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.
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?
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