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

From: Avi Kivity
Date: Wed Jun 22 2011 - 07:24:47 EST

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.

error compiling committee.c: too many arguments to function

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