Re: [PATCH v2 12/15] KVM: MMU: allow locklessly access shadow pagetable out of vcpu thread

[Marcelo Tosatti]

On Tue, Oct 08, 2013 at 12:02:32PM +0800, Xiao Guangrong wrote:
> 
> Hi Marcelo,
> 
> On Oct 8, 2013, at 9:23 AM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> 
> >> 
> >> +	if (kvm->arch.rcu_free_shadow_page) {
> >> +		kvm_mmu_isolate_pages(invalid_list);
> >> +		sp = list_first_entry(invalid_list, struct kvm_mmu_page, link);
> >> +		list_del_init(invalid_list);
> >> +		call_rcu(&sp->rcu, free_pages_rcu);
> >> +		return;
> >> +	}
> > 
> > This is unbounded (there was a similar problem with early fast page fault
> > implementations):
> > 
> > From RCU/checklist.txt:
> > 
> > "        An especially important property of the synchronize_rcu()
> >        primitive is that it automatically self-limits: if grace periods
> >        are delayed for whatever reason, then the synchronize_rcu()
> >        primitive will correspondingly delay updates.  In contrast,
> >        code using call_rcu() should explicitly limit update rate in
> >        cases where grace periods are delayed, as failing to do so can
> >        result in excessive realtime latencies or even OOM conditions.
> > "
> 
> I understand what you are worrying aboutâ Hmm, can it be avoided by
> just using kvm->arch.rcu_free_shadow_page in a small window? - Then
> there are slight chance that the page need to be freed by call_rcu.

The point that must be addressed is that you cannot allow an unlimited
number of sp's to be freed via call_rcu between two grace periods.

So something like:

- For every 17MB worth of shadow pages.
- Guarantee a grace period has passed.

If you control kvm->arch.rcu_free_shadow_page, you could periodically
verify how many MBs worth of shadow pages are in the queue for RCU
freeing and force grace period after a certain number.

> > Moreover, freeing pages differently depending on some state should 
> > be avoided.
> > 
> > Alternatives:
> > 
> > - Disable interrupts at write protect sites.
> 
> The write-protection can be triggered by KVM ioctl that is not in the VCPU
> context, if we do this, we also need to send IPI to the KVM thread when do
> TLB flush.

Yes. However for the case being measured, simultaneous page freeing by vcpus 
should be minimal (therefore not affecting the latency of GET_DIRTY_LOG).

> And we can not do much work while interrupt is disabled due to
> interrupt latency.
> 
> > - Rate limit the number of pages freed via call_rcu
> > per grace period.
> 
> Seems complex. :(
> 
> > - Some better alternative.
> 
> Gleb has a idea that uses RCU_DESTORY to protect the shadow page table
> and encodes the page-level into the spte (since we need to check if the spte
> is the last-spte. ).  How about this?

Pointer please? Why is DESTROY_SLAB_RCU any safer than call_rcu with
regards to limitation? (maybe it is).

> I planned to do it after this patchset merged, if you like it and if you think
> that "using kvm->arch.rcu_free_shadow_page in a small window" can not avoid
> the issue, i am happy to do it in the next version. :)

Unfortunately the window can be large (as it depends on the size of the
memslot), so it would be best if this problem can be addressed before 
merging. What is your idea for reducing rcu_free_shadow_page=1 window?

Thank you for the good work.

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