From: Oliver Upton <oupton@kernel•org>
To: Hyunwoo Kim <imv4bel@gmail•com>
Cc: maz@kernel•org, joey.gouly@arm•com, seiden@linux•ibm.com,
suzuki.poulose@arm•com, yuzenghui@huawei•com,
catalin.marinas@arm•com, will@kernel•org,
christoffer.dall@arm•com, linux-arm-kernel@lists•infradead.org,
kvmarm@lists•linux.dev
Subject: Re: [PATCH] KVM: arm64: Reallocate the nested_mmus array under the mmu_lock
Date: Thu, 4 Jun 2026 15:27:16 -0700 [thread overview]
Message-ID: <aiH7xKnprU9bUap5@kernel.org> (raw)
In-Reply-To: <aiHEKOeZMVwsRlvP@v4bel>
Hi,
The shortlog is very confusing, since "allocate behind $LOCK" is usually
something alarming. Maybe instead:
KVM: arm64: Reassign nested_mmus array behind mmu_lock
On Fri, Jun 05, 2026 at 03:30:00AM +0900, Hyunwoo Kim wrote:
> Code that walks kvm->arch.nested_mmus[] holds kvm->mmu_lock. By contrast,
> kvm_vcpu_init_nested() reallocates the array and frees the old buffer while
> holding only kvm->arch.config_lock, so a walker can reference the freed
> array.
It wouldn't hurt to share slightly more information here. Are you
dealing with a concurrent MMU notifier?
> Allocate the new array outside the lock, as the allocation can sleep, and
> do only the copy and the pointer swap under the mmu_lock. After the swap no
> walker can reach the old buffer, so free it once the lock has been
> released.
>
> Fixes: 4f128f8e1aaac ("KVM: arm64: nv: Support multiple nested Stage-2 mmu structures")
> Signed-off-by: Hyunwoo Kim <imv4bel@gmail•com>
The diff itself LGTM
Reviewed-by: Oliver Upton <oupton@kernel•org>
Thanks,
Oliver
> ---
> arch/arm64/kvm/nested.c | 33 ++++++++++++++++++++-------------
> 1 file changed, 20 insertions(+), 13 deletions(-)
>
> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> index 38f672e940878..6f7bc9a9992e0 100644
> --- a/arch/arm64/kvm/nested.c
> +++ b/arch/arm64/kvm/nested.c
> @@ -89,21 +89,28 @@ int kvm_vcpu_init_nested(struct kvm_vcpu *vcpu)
> * again, and there is no reason to affect the whole VM for this.
> */
> num_mmus = atomic_read(&kvm->online_vcpus) * S2_MMU_PER_VCPU;
> - tmp = kvrealloc(kvm->arch.nested_mmus,
> - size_mul(sizeof(*kvm->arch.nested_mmus), num_mmus),
> - GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> - if (!tmp)
> - return -ENOMEM;
>
> - swap(kvm->arch.nested_mmus, tmp);
> + if (num_mmus > kvm->arch.nested_mmus_size) {
> + tmp = kvcalloc(num_mmus, sizeof(*tmp), GFP_KERNEL_ACCOUNT);
> + if (!tmp)
> + return -ENOMEM;
>
> - /*
> - * If we went through a realocation, adjust the MMU back-pointers in
> - * the previously initialised kvm_pgtable structures.
> - */
> - if (kvm->arch.nested_mmus != tmp)
> - for (int i = 0; i < kvm->arch.nested_mmus_size; i++)
> - kvm->arch.nested_mmus[i].pgt->mmu = &kvm->arch.nested_mmus[i];
> + write_lock(&kvm->mmu_lock);
> +
> + if (kvm->arch.nested_mmus_size) {
> + memcpy(tmp, kvm->arch.nested_mmus,
> + size_mul(sizeof(*tmp), kvm->arch.nested_mmus_size));
> +
> + for (int i = 0; i < kvm->arch.nested_mmus_size; i++)
> + tmp[i].pgt->mmu = &tmp[i];
> + }
> +
> + swap(kvm->arch.nested_mmus, tmp);
> +
> + write_unlock(&kvm->mmu_lock);
> +
> + kvfree(tmp);
> + }
>
> for (int i = kvm->arch.nested_mmus_size; !ret && i < num_mmus; i++)
> ret = init_nested_s2_mmu(kvm, &kvm->arch.nested_mmus[i]);
> --
> 2.43.0
>
next prev parent reply other threads:[~2026-06-04 22:27 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-06-04 18:30 [PATCH] KVM: arm64: Reallocate the nested_mmus array under the mmu_lock Hyunwoo Kim
2026-06-04 22:27 ` Oliver Upton [this message]
2026-06-04 22:58 ` Sean Christopherson
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