From: Nai Xia <nai.xia@gmail.com> To: Avi Kivity <avi@redhat.com> Cc: Izik Eidus <izik.eidus@ravellosystems.com>, Andrew Morton <akpm@linux-foundation.org>, Andrea Arcangeli <aarcange@redhat.com>, Hugh Dickins <hughd@google.com>, Chris Wright <chrisw@sous-sol.org>, Rik van Riel <riel@redhat.com>, "linux-mm" <linux-mm@kvack.org>, Johannes Weiner <hannes@cmpxchg.org>, "linux-kernel" <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org> 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 [thread overview] Message-ID: <201106221933.37691.nai.xia@gmail.com> (raw) In-Reply-To: <4E01D1C8.2050707@redhat.com> 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. > >
WARNING: multiple messages have this Message-ID (diff)
From: Nai Xia <nai.xia@gmail.com> To: Avi Kivity <avi@redhat.com> Cc: Izik Eidus <izik.eidus@ravellosystems.com>, Andrew Morton <akpm@linux-foundation.org>, Andrea Arcangeli <aarcange@redhat.com>, Hugh Dickins <hughd@google.com>, Chris Wright <chrisw@sous-sol.org>, Rik van Riel <riel@redhat.com>, "linux-mm" <linux-mm@kvack.org>, Johannes Weiner <hannes@cmpxchg.org>, "linux-kernel" <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org> 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 [thread overview] Message-ID: <201106221933.37691.nai.xia@gmail.com> (raw) In-Reply-To: <4E01D1C8.2050707@redhat.com> 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: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2011-06-22 11:33 UTC|newest] Thread overview: 96+ messages / expand[flat|nested] mbox.gz Atom feed top 2011-06-21 12:55 [PATCH 0/2 V2] ksm: take dirty bit as reference to avoid volatile pages scanning Nai Xia 2011-06-21 12:55 ` Nai Xia 2011-06-21 13:26 ` [PATCH 1/2 " Nai Xia 2011-06-21 13:26 ` Nai Xia 2011-06-21 21:42 ` Chris Wright 2011-06-21 21:42 ` Chris Wright 2011-06-22 0:02 ` Nai Xia 2011-06-22 0:02 ` Nai Xia 2011-06-22 0:42 ` Chris Wright 2011-06-22 0:42 ` Chris Wright 2011-06-21 13:32 ` [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte and mmu notifier to help KSM dirty bit tracking Nai Xia 2011-06-21 13:32 ` Nai Xia 2011-06-22 0:21 ` Chris Wright 2011-06-22 0:21 ` Chris Wright 2011-06-22 4:43 ` Nai Xia 2011-06-22 4:43 ` Nai Xia 2011-06-22 6:15 ` Izik Eidus 2011-06-22 6:15 ` Izik Eidus 2011-06-22 6:38 ` Nai Xia 2011-06-22 6:38 ` Nai Xia 2011-06-22 15:46 ` Chris Wright 2011-06-22 15:46 ` Chris Wright 2011-06-22 10:43 ` Avi Kivity 2011-06-22 10:43 ` Avi Kivity 2011-06-22 11:05 ` Izik Eidus 2011-06-22 11:05 ` Izik Eidus 2011-06-22 11:10 ` Avi Kivity 2011-06-22 11:10 ` Avi Kivity 2011-06-22 11:19 ` Izik Eidus 2011-06-22 11:19 ` Izik Eidus 2011-06-22 11:24 ` Avi Kivity 2011-06-22 11:24 ` Avi Kivity 2011-06-22 11:28 ` Avi Kivity 2011-06-22 11:28 ` Avi Kivity 2011-06-22 11:31 ` Avi Kivity 2011-06-22 11:31 ` Avi Kivity 2011-06-22 11:33 ` Nai Xia [this message] 2011-06-22 11:33 ` Nai Xia 2011-06-22 11:39 ` Izik Eidus 2011-06-22 11:39 ` Izik Eidus 2011-06-22 15:39 ` Rik van Riel 2011-06-22 15:39 ` Rik van Riel 2011-06-22 16:55 ` Andrea Arcangeli 2011-06-22 16:55 ` Andrea Arcangeli 2011-06-22 23:37 ` Nai Xia 2011-06-22 23:37 ` Nai Xia 2011-06-22 23:59 ` Andrea Arcangeli 2011-06-22 23:59 ` Andrea Arcangeli 2011-06-23 0:31 ` Nai Xia 2011-06-23 0:31 ` Nai Xia 2011-06-23 0:44 ` Andrea Arcangeli 2011-06-23 0:44 ` Andrea Arcangeli 2011-06-23 1:36 ` Nai Xia 2011-06-23 1:36 ` Nai Xia 2011-06-23 0:00 ` Rik van Riel 2011-06-23 0:00 ` Rik van Riel 2011-06-23 0:42 ` Nai Xia 2011-06-23 0:42 ` Nai Xia 2011-06-22 23:13 ` Nai Xia 2011-06-22 23:13 ` Nai Xia 2011-06-22 23:25 ` Andrea Arcangeli 2011-06-22 23:25 ` Andrea Arcangeli 2011-06-23 1:30 ` Nai Xia 2011-06-23 1:30 ` Nai Xia 2011-06-22 23:28 ` Rik van Riel 2011-06-22 23:28 ` Rik van Riel 2011-06-23 0:52 ` Nai Xia 2011-06-23 0:52 ` Nai Xia 2011-06-22 11:24 ` Nai Xia 2011-06-22 15:03 ` Andrea Arcangeli 2011-06-22 15:03 ` Andrea Arcangeli 2011-06-22 15:19 ` Izik Eidus 2011-06-22 15:19 ` Izik Eidus 2011-06-22 23:19 ` Nai Xia 2011-06-22 23:19 ` Nai Xia 2011-06-22 23:44 ` Andrea Arcangeli 2011-06-22 23:44 ` Andrea Arcangeli 2011-06-23 0:14 ` Nai Xia 2011-06-23 0:14 ` Nai Xia 2011-06-22 23:42 ` Nai Xia 2011-06-22 23:42 ` Nai Xia 2011-06-21 13:36 ` [PATCH 2/2 V2] ksm: take dirty bit as reference to avoid volatile pages scanning Nai Xia 2011-06-21 13:36 ` Nai Xia 2011-06-21 22:38 ` Chris Wright 2011-06-21 22:38 ` Chris Wright 2011-06-22 0:04 ` Nai Xia 2011-06-22 0:04 ` Nai Xia 2011-06-22 0:35 ` Chris Wright 2011-06-22 0:35 ` Chris Wright 2011-06-22 4:47 ` Nai Xia 2011-06-22 4:47 ` Nai Xia 2011-06-22 10:55 ` Nai Xia 2011-06-22 10:55 ` Nai Xia 2011-06-22 0:46 ` [PATCH 0/2 " Chris Wright 2011-06-22 0:46 ` Chris Wright 2011-06-22 4:15 ` Nai Xia
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=201106221933.37691.nai.xia@gmail.com \ --to=nai.xia@gmail.com \ --cc=aarcange@redhat.com \ --cc=akpm@linux-foundation.org \ --cc=avi@redhat.com \ --cc=chrisw@sous-sol.org \ --cc=hannes@cmpxchg.org \ --cc=hughd@google.com \ --cc=izik.eidus@ravellosystems.com \ --cc=kvm@vger.kernel.org \ --cc=linux-kernel@vger.kernel.org \ --cc=linux-mm@kvack.org \ --cc=riel@redhat.com \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is an external index of several public inboxes, see mirroring instructions on how to clone and mirror all data and code used by this external index.