From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail144.messagelabs.com (mail144.messagelabs.com [216.82.254.51]) by kanga.kvack.org (Postfix) with SMTP id 86972900185 for ; Wed, 22 Jun 2011 07:24:39 -0400 (EDT) Message-ID: <4E01D0E3.9080508@redhat.com> Date: Wed, 22 Jun 2011 14:24:19 +0300 From: Avi Kivity MIME-Version: 1.0 Subject: Re: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte and mmu notifier to help KSM dirty bit tracking References: <201106212055.25400.nai.xia@gmail.com> <201106212132.39311.nai.xia@gmail.com> <4E01C752.10405@redhat.com> <4E01CC77.10607@ravellosystems.com> <4E01CDAD.3070202@redhat.com> <4E01CFD2.6000404@ravellosystems.com> In-Reply-To: <4E01CFD2.6000404@ravellosystems.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Izik Eidus Cc: nai.xia@gmail.com, Andrew Morton , Andrea Arcangeli , Hugh Dickins , Chris Wright , Rik van Riel , linux-mm , Johannes Weiner , linux-kernel , kvm 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. > > So what we say here is: it is better to have little junk in the > unstable tree that get flushed eventualy anyway, instead of make the > guest slower.... > this race is something that does not reflect accurate of ksm anyway > due to the full memcmp that we will eventualy perform... > > Ofcurse we trust that in most cases, beacuse it take ksm to get into a > random virtual address in real systems few minutes, there will be > already tlb flush performed. > > What you think about having 2 calls: one that does the expected > behivor and does flush the tlb, and one that clearly say it doesnt > flush the tlb > and expline its use case for ksm? Yes. And if the unstable/fast callback is not provided, have the common code fall back to the stable/slow callback instead. Or have a parameter that allows inaccurate results to be returned more quickly. -- error compiling committee.c: too many arguments to function -- 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