linux-kernel - Re: [PATCH -mm v9 5/8] mmu-notifier: add clear

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Date:	Wed, 22 Jul 2015 19:33:17 +0300
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	"Greg Thelen" <gthelen@...gle.com>,
	Michel Lespinasse <walken@...gle.com>,
	"David Rientjes" <rientjes@...gle.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Jonathan Corbet <corbet@....net>, <linux-api@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, <linux-mm@...ck.org>,
	<cgroups@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm v9 5/8] mmu-notifier: add clear_young callback

Hi Andrew,

Would you mind merging this incremental patch to the original one? Or
should I better resubmit the whole series with all the fixes?

On Tue, Jul 21, 2015 at 11:51:08AM +0300, Vladimir Davydov wrote:
> On Mon, Jul 20, 2015 at 11:34:21AM -0700, Andres Lagar-Cavilla wrote:
> > On Sun, Jul 19, 2015 at 5:31 AM, Vladimir Davydov <vdavydov@...allels.com>
> [...]
> > > +static int kvm_mmu_notifier_clear_young(struct mmu_notifier *mn,
> > > +                                       struct mm_struct *mm,
> > > +                                       unsigned long start,
> > > +                                       unsigned long end)
> > > +{
> > > +       struct kvm *kvm = mmu_notifier_to_kvm(mn);
> > > +       int young, idx;
> > > +
> > >
> > If you need to cut out another version please add comments as to the two
> > issues raised:
> > - This doesn't proactively flush TLBs -- not obvious if it should.
> > - This adversely affects performance in Pre_haswell Intel EPT.
> 
> Oops, I stopped reading your e-mail in reply to the previous version of
> this patch as soon as I saw the Reviewed-by tag, so I missed your
> request for the comment, sorry about that.
> 
> Here it goes (incremental):
> ---
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index ff4173ce6924..e69a5cb99571 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -397,6 +397,19 @@ static int kvm_mmu_notifier_clear_young(struct mmu_notifier *mn,
>  
>  	idx = srcu_read_lock(&kvm->srcu);
>  	spin_lock(&kvm->mmu_lock);
> +	/*
> +	 * Even though we do not flush TLB, this will still adversely
> +	 * affect performance on pre-Haswell Intel EPT, where there is
> +	 * no EPT Access Bit to clear so that we have to tear down EPT
> +	 * tables instead. If we find this unacceptable, we can always
> +	 * add a parameter to kvm_age_hva so that it effectively doesn't
> +	 * do anything on clear_young.
> +	 *
> +	 * Also note that currently we never issue secondary TLB flushes
> +	 * from clear_young, leaving this job up to the regular system
> +	 * cadence. If we find this inaccurate, we might come up with a
> +	 * more sophisticated heuristic later.
> +	 */
>  	young = kvm_age_hva(kvm, start, end);
>  	spin_unlock(&kvm->mmu_lock);
>  	srcu_read_unlock(&kvm->srcu, idx);
--
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