Re: [PATCH] KVM: x86/mmu: Ensure NX huge page recovery thread is alive before waking

[Sean Christopherson <seanjc@google.com>]

On Mon, Jan 27, 2025, Keith Busch wrote:
> On Mon, Jan 27, 2025 at 08:48:03AM -0800, Sean Christopherson wrote:
> > If vhost_task_create() fails, then the call_once() will "succeed" and mark the
> > structure as ONCE_COMPLETED.  The first KVM_RUN will fail with -ENOMEM, but any
> > subsequent calls will succeed, including in-flight KVM_RUNs on other threads.
> 
> The criteria for returning -ENOMEM for any KVM_RUN is if we have a NULL
> nx_huge_page_recovery_thread vhost_task. So I think that part, at least,
> is fine.
> 
> The call_once is just needed to ensure that only the very first KVM_RUN
> even tries to create it. If the vhost_task_create fails, then all the
> KVM_RUN threads will see the NULL nx_huge_page_recovery_thread and
> return -ENOMEM.

Ah, duh, because the check is performed by the caller, outside of the "once"
protection.

> What you're suggesting here will allow a subsequent thread to attempt
> creating the vhost task if the first one failed. Maybe you do want to
> try again, but the current upstream code doesn't retry this, so I
> thought it best to keep that behavior.

No strong opinion.  In practice, it's a moot point because the odds of a VM being
able to make forward progress if task creation hits an OOM are basically nil.

I'll defer to Paolo on what he thinks is best for the call_once() API.