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

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Message-ID: <Z5e8umkPeRri0Z_p@kbusch-mbp>
Date: Mon, 27 Jan 2025 10:04:58 -0700
From: Keith Busch <kbusch@...nel.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: Ensure NX huge page recovery thread is
 alive before waking

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.

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.