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 02:19 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.

-- 
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