From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f42.google.com (mail-pa0-f42.google.com [209.85.220.42]) by kanga.kvack.org (Postfix) with ESMTP id 47EAA6B0031 for ; Wed, 9 Oct 2013 17:02:21 -0400 (EDT) Received: by mail-pa0-f42.google.com with SMTP id lj1so1670581pab.1 for ; Wed, 09 Oct 2013 14:02:20 -0700 (PDT) Message-ID: <1381352311.5429.115.camel@misato.fc.hp.com> Subject: Re: [PATCH part1 v6 4/6] x86/mem-hotplug: Support initialize page tables in bottom-up From: Toshi Kani Date: Wed, 09 Oct 2013 14:58:31 -0600 In-Reply-To: <20131009192040.GA5592@mtj.dyndns.org> References: <524E2032.4020106@gmail.com> <524E2127.4090904@gmail.com> <5251F9AB.6000203@zytor.com> <525442A4.9060709@gmail.com> <20131009164449.GG22495@htj.dyndns.org> <52558EEF.4050009@gmail.com> <20131009192040.GA5592@mtj.dyndns.org> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Tejun Heo Cc: Zhang Yanfei , "H. Peter Anvin" , Andrew Morton , "Rafael J . Wysocki" , lenb@kernel.org, Thomas Gleixner , mingo@elte.hu, Wanpeng Li , Thomas Renninger , Yinghai Lu , Jiang Liu , Wen Congyang , Lai Jiangshan , isimatu.yasuaki@jp.fujitsu.com, izumi.taku@jp.fujitsu.com, Mel Gorman , Minchan Kim , mina86@mina86.com, gong.chen@linux.intel.com, vasilis.liaskovitis@profitbricks.com, lwoodman@redhat.com, Rik van Riel , jweiner@redhat.com, prarit@redhat.com, "x86@kernel.org" , linux-doc@vger.kernel.org, "linux-kernel@vger.kernel.org" , Linux MM , linux-acpi@vger.kernel.org, imtangchen@gmail.com, Zhang Yanfei , Tang Chen On Wed, 2013-10-09 at 15:20 -0400, Tejun Heo wrote: > Hello, > > On Thu, Oct 10, 2013 at 01:14:23AM +0800, Zhang Yanfei wrote: > > >> You meant that the memory size is about few megs. But here, page tables > > >> seems to be large enough in big memory machines, so that page tables will > > > > > > Hmmm? Even with 4k mappings and, say, 16Gigs of memory, it's still > > > somewhere above 32MiB, right? And, these physical mappings don't > > > usually use 4k mappings to begin with. Unless we're worrying about > > > ISA DMA limit, I don't think it'd be problematic. > > > > I think Peter meant very huge memory machines, say 2T memory? In the worst > > case, this may need 2G memory for page tables, seems huge.... > > Realistically tho, why would people be using 4k mappings on 2T > machines? For the sake of argument, let's say 4k mappings are > required for some weird reason, even then, doing SRAT parsing early > doesn't necessarily solve the problem in itself. It'd still need > heuristics to avoid occupying too much of 32bit memory because it > isn't difficult to imagine specific NUMA settings which would drive > page table allocation into low address. > > No matter what we do, there's no way around the fact that this whole > effort is mostly an incomplete solution in its nature and that's why I > think we better keep things isolated and simple. It isn't a good idea > to make structural changes to accomodate something which isn't and > doesn't have much chance of becoming a full solution. In addition, > the problem itself is niche to begin with. Let's not assume that memory hotplug is always a niche feature for huge & special systems. It may be a niche to begin with, but it could be supported on VMs, which allows anyone to use. Vasilis has been working on KVM to support memory hotplug. Thanks, -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