Re: [PATCH v2 4/5] KVM: s390: vsie: Add missing radix_tree_preload() in _gaccess_shadow_fault()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Ignoring the return value of radix_tree_preload() causes an unbalanced unlock and preempt count underflow if preloading fails.
--

commit c5536624d068cf6e0fec6f8851a47bfb6f4371b3
Author: Claudio Imbrenda <imbrenda@linux.ibm.com>

KVM: s390: vsie: Add missing radix_tree_preload() in _gaccess_shadow_fault()

This commit adds a missing radix_tree_preload() to _gaccess_shadow_fault()
to guarantee forward progress. It also splits the core of the function
into ___gaccess_shadow_fault() to simplify locking.

CC: stable@vger.kernel.org # 7.1

> diff --git a/arch/s390/kvm/gaccess.c b/arch/s390/kvm/gaccess.c
> index 20e28b183c1ac..c072b6872bf8b 100644
> --- a/arch/s390/kvm/gaccess.c
> +++ b/arch/s390/kvm/gaccess.c

[ ... ]

> @@ -1582,35 +1582,46 @@ static int _gaccess_do_shadow(struct kvm_s390_mmu_cache *mc, struct gmap *sg,

[ ... ]

>  static inline int _gaccess_shadow_fault(struct kvm_vcpu *vcpu, struct gmap *sg, gpa_t saddr,
>  					unsigned long seq, struct pgtwalk *walk)
>  {
>  	int rc;
>  
>  	if (kvm_s390_array_needs_retry_unsafe(vcpu->kvm, seq, walk->raw_entries))
>  		return -EAGAIN;
>  	rc = kvm_s390_mmu_cache_topup(vcpu->arch.mc);
>  	if (rc)
>  		return rc;
> +
> +	do {
> +		radix_tree_preload(GFP_KERNEL);
> +		scoped_guard(read_lock, &vcpu->kvm->mmu_lock)
> +			rc = ___gaccess_shadow_fault(vcpu, sg, saddr, seq, walk);
> +		radix_tree_preload_end();

[Severity: High]
Does this code handle radix_tree_preload() allocation failures correctly?

radix_tree_preload() can return -ENOMEM under memory pressure. When it fails,
it returns without disabling preemption or acquiring the local lock.

Since the return value is ignored here, radix_tree_preload_end() is
unconditionally called at the end of the loop iteration. Could this result
in an unbalanced unlock and preempt count underflow if the preload fails?

> +	} while (rc == -ENOMEM);
> +
>  	return rc;
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260610165247.238366-1-imbrenda@linux.ibm.com?part=4