwise hot-remove should fail. Could they be migrated > > alternatively e.g. to node 0 memory? But Iiuc direct mapped pages cannot be > > migrated, correct? > > I think we have unmapped the local pagetables. in functions > free_pud/pmd/pte_table(), we cleared pud, pmd, and pte. We just didn't > free the pagetable pages to buddy. > > But when we are not hot-removing the whole node, it is still problematic. > This is true, and it is my design problem. > > > > > What is the original reason for local node pagetable allocation with regards > > to memory hotplug? I assume we want to have hotplugged nodes use only their local > > memory, so that there are no inter-node memory dependencies for hot-add/remove. > > Are there other reasons that I am missing? > > I think the original reason to do local node pagetable is to improve > performance. > Using local pagetable, vmemmap and so on will be faster. > > But actually I think there is no particular reason to implement memory hot-remove > and local node pagetable at the same time. And before this patch-set, I also > suggested once that implement memory hot-remove first, and then improve it to > local pagetable. But Yinghai has done the local pagetable work in has > patches (part1). > And my work is based on his patches. So I just did it. > > But obviously it is more complicated than I thought. > > And now, it seems tj has some more thinking on part1. > > So how about the following plan: > 1. Implement arranging hotpluggable memory with SRAT first, without > local pagetable. > (The main work in part2. And of course, need some patches in part1.) > 2. Do the local device pagetable work, not local node. > 3. Improve memory hotplug to support local device pagetable. Sounds like a good plan. Thanks Tang! -Toshi -- 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