From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail6.bemta12.messagelabs.com (mail6.bemta12.messagelabs.com [216.82.250.247]) by kanga.kvack.org (Postfix) with ESMTP id D544A900185 for ; Wed, 22 Jun 2011 07:33:55 -0400 (EDT) Received: by iwn8 with SMTP id 8so797261iwn.14 for ; Wed, 22 Jun 2011 04:33:53 -0700 (PDT) From: Nai Xia Reply-To: nai.xia@gmail.com Subject: Re: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte and mmu notifier to help KSM dirty bit tracking Date: Wed, 22 Jun 2011 19:33:37 +0800 References: <201106212055.25400.nai.xia@gmail.com> <4E01D0E3.9080508@redhat.com> <4E01D1C8.2050707@redhat.com> In-Reply-To: <4E01D1C8.2050707@redhat.com> MIME-Version: 1.0 Content-Type: Text/Plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Message-Id: <201106221933.37691.nai.xia@gmail.com> Sender: owner-linux-mm@kvack.org List-ID: To: Avi Kivity Cc: Izik Eidus , Andrew Morton , Andrea Arcangeli , Hugh Dickins , Chris Wright , Rik van Riel , linux-mm , Johannes Weiner , linux-kernel , kvm On Wednesday 22 June 2011 19:28:08 Avi Kivity wrote: > On 06/22/2011 02:24 PM, Avi Kivity wrote: > > On 06/22/2011 02:19 PM, Izik Eidus wrote: > >> On 6/22/2011 2:10 PM, Avi Kivity wrote: > >>> On 06/22/2011 02:05 PM, Izik Eidus wrote: > >>>>>> + spte = rmap_next(kvm, rmapp, NULL); > >>>>>> + while (spte) { > >>>>>> + int _dirty; > >>>>>> + u64 _spte = *spte; > >>>>>> + BUG_ON(!(_spte& PT_PRESENT_MASK)); > >>>>>> + _dirty = _spte& PT_DIRTY_MASK; > >>>>>> + if (_dirty) { > >>>>>> + dirty = 1; > >>>>>> + clear_bit(PT_DIRTY_SHIFT, (unsigned long *)spte); > >>>>>> + } > >>>>> > >>>>> Racy. Also, needs a tlb flush eventually. > >>>> + > >>>> > >>>> Hi, one of the issues is that the whole point of this patch is not > >>>> do tlb flush eventually, > >>>> But I see your point, because other users will not expect such > >>>> behavior, so maybe there is need into a parameter > >>>> flush_tlb=?, or add another mmu notifier call? > >>>> > >>> > >>> If you don't flush the tlb, a subsequent write will not see that > >>> spte.d is clear and the write will happen. So you'll see the page > >>> as clean even though it's dirty. That's not acceptable. > >>> > >> > >> Yes, but this is exactly what we want from this use case: > >> Right now ksm calculate the page hash to see if it was changed, the > >> idea behind this patch is to use the dirty bit instead, > >> however the guest might not really like the fact that we will flush > >> its tlb over and over again, specially in periodically scan like ksm > >> does. > > > > I see. > > Actually, this is dangerous. If we use the dirty bit for other things, > we will get data corruption. Yeah,yeah, I actually clarified in a reply letter to Chris about his similar concern that we are currently the _only_ user. :) We can add the flushing when someone else should rely on this bit. > > For example we might want to map clean host pages as writeable-clean in > the spte on a read fault so that we don't get a page fault when they get > eventually written. > > -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org