Re: [syzbot] [kvm?] WARNING: locking bug in __refill_objects_node

[Paolo Bonzini]

On Mon, Nov 2, 2026 at 6:43 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > If so, we need to make that a thread for RT kernels if you want to call
> > kzalloc_obj().
>
> Eww.  I can't imagine there's a sane usecase for memory overcommitting a real-time
> guest.

While I agree with the feeling, the dummy allocation path should be
mostly dead, because page fault should never (?) enable interrupts
before waiting:

#PF interrupt gate
  exc_page_fault()
    kvm_handle_async_pf()
      __kvm_handle_async_pf()
        irqentry_enter()
        kvm_async_pf_task_wait_schedule()
          kvm_async_pf_queue_task() <- registers waiter
          prepare_to_swait_exclusive()
          local_irq_enable()

The dummy allocation path is only reachable if it's an L1 hypervisor
that is getting an async page fault while L2 is running, because it
enables irq before the call to kvm_async_pf_task_wait_schedule(). I
think we can drop the dummy allocation for good, which would be even
better.

Whether you would still want to disable APF for RT kernels is up to you.

Paolo