Re: [PATCH v2 1/2] KVM: arm64: Fix spurious warning for benign stage 2 teardown race

[Yao Yuan]

On Sat, Aug 22, 2026 at 06:46:53PM +0800, Lorenzo Stoakes (ARM) wrote:
> A batch of kernel warnings were triggered in the L0 host kernel when using
> kvmtool to experiment with nested virtualisation.

...

> Fixes: ec14c272408a ("KVM: arm64: nv: Unmap/flush shadow stage 2 page tables")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> ---
>  arch/arm64/kvm/mmu.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 74e7e7f7564c..31e049ded093 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -59,19 +59,25 @@ static phys_addr_t stage2_range_addr_end(phys_addr_t addr, phys_addr_t end)
>   * long will also starve other vCPUs. We have to also make sure that the page
>   * tables are not freed while we released the lock.
>   */
> -static int stage2_apply_range(struct kvm_s2_mmu *mmu, phys_addr_t addr,
> +static int stage2_apply_range(struct kvm_s2_mmu *mmu, phys_addr_t start,
>  			      phys_addr_t end,
>  			      int (*fn)(struct kvm_pgtable *, u64, u64),
>  			      bool resched)
>  {
>  	struct kvm *kvm = kvm_s2_mmu_to_kvm(mmu);
> +	phys_addr_t addr = start;
>  	int ret;
>  	u64 next;
>
>  	do {
>  		struct kvm_pgtable *pgt = mmu->pgt;
> +		/*
> +		 * We may be raced on PGT teardown when we release the
> +		 * kvm->mmu_lock. That's fine as the PGT is legitimately no
> +		 * longer present.
> +		 */
>  		if (!pgt)
> -			return -EINVAL;
> +			return resched && addr > start ? 0 : -EINVAL;

Hi,

I can understand the addr checking makes sure only return 0
when the mmu_lock is dropped at least once and get pgt =
NULL, may a variable like drop_lock = true when
cond_resched_rwlock_write(&kvm->mmu_lock) happens can have
better readability, but depends on you and others' opinion.

Reviewed-by: Yuan Yao <yaoyuan@xxxxxxxxxxxxxxxxx>

>
>  		next = stage2_range_addr_end(addr, end);
>  		ret = fn(pgt, addr, next - addr);
>
> --
> 2.55.0