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

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Date: Tue, 2 Apr 2024 00:06:56 -0400
From: Yu Zhao <yuzhao@...gle.com>
To: James Houghton <jthoughton@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Paolo Bonzini <pbonzini@...hat.com>, 
	David Matlack <dmatlack@...gle.com>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, Sean Christopherson <seanjc@...gle.com>, 
	Jonathan Corbet <corbet@....net>, James Morse <james.morse@....com>, 
	Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu <yuzenghui@...wei.com>, 
	Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Shaoqin Huang <shahuang@...hat.com>, 
	Gavin Shan <gshan@...hat.com>, Ricardo Koller <ricarkol@...gle.com>, 
	Raghavendra Rao Ananta <rananta@...gle.com>, Ryan Roberts <ryan.roberts@....com>, 
	David Rientjes <rientjes@...gle.com>, Axel Rasmussen <axelrasmussen@...gle.com>, 
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	kvmarm@...ts.linux.dev, kvm@...r.kernel.org, linux-mm@...ck.org, 
	linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v3 6/7] KVM: arm64: Participate in bitmap-based PTE aging

On Mon, Apr 1, 2024 at 7:30 PM James Houghton <jthoughton@...gle.com> wrote:
>
> Participate in bitmap-based aging while grabbing the KVM MMU lock for
> reading. Ideally we wouldn't need to grab this lock at all, but that
> would require a more intrustive and risky change.
                       ^^^^^^^^^^ intrusive
This sounds subjective -- I'd just present the challenges and let
reviewers make their own judgements.

> Also pass
> KVM_PGTABLE_WALK_SHARED, as this software walker is safe to run in
> parallel with other walkers.
>
> It is safe only to grab the KVM MMU lock for reading as the kvm_pgtable
> is destroyed while holding the lock for writing, and freeing of the page
> table pages is either done while holding the MMU lock for writing or
> after an RCU grace period.
>
> When mkold == false, record the young pages in the passed-in bitmap.
>
> When mkold == true, only age the pages that need aging according to the
> passed-in bitmap.
>
> Suggested-by: Yu Zhao <yuzhao@...gle.com>

Thanks but I did not suggest this.

What I have in v2 is RCU based. I hope Oliver or someone else can help
make that work. Otherwise we can just drop this for now and revisit
later.

(I have no problems with this patch if the Arm folks think the
RCU-based version doesn't have a good ROI.)