Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759352Ab2JKUtN (ORCPT ); Thu, 11 Oct 2012 16:49:13 -0400 Received: from ozlabs.org ([203.10.76.45]:37246 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759263Ab2JKUtJ (ORCPT ); Thu, 11 Oct 2012 16:49:09 -0400 From: Rusty Russell To: Andrew Morton , Jeremy Fitzhardinge Cc: KY Srinivasan , "gregkh\@linuxfoundation.org" , "linux-kernel\@vger.kernel.org" , "devel\@linuxdriverproject.org" , "olaf\@aepfle.de" , "apw\@canonical.com" , "andi\@firstfloor.org" Subject: Re: [PATCH 2/2] Drivers: hv: Add Hyper-V balloon driver In-Reply-To: <20121010165616.ffb38603.akpm@linux-foundation.org> References: <1349654347-18337-1-git-send-email-kys@microsoft.com> <1349654386-18378-1-git-send-email-kys@microsoft.com> <1349654386-18378-2-git-send-email-kys@microsoft.com> <20121009124449.f54bf8cb.akpm@linux-foundation.org> <426367E2313C2449837CD2DE46E7EAF930A33CC4@SN2PRD0310MB382.namprd03.prod.outlook.com> <20121009181458.e59e7094.akpm@linux-foundation.org> <5076060D.1030101@goop.org> <20121010165616.ffb38603.akpm@linux-foundation.org> User-Agent: Notmuch/0.13.2 (http://notmuchmail.org) Emacs/23.3.1 (i686-pc-linux-gnu) Date: Thu, 11 Oct 2012 18:35:45 +1030 Message-ID: <87r4p577s6.fsf@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2746 Lines: 60 Andrew Morton writes: > On Wed, 10 Oct 2012 16:34:37 -0700 > Jeremy Fitzhardinge wrote: > >> On 10/09/2012 06:14 PM, Andrew Morton wrote: >> > On Wed, 10 Oct 2012 00:09:12 +0000 KY Srinivasan wrote: >> > >> >>>> + if (!pg) { >> >>>> + *alloc_error = true; >> >>>> + return i * alloc_unit; >> >>>> + } >> >>>> + >> >>>> + totalram_pages -= alloc_unit; >> >>> Well, I'd consider totalram_pages to be an mm-private thing which drivers >> >>> shouldn't muck with. Why is this done? >> >> By modifying the totalram_pages, the information presented in /proc/meminfo >> >> correctly reflects what is currently assigned to the guest (MemTotal). >> > eh? /proc/meminfo:MemTotal tells you the total memory in the machine. >> > The only thing which should change it after boot is memory hotplug. >> [...] >> > Why on earth do balloon drivers do this? If the amount of memory which >> > is consumed by balloons is interesting then it should be exported via a >> > standalone metric, not by mucking with totalram_pages. >> >> Balloon drivers are trying to fake a form of page-by-page memory >> hotplug. When they allocate memory from the kernel, they're actually >> giving the pages back to the hypervisor to redistribute to other >> guests. They reduce totalram_pages to try and reflect that the memory >> is no longer the kernel (in Xen, at least, the pfns will no longer have >> any physical page underlying them). >> >> I agree this is pretty ugly; it would be nice to have some better >> interface to indicate what's going on. At one point I tried to use the >> memory hotplug interfaces for larger-scale dynamic transfers of memory >> between a domain and the host, but when I last looked at it, it was too >> coarse grained and heavyweight to replace the balloon mechanism. >> > > urgh. > > I suppose the least we can do here would be to stop directly dinking > with totalram_pages and create some sort of interface for this > operation. That interface would run the memory hotplug notifier so > that code which cares about changes in the amount of physical memory > can take appropriate steps. The implications would be that the balloon > drivers would need to call this interface at low frequency (ie: batch > the pages) and in some reasonably lock-free context. > > I guess that's solving a non-problem at this stage. Yep. drivers/virtio/virtio_balloon manipulates it too. This, it's best practice! Cheers, Rusty. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/