Re: [PATCH v3 6/7] KVM: arm64: Participate in bitmap-based PTE aging

From: Oliver Upton <oliver.upton@linux.dev>
To: Yu Zhao <yuzhao@google.com>
Cc: James Houghton <jthoughton@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	David Matlack <dmatlack@google.com>,
	Marc Zyngier <maz@kernel.org>,
	Sean Christopherson <seanjc@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Shaoqin Huang <shahuang@redhat.com>,
	Gavin Shan <gshan@redhat.com>,
	Ricardo Koller <ricarkol@google.com>,
	Raghavendra Rao Ananta <rananta@google.com>,
	Ryan Roberts <ryan.roberts@arm.com>,
	David Rientjes <rientjes@google.com>,
	Axel Rasmussen <axelrasmussen@google.com>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	kvm@vger.kernel.org, linux-mm@kvack.org,
	linux-trace-kernel@vger.kernel.org
Subject: Re: [PATCH v3 6/7] KVM: arm64: Participate in bitmap-based PTE aging
Date: Tue, 2 Apr 2024 00:00:12 -0700	[thread overview]
Message-ID: <Zgus_A_cJ68f6glV@linux.dev> (raw)
In-Reply-To: <CAOUHufaQ-g6L5roB-3K0GamuS3p9ACpPj9XM-NF67GgrjoTj_A@mail.gmail.com>

On Tue, Apr 02, 2024 at 12:06:56AM -0400, Yu Zhao wrote:
> On Mon, Apr 1, 2024 at 7:30 PM James Houghton <jthoughton@google.com> wrote:
> > Suggested-by: Yu Zhao <yuzhao@google.com>
> 
> Thanks but I did not suggest this.

Entirely up to you, but I would still want to credit everyone who
contributed to a feature even if the underlying implementation has
changed since the original attempt.

> What I have in v2 is RCU based. I hope Oliver or someone else can help
> make that work.

Why? If there's data to show that RCU has a material benefit over taking
the MMU lock for read then I'm all for it. Otherwise, the work required
to support page-table walkers entirely outside of the MMU lock isn't
justified.

In addition to ensuring that page table teardown is always RCU-safe,
we'd need to make sure all of the walkers that take the write lock are
prepared to handle races.

> Otherwise we can just drop this for now and revisit
> later.

I really wouldn't get hung up on the locking as the make-or-break for
whether arm64 supports this MMU notifier.

-- 
Thanks,
Oliver