Re: [PATCH v2] KVM: x86/mmu: Do not recover dirty-tracked NX Huge Pages

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/7/22 22:21, Sean Christopherson wrote:
> 
> Hmm, and the memslot heuristic doesn't address the recovery worker holding mmu_lock
> for write.  On a non-preemptible kernel, rwlock_needbreak() is always false, e.g.
> the worker won't yield to vCPUs that are trying to handle non-fast page faults.
> The worker should eventually reach steady state by unaccounting everything, but
> that might take a while.

I'm not sure what you mean here?  The recovery worker will still 
decrease to_zap by 1 on every unaccounted NX hugepage, and go to sleep 
after it reaches 0.

Also, David's test used a 10-second halving time for the recovery 
thread.  With the 1 hour time the effect would Perhaps the 1 hour time 
used by default by KVM is overly conservative, but 1% over 10 seconds is 
certainly a lot larger an effect, than 1% over 1 hour.

So, I'm queuing the patch.

Paolo

> An alternative idea to the memslot heuristic would be to add a knob to allow
> disabling the recovery thread on a per-VM basis.  Userspace should know that it's
> dirty logging a given VM for migration.