linux-kernel - Re: [PATCH] KVM: x86/mmu: Make page tables for eager page splitting NUMA aware

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Date:   Wed, 3 Aug 2022 17:08:25 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     David Matlack <dmatlack@...gle.com>,
        Vipin Sharma <vipinsh@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: Make page tables for eager page splitting
 NUMA aware

On 8/2/22 21:12, Sean Christopherson wrote:
> Ah crud, I misread the patch.  I was thinking tdp_mmu_spte_to_nid() was getting
> the node for the existing shadow page, but it's actually getting the node for the
> underlying physical huge page.
> 
> That means this patch is broken now that KVM doesn't require a "struct page" to
> create huge SPTEs (commit  a8ac499bb6ab ("KVM: x86/mmu: Don't require refcounted
> "struct page" to create huge SPTEs").  I.e. this will explode as pfn_to_page()
> may return garbage.
> 
> 	return page_to_nid(pfn_to_page(spte_to_pfn(spte)));

I was about to say that yesterday.  However my knowledge of struct page 
things has been proved to be spotty enough, that I wasn't 100% sure of 
that.  But yeah, with a fresh mind it's quite obvious that anything that 
goes through hva_to_pfn_remap and similar paths will fail.

> That said, I still don't like this patch, at least not as a one-off thing.  I do
> like the idea of having page table allocations follow the node requested for the
> target pfn, what I don't like is having one-off behavior that silently overrides
> userspace policy.

Yes, I totally agree with making it all or nothing.

> I would much rather we solve the problem for all page table allocations, maybe
> with a KVM_CAP or module param?  Unfortunately, that's a very non-trivial change
> because it will probably require having a per-node cache in each of the per-vCPU
> caches.

A module parameter is fine.  If you care about performance to this 
level, your userspace is probably homogeneous enough.

Paolo

> Hmm, actually, a not-awful alternative would be to have the fault path fallback
> to the current task's policy if allocation fails.  I.e. make it best effort.
> 
> E.g. as a rough sketch...
> 
> ---
>   arch/x86/kvm/mmu/tdp_mmu.c | 37 ++++++++++++++++++++++++++++++-------
>   1 file changed, 30 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index bf2ccf9debca..e475f5b55794 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -273,10 +273,11 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
> 
>   static struct kvm_mmu_page *tdp_mmu_alloc_sp(struct kvm_vcpu *vcpu)
>   {
> +	struct kvm_mmu_memory_cache *cache = &vcpu->arch.mmu_shadow_page_cache;
>   	struct kvm_mmu_page *sp;
> 
>   	sp = kvm_mmu_memory_cache_alloc(&vcpu->arch.mmu_page_header_cache);
> -	sp->spt = kvm_mmu_memory_cache_alloc(&vcpu->arch.mmu_shadow_page_cache);
> +	sp->spt = kvm_mmu_alloc_shadow_page_table(cache, GFP_NOWAIT, pfn);
> 
>   	return sp;
>   }
> @@ -1190,7 +1191,7 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>   			if (is_removed_spte(iter.old_spte))
>   				break;
> 
> -			sp = tdp_mmu_alloc_sp(vcpu);
> +			sp = tdp_mmu_alloc_sp(vcpu, fault->pfn);
>   			tdp_mmu_init_child_sp(sp, &iter);
> 
>   			if (tdp_mmu_link_sp(vcpu->kvm, &iter, sp, account_nx, true)) {
> @@ -1402,17 +1403,39 @@ bool kvm_tdp_mmu_wrprot_slot(struct kvm *kvm,
>   	return spte_set;
>   }
> 
> -static struct kvm_mmu_page *__tdp_mmu_alloc_sp_for_split(gfp_t gfp)
> +void *kvm_mmu_alloc_shadow_page_table(struct kvm_mmu_memory_cache *cache,
> +				      gfp_t gfp, kvm_pfn_t pfn)
> +{
> +	struct page *page = kvm_pfn_to_refcounted_page(pfn);
> +	struct page *spt_page;
> +	int node;
> +
> +	gfp |= __GFP_ZERO | __GFP_ACCOUNT;
> +
> +	if (page) {
> +		spt_page = alloc_pages_node(page_to_nid(page), gfp, 0);
> +		if (spt_page)
> +			return page_address(spt_page);
> +	} else if (!cache) {
> +		return (void *)__get_free_page(gfp);
> +	}
> +
> +	if (cache)
> +		return kvm_mmu_memory_cache_alloc(cache);
> +
> +	return NULL;
> +}
> +
> +static struct kvm_mmu_page *__tdp_mmu_alloc_sp_for_split(gfp_t gfp,
> +							 kvm_pfn_t pfn)
>   {
>   	struct kvm_mmu_page *sp;
> 
> -	gfp |= __GFP_ZERO;
> -
>   	sp = kmem_cache_alloc(mmu_page_header_cache, gfp);
>   	if (!sp)
>   		return NULL;
> 
> -	sp->spt = (void *)__get_free_page(gfp);
> +	sp->spt = kvm_mmu_alloc_shadow_page_table(NULL, gfp, pfn);
>   	if (!sp->spt) {
>   		kmem_cache_free(mmu_page_header_cache, sp);
>   		return NULL;
> @@ -1436,7 +1459,7 @@ static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(struct kvm *kvm,
>   	 * If this allocation fails we drop the lock and retry with reclaim
>   	 * allowed.
>   	 */
> -	sp = __tdp_mmu_alloc_sp_for_split(GFP_NOWAIT | __GFP_ACCOUNT);
> +	sp = __tdp_mmu_alloc_sp_for_split(GFP_NOWAIT);
>   	if (sp)
>   		return sp;
> 
> 
> base-commit: f8990bfe1eab91c807ca8fc0d48705e8f986b951
> --
>