Re: [PATCH 4/4] KVM: Nullify async #PF worker's "apf" pointer as soon as it might be freed

Vitaly Kuznetsov
2024-01-26T16:30:39Z

Sean Christopherson <seanjc@google.com> writes:

> Nullify the async #PF worker's local "apf" pointer immediately after the
> point where the structure can be freed by the vCPU. The existing comment
> is helpful, but easy to overlook as there is no associated code.
>
> Update the comment to clarify that it can be freed by as soon as the lock
> is dropped, as "after this point" isn't strictly accurate, nor does it
> help understand what prevents the structure from being freed earlier.
>

"No functional change intended." must be made a requirement, especially
for those who made it their trademark)

> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> virt/kvm/async_pf.c | 11 ++++++-----
> 1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
> index c3f4f351a2ae..1088c6628de9 100644
> --- a/virt/kvm/async_pf.c
> +++ b/virt/kvm/async_pf.c
> @@ -83,13 +83,14 @@ static void async_pf_execute(struct work_struct *work)
> apf->vcpu = NULL;
> spin_unlock(&vcpu->async_pf.lock);
>
> - if (!IS_ENABLED(CONFIG_KVM_ASYNC_PF_SYNC) && first)
> - kvm_arch_async_page_present_queued(vcpu);
> -
> /*
> - * apf may be freed by kvm_check_async_pf_completion() after
> - * this point
> + * The apf struct may freed by kvm_check_async_pf_completion() as soon

Nit: "may be freed"/"may get freed" maybe?

> + * as the lock is dropped. Nullify it to prevent improper usage.
> */
> + apf = NULL;
> +
> + if (!IS_ENABLED(CONFIG_KVM_ASYNC_PF_SYNC) && first)
> + kvm_arch_async_page_present_queued(vcpu);
>
> trace_kvm_async_pf_completed(addr, cr2_or_gpa);

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

--
Vitaly