linux-kernel - Re: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte and mmu notifier to help KSM dirty bit tracking

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Date:	Wed, 22 Jun 2011 14:10:37 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Izik Eidus <izik.eidus@...ellosystems.com>
CC:	nai.xia@...il.com, Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Chris Wright <chrisw@...s-sol.org>,
	Rik van Riel <riel@...hat.com>,
	linux-mm <linux-mm@...ck.org>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte
 and mmu notifier to help KSM dirty bit tracking

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.

-- 
error compiling committee.c: too many arguments to function

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