Re: [PATCH v4 12/23] KVM: Destroy memslots immediately after mmu_notifiers are unregistered

[Sean Christopherson <seanjc@google.com>]

+Ritesh as well

On Mon, Nov 02, 2026, Sean Christopherson wrote:
> +Gautam and PPC lists/maintainers
> 
> On Fri, Oct 30, 2026, sashiko-bot@kernel.org wrote:
> > Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> > - [Medium] KVM: PPC: Book3S HV: rmap_nested memory leak in kvmhv_release_all_nested()
> > --
> > 
> > commit 9c25a1b4a76e8526eb8a2b11e1b9e63893b0de80
> > Author: Sean Christopherson <seanjc@google.com>
> > 
> > KVM: Destroy memslots immediately after mmu_notifiers are unregistered
> > 
> > This commit replaces active memslots with an empty memslot set and frees all
> > memslots immediately after unregistering the MMU notifier in kvm_destroy_vm(),
> > prior to calling kvm_arch_destroy_vm().
> > 
> > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > > index fa548b12228ff..2eda12a52bf4b 100644
> > > --- a/virt/kvm/kvm_main.c
> > > +++ b/virt/kvm/kvm_main.c
> > 
> > [ ... ]
> > 
> > > @@ -1310,12 +1329,10 @@ static void kvm_destroy_vm(struct kvm *kvm)
> > >  		kvm->mn_active_invalidate_count = 0;
> > >  	else
> > >  		WARN_ON(kvm->mmu_invalidate_in_progress);
> > > +	kvm_destroy_memslots(kvm);
> > > +
> > >  	kvm_arch_destroy_vm(kvm);
> > >  	kvm_destroy_devices(kvm);
> > > -	for (i = 0; i < kvm_arch_nr_memslot_as_ids(kvm); i++) {
> > > -		kvm_free_memslots(kvm, &kvm->__memslots[i][0]);
> > > -		kvm_free_memslots(kvm, &kvm->__memslots[i][1]);
> > > -	}
> > 
> > [Severity: Medium]
> > Does hoisting kvm_destroy_memslots() above kvm_arch_destroy_vm() leak
> > struct rmap_nested allocations in slot->arch.rmap on PowerPC Book3S HV when
> > destroying a VM with nested virtualization enabled?
> 
> I don't think so?  Patch 3[*], "KVM: PPC: Flush/zap all memslots on kvm_arch_flush_shadow_all()",
> should free the rmaps in response to kvm_arch_flush_shadow_all().   From what I
> can tell, the page tables have to be in radix mode, and so the rmaps are freed via:
> 
>   kvmppc_radix_flush_memslot()
>   |
>   -> kvmppc_unmap_pte()
>      |
>      -> kvmhv_remove_nest_rmap_range()
>         |
>         -> kvmhv_remove_nest_rmap_list()
> 
> And if that doesn't work, it means memslot deletion on an active VM is completely
> broken.
> 
> However, it means the memslot code in kvmhv_release_all_nested() is dead code, so
> this as follow-up to patch 3?  Or maybe folded in?
> 
> diff --git a/arch/powerpc/kvm/book3s_hv_nested.c b/arch/powerpc/kvm/book3s_hv_nested.c
> index a6ff42d7666c..27985210f972 100644
> --- a/arch/powerpc/kvm/book3s_hv_nested.c
> +++ b/arch/powerpc/kvm/book3s_hv_nested.c
> @@ -802,8 +802,6 @@ void kvmhv_release_all_nested(struct kvm *kvm)
>  	int lpid;
>  	struct kvm_nested_guest *gp;
>  	struct kvm_nested_guest *freelist = NULL;
> -	struct kvm_memory_slot *memslot;
> -	int srcu_idx, bkt;
>  
>  	spin_lock(&kvm->mmu_lock);
>  	idr_for_each_entry(&kvm->arch.kvm_nested_guest_idr, gp, lpid) {
> @@ -820,11 +818,6 @@ void kvmhv_release_all_nested(struct kvm *kvm)
>  		freelist = gp->next;
>  		kvmhv_release_nested(gp);
>  	}
> -
> -	srcu_idx = srcu_read_lock(&kvm->srcu);
> -	kvm_for_each_memslot(memslot, bkt, kvm_memslots(kvm))
> -		kvmhv_free_memslot_nest_rmap(memslot);
> -	srcu_read_unlock(&kvm->srcu, srcu_idx);
>  }
>  
>  /* caller must hold gp->tlb_lock */
> 			
> 
> [*] https://lore.kernel.org/all/20261030212112.2598640-4-seanjc@google.com
> 
> > In virt/kvm/kvm_main.c, kvm_destroy_memslots() replaces kvm->memslots[i]
> > with kvm_empty_memslots and frees each memslot via
> > kvm_free_memslot() -> kvm_arch_free_memslot() -> kvmppc_core_free_memslot_hv()
> > in arch/powerpc/kvm/book3s_hv.c:
> > 
> >     static void kvmppc_core_free_memslot_hv(struct kvm_memory_slot *slot)
> >     {
> >             vfree(slot->arch.rmap);
> >             slot->arch.rmap = NULL;
> >     }
> > 
> > which frees the slot->arch.rmap array without freeing the linked
> > struct rmap_nested entries.
> > 
> > Previously, those entries were freed before memslot destruction via
> > kvm_arch_destroy_vm() -> kvmppc_core_destroy_vm_hv() ->
> > kvmhv_release_all_nested() in arch/powerpc/kvm/book3s_hv_nested.c:
> > 
> >     srcu_idx = srcu_read_lock(&kvm->srcu);
> >     kvm_for_each_memslot(memslot, bkt, kvm_memslots(kvm))
> >             kvmhv_free_memslot_nest_rmap(memslot);
> >     srcu_read_unlock(&kvm->srcu, srcu_idx);
> > 
> > With kvm_destroy_memslots() running before kvm_arch_destroy_vm(),
> > kvm_memslots(kvm) now returns kvm_empty_memslots and the memslots have
> > already been freed when kvmhv_release_all_nested() runs, so
> > kvmhv_free_memslot_nest_rmap() is never called. Any struct rmap_nested
> > entries remaining in slot->arch.rmap at teardown (for example, when
> > KVMPPC_SECURE_INIT_DONE causes kvmppc_radix_flush_memslot() to return early,
> > or when 4KB PTEs were replaced by a huge page in kvmppc_unmap_free_pte())
> > will be leaked.
> > 
> > -- 
> > Sashiko AI review · https://sashiko.dev/#/patchset/20261030212112.2598640-1-seanjc@google.com?part=12
> > 
>