Re: [PATCH v4 2/3] KVM: PPC: Book3S HV: Add preempt_disable() around virtual-mode HPTE bit-lock users

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] kvm: deadlock in kvm_test_age_rmapp() due to missing preempt_disable()
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commit 6b1add31442320e6307f450d080fd72f64ee2368
Author: Amit Machhiwal <amachhiw@linux.ibm.com>

KVM: PPC: Book3S HV: Add preempt_disable() around virtual-mode HPTE bit-lock users

This commit enforces that threads acquiring HPTE or RMAP bit-locks run with
preemption disabled to prevent CPU hangs. It adds preempt_disable() and
preempt_enable() pairs around various bit-lock acquisition paths.

> diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> index 2ccb3d138f46c..e8f73c8f97803 100644
> --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
> +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> @@ -890,6 +913,7 @@ static bool kvm_age_rmapp(struct kvm *kvm, struct kvm_memory_slot *memslot,
>  
>  	rmapp = &memslot->arch.rmap[gfn - memslot->base_gfn];
>   retry:
> +	preempt_disable();
>  	lock_rmap(rmapp);
>  	if (*rmapp & KVMPPC_RMAP_REFERENCED) {
>  		*rmapp &= ~KVMPPC_RMAP_REFERENCED;

[Severity: High]
Does kvm_test_age_rmapp() also need to disable preemption before acquiring
the rmap lock?

Looking at arch/powerpc/kvm/book3s_64_mmu_hv.c:kvm_test_age_rmapp():

	lock_rmap(rmapp);
	if (*rmapp & KVMPPC_RMAP_REFERENCED)
		goto out;

This MMU notifier callback still acquires the KVMPPC_RMAP_LOCK_BIT without
disabling preemption.

If a thread is preempted while holding this bit-lock, and another thread on
the same CPU begins spinning for the same lock with preemption disabled
(such as in the newly patched paths), couldn't this result in a permanent
CPU hang?

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Sashiko AI review · https://sashiko.dev/#/patchset/20261007182116.12479-1-amachhiw@linux.ibm.com?part=2