On Tue, 14 Aug 2012 12:17:12 -0300
Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> - if (kvm->arch.n_used_mmu_pages > 0) {
> - if (!nr_to_scan--)
> - break;
-- (*1)
> + if (!kvm->arch.n_used_mmu_pages)
> continue;
-- (*2)
> - }
>
> idx = srcu_read_lock(&kvm->srcu);
> spin_lock(&kvm->mmu_lock);
>
> This patch removes the maximum (successful) loops, which is nr_scan ==
> sc->nr_to_scan.
IIUC, there was no successful loop from the beginning:
if (kvm->arch.n_used_mmu_pages > 0) {
if (!nr_to_scan--)
break; -- (*1)
continue; -- (*2)
}
Before the patch even when we find a VM with kvm->arch.n_used_mmu_pages
greater than 0, we just do either:
skip it (*2) or
break (*1) if nr_to_scan becomes 0.
We only reach to
kvm_mmu_remove_some_alloc_mmu_pages(kvm, &invalid_list);
kvm_mmu_commit_zap_page(kvm, &invalid_list);
when (kvm->arch.n_used_mmu_pages == 0) that is probably why
commit 85b7059169e128c57a3a8a3e588fb89cb2031da1
KVM: MMU: fix shrinking page from the empty mmu
could hit the very unlikely condition so easily.
So we are just looping for trying to free from empty MMUs.
> The description above where you say 'possibility that we see
> "n_used_mmu_pages == 0" 128 times' does not match the patch above.
Sorry about that.
> If the patch is correct, then please explain it clearly in the
> changelog.
Yes, I will do so.
> What is the reasoning to remove nr_to_scan? What tests did you perform?
I just confirmed:
- mmu_shrink() did not free any pages:
just checked all VMs and did "continue"
- with my patch, it could free from the first VM with (n_used_mmu_pages > 0)
About nr_to_scan:
If my explanation above is right, this is not functioning at all.
But since it will not hurt anyone and may help us when we change
our batch size, I won't remove it in the next version.
Thanks,
Takuya
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