Re: [PATCH 2/2] KVM: x86/mmu: Recover NX Huge pages belonging to TDP MMU under MMU read lock

[Sean Christopherson <seanjc@google.com>]

On Mon, Aug 12, 2024, Vipin Sharma wrote:
> @@ -1807,7 +1822,7 @@ ulong kvm_tdp_mmu_recover_nx_huge_pages(struct kvm *kvm, ulong to_zap)
>  	struct kvm_mmu_page *sp;
>  	bool flush = false;
>  
> -	lockdep_assert_held_write(&kvm->mmu_lock);
> +	lockdep_assert_held_read(&kvm->mmu_lock);
>  	/*
>  	 * Zapping TDP MMU shadow pages, including the remote TLB flush, must
>  	 * be done under RCU protection, because the pages are freed via RCU
> @@ -1821,7 +1836,6 @@ ulong kvm_tdp_mmu_recover_nx_huge_pages(struct kvm *kvm, ulong to_zap)
>  		if (!sp)
>  			break;
>  
> -		WARN_ON_ONCE(!sp->nx_huge_page_disallowed);
>  		WARN_ON_ONCE(!sp->role.direct);
>  
>  		/*
> @@ -1831,12 +1845,17 @@ ulong kvm_tdp_mmu_recover_nx_huge_pages(struct kvm *kvm, ulong to_zap)
>  		 * recovered, along with all the other huge pages in the slot,
>  		 * when dirty logging is disabled.
>  		 */
> -		if (kvm_mmu_sp_dirty_logging_enabled(kvm, sp))
> +		if (kvm_mmu_sp_dirty_logging_enabled(kvm, sp)) {
> +			spin_lock(&kvm->arch.tdp_mmu_pages_lock);
>  			unaccount_nx_huge_page(kvm, sp);
> -		else
> -			flush |= kvm_tdp_mmu_zap_sp(kvm, sp);
> -
> -		WARN_ON_ONCE(sp->nx_huge_page_disallowed);
> +			spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
> +			to_zap--;
> +			WARN_ON_ONCE(sp->nx_huge_page_disallowed);
> +		} else if (tdp_mmu_zap_sp(kvm, sp)) {
> +			flush = true;
> +			to_zap--;

This is actively dangerous.  In the old code, tdp_mmu_zap_sp() could fail only
in a WARN-able scenario, i.e. practice was guaranteed to succeed.  And, the
for-loop *always* decremented to_zap, i.e. couldn't get stuck in an infinite
loop.

Neither of those protections exist in this version.  Obviously it shouldn't happen,
but it's possible this could flail on the same SP over and over, since nothing
guarnatees forward progress.  The cond_resched() would save KVM from true pain,
but it's still a wart in the implementation.

Rather than loop on to_zap, just do

	list_for_each_entry(...) {

		if (!to_zap)
			break;
	}

And if we don't use separate lists, that'll be an improvement too, as it KVM
will only have to skip "wrong" shadow pages once, whereas this approach means
every iteration of the loop has to walk past the "wrong" shadow pages.

But I'd still prefer to use separate lists.