Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755681Ab2F0JCL (ORCPT ); Wed, 27 Jun 2012 05:02:11 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55676 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754730Ab2F0JCH (ORCPT ); Wed, 27 Jun 2012 05:02:07 -0400 Date: Wed, 27 Jun 2012 12:02:01 +0300 From: "Michael S. Tsirkin" To: Frank Swiderski Cc: Rik van Riel , Rusty Russell , Andrea Arcangeli , virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, mikew@google.com, Ying Han , Rafael Aquini Subject: Re: [PATCH] Add a page cache-backed balloon device driver. Message-ID: <20120627090201.GA17507@redhat.com> References: <1340742778-11282-1-git-send-email-fes@google.com> <4FEA1E2E.4020806@redhat.com> <20120626214709.GA15406@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3581 Lines: 86 On Tue, Jun 26, 2012 at 04:21:58PM -0700, Frank Swiderski wrote: > On Tue, Jun 26, 2012 at 2:47 PM, Michael S. Tsirkin wrote: > > On Tue, Jun 26, 2012 at 02:31:26PM -0700, Frank Swiderski wrote: > >> On Tue, Jun 26, 2012 at 1:40 PM, Rik van Riel wrote: > >> > On 06/26/2012 04:32 PM, Frank Swiderski wrote: > >> >> > >> >> This implementation of a virtio balloon driver uses the page cache to > >> >> "store" pages that have been released to the host. ?The communication > >> >> (outside of target counts) is one way--the guest notifies the host when > >> >> it adds a page to the page cache, allowing the host to madvise(2) with > >> >> MADV_DONTNEED. ?Reclaim in the guest is therefore automatic and implicit > >> >> (via the regular page reclaim). ?This means that inflating the balloon > >> >> is similar to the existing balloon mechanism, but the deflate is > >> >> different--it re-uses existing Linux kernel functionality to > >> >> automatically reclaim. > >> >> > >> >> Signed-off-by: Frank Swiderski > >> > > >> > > >> > It is a great idea, but how can this memory balancing > >> > possibly work if someone uses memory cgroups inside a > >> > guest? > >> > >> Thanks and good point--this isn't something that I considered in the > >> implementation. > >> > >> > Having said that, we currently do not have proper > >> > memory reclaim balancing between cgroups at all, so > >> > requiring that of this balloon driver would be > >> > unreasonable. > >> > > >> > The code looks good to me, my only worry is the > >> > code duplication. We now have 5 balloon drivers, > >> > for 4 hypervisors, all implementing everything > >> > from scratch... > >> > >> Do you have any recommendations on this? ?I could (I think reasonably > >> so) modify the existing virtio_balloon.c and have it change behavior > >> based on a feature bit or other configuration. ?I'm not sure that > >> really addresses the root of what you're pointing out--it's still > >> adding a different implementation, but doing so as an extension of an > >> existing one. > >> > >> fes > > > > Let's assume it's a feature bit: how would you > > formulate what the feature does *from host point of view*? > > > > -- > > MST > > In this implementation, the host doesn't keep track of pages in the > balloon, as there is no explicit deflate path. The host device for > this implementation should merely, for example, MADV_DONTNEED on the > pages sent in an inflate. Thus, the inflate becomes a notification > that the guest doesn't need those pages mapped in, but that they > should be available if the guest touches them. So guest access removes the page from the balloon, since it cancels MADV_DONTNEED, right? Okay. But what is the meaning of num_pages then? For example, let's assume I set num_pages to 1, then guest gives me a page and later accesses this page. Is guest also required to give me another page now? Later I send a config interrupt without changing num_pages. Is guest required to give me another page now? > In that sense, it's > not a rigid shrink of guest memory. I'm not sure what I'd call the > feature bit though. > > Was that the question you were asking, or did I misread? > > fes Yes. It would be a good idea for you to try and write a spec IMO. Send a patch to virtio.lyx -- MST -- 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/