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

[Avi Kivity <avi@redhat.com>]

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.

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.

-- 
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