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

[Nai Xia <nai.xia@gmail.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.
> 
> 

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