linux-kernel - Re: [PATCH] KVM: selftests: Wait mprotect_ro_done before write to RO in mmu_stress

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Message-ID: <Z750LaPTDS6z6DAK@google.com>
Date: Tue, 25 Feb 2025 17:53:49 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Rick P Edgecombe <rick.p.edgecombe@...el.com>
Cc: Yan Y Zhao <yan.y.zhao@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: selftests: Wait mprotect_ro_done before write to RO
 in mmu_stress_test

On Tue, Feb 11, 2025, Rick P Edgecombe wrote:
> On Tue, 2025-02-11 at 09:42 +0800, Yan Zhao wrote:
> > > On the fix though, doesn't this remove the coverage of writing to a
> > > region that is in the process of being made RO? I'm thinking about
> > > warnings, etc that may trigger intermittently based on bugs with a race
> > > component. I don't know if we could fix the test and still leave the
> > > write while the "mprotect(PROT_READ) is underway". It seems to be
> > > deliberate.
> > Write before "mprotect(PROT_READ)" has been tested in stage 0.
> > Not sure it's deliberate to test write in the process of being made RO.

Writing while VMAs are being made RO is 100% intended.  The goal is to stress
KVM's interactions with the mmu_notifier, and to verify KVM delivers -EFAULT to
userspace.

Something isn't quite right in the original analysis.  We need to drill down on
that before change anything.

FWIW, I run this test frequently on large systems and have never observed failures.
Maybe Rick and I should go buy lottery tickets?