From mboxrd@z Thu Jan 1 00:00:00 1970 From: Izik Eidus Subject: Re: [PATCH 0/4] ksm - dynamic page sharing driver for linux Date: Tue, 11 Nov 2008 21:18:23 +0200 Message-ID: <4919DA7F.5090106@redhat.com> References: <1226409701-14831-1-git-send-email-ieidus@redhat.com> <20081111103051.979aea57.akpm@linux-foundation.org> <4919D370.7080301@redhat.com> <20081111111110.decc0f06.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Avi Kivity , linux-kernel@vger.kernel.org, linux-mm@kvack.org, kvm@vger.kernel.org, aarcange@redhat.com, chrisw@redhat.com To: Andrew Morton Return-path: Received: from mx2.redhat.com ([66.187.237.31]:59621 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751367AbYKKTSy (ORCPT ); Tue, 11 Nov 2008 14:18:54 -0500 In-Reply-To: <20081111111110.decc0f06.akpm@linux-foundation.org> Sender: kvm-owner@vger.kernel.org List-ID: Andrew Morton wrote: > On Tue, 11 Nov 2008 20:48:16 +0200 > Avi Kivity wrote: > > >> Andrew Morton wrote: >> >>> The whole approach seems wrong to me. The kernel lost track of these >>> pages and then we run around post-facto trying to fix that up again. >>> Please explain (for the changelog) why the kernel cannot get this right >>> via the usual sharing, refcounting and COWing approaches. >>> >>> >> For kvm, the kernel never knew those pages were shared. They are loaded >> from independent (possibly compressed and encrypted) disk images. These >> images are different; but some pages happen to be the same because they >> came from the same installation media. >> > > What userspace-only changes could fix this? Identify the common data, > write it to a flat file and mmap it, something like that? > > >> For OpenVZ the situation is less clear, but if you allow users to >> independently upgrade their chroots you will eventually arrive at the >> same scenario (unless of course you apply the same merging strategy at >> the filesystem level). >> > > hm. > > There has been the occasional discussion about idenfifying all-zeroes > pages and scavenging them, repointing them at the zero page. Could > this infrastructure be used for that? (And how much would we gain from > it?) > > [I'm looking for reasons why this is more than a muck-up-the-vm-for-kvm > thing here ;) ] KSM is separate driver , it doesn't change anything in the VM but adding two helper functions.