linux-kernel - Re: [PATCH 1/3] KVM: x86/mmu: Dynamically allocate shadow MMU's hashed page list

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Message-ID: <Z9hXpERDYZX9pj6V@google.com>
Date: Mon, 17 Mar 2025 10:11:00 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Vipin Sharma <vipinsh@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] KVM: x86/mmu: Dynamically allocate shadow MMU's
 hashed page list

On Mon, Mar 17, 2025, Vipin Sharma wrote:
> On 2025-03-14 19:40:08, Sean Christopherson wrote:
> > Dynamically allocate the (massive) array of hashed lists used to track
> > shadow pages, as the array itself is 32KiB, i.e. is an order-3 allocation
> > all on its own, and is *exactly* an order-3 allocation.  Dynamically
> > allocating the array will allow allocating "struct kvm" using regular
> > kmalloc(), and will also allow deferring allocation of the array until
> > it's actually needed, i.e. until the first shadow root is allocated.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >  arch/x86/include/asm/kvm_host.h |  4 ++--
> >  arch/x86/kvm/mmu/mmu.c          | 21 ++++++++++++++++++++-
> >  arch/x86/kvm/x86.c              |  5 ++++-
> >  3 files changed, 26 insertions(+), 4 deletions(-)
> > 
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -6673,13 +6685,19 @@ static void kvm_mmu_zap_all_fast(struct kvm *kvm)
> >  		kvm_tdp_mmu_zap_invalidated_roots(kvm, true);
> >  }
> >  
> > -void kvm_mmu_init_vm(struct kvm *kvm)
> > +int kvm_mmu_init_vm(struct kvm *kvm)
> >  {
> > +	int r;
> > +
> >  	kvm->arch.shadow_mmio_value = shadow_mmio_value;
> >  	INIT_LIST_HEAD(&kvm->arch.active_mmu_pages);
> >  	INIT_LIST_HEAD(&kvm->arch.possible_nx_huge_pages);
> >  	spin_lock_init(&kvm->arch.mmu_unsync_pages_lock);
> >  
> > +	r = kvm_mmu_alloc_page_hash(kvm);
> > +	if (r)
> > +		return r;
> > +
> 
> In the patch 3, shouldn't this be moved to else part of the below 
> 'if (tdp_mmu_enabled)' line? Otherwise, this hash array will always get
> allocated.

Ugh, I botched the rebase, and didn't point test that the allocations actually
went away.

Before commit 0df9dab891ff ("KVM: x86/mmu: Stop zapping invalidated TDP MMU roots
asynchronously"), kvm_mmu_init_tdp_mmu() returned a value and so the code was:

	if (tdp_mmu_enabled)
		r = kvm_mmu_init_tdp_mmu(kvm);
	else
		r = kvm_mmu_alloc_page_hash(kvm);
	if (r < 0)
		return r;

I suppose the least ugly approach is:

	if (tdp_mmu_enabled) {
		kvm_mmu_init_tdp_mmu(kvm);
	} else {
		r = kvm_mmu_alloc_page_hash(kvm);
		if (r)
			return r;
	}

> >  	if (tdp_mmu_enabled)
> >  		kvm_mmu_init_tdp_mmu(kvm);
> >  
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -12704,7 +12704,9 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
> >  	if (ret)
> >  		goto out;
> >  
> > -	kvm_mmu_init_vm(kvm);
> > +	ret = kvm_mmu_init_vm(kvm);
> > +	if (ret)
> > +		goto out_cleanup_page_track;
> >  
> >  	ret = kvm_x86_call(vm_init)(kvm);
> >  	if (ret)
> > @@ -12757,6 +12759,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
> >  
> >  out_uninit_mmu:
> >  	kvm_mmu_uninit_vm(kvm);
> > +out_cleanup_page_track:
> 
> I think there is a memory leak in this series.

/facepalm

Good job, me.

Thanks for the review!