linux-kernel - Re: [PATCH v5 4/9] mm: Add test_clear_young_fast

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Message-ID: <ZmiqXUwMXtUGanQc@google.com>
Date: Tue, 11 Jun 2024 12:49:49 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Oliver Upton <oliver.upton@...ux.dev>
Cc: James Houghton <jthoughton@...gle.com>, Yu Zhao <yuzhao@...gle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Paolo Bonzini <pbonzini@...hat.com>, 
	Ankit Agrawal <ankita@...dia.com>, Axel Rasmussen <axelrasmussen@...gle.com>, 
	Catalin Marinas <catalin.marinas@....com>, David Matlack <dmatlack@...gle.com>, 
	David Rientjes <rientjes@...gle.com>, James Morse <james.morse@....com>, 
	Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>, 
	Raghavendra Rao Ananta <rananta@...gle.com>, Ryan Roberts <ryan.roberts@....com>, 
	Shaoqin Huang <shahuang@...hat.com>, Suzuki K Poulose <suzuki.poulose@....com>, 
	Wei Xu <weixugc@...gle.com>, Will Deacon <will@...nel.org>, Zenghui Yu <yuzenghui@...wei.com>, 
	kvmarm@...ts.linux.dev, kvm@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v5 4/9] mm: Add test_clear_young_fast_only MMU notifier

On Tue, Jun 11, 2024, Oliver Upton wrote:
> On Tue, Jun 11, 2024 at 09:49:59AM -0700, James Houghton wrote:
> > I think consolidating the callbacks is cleanest, like you had it in
> > v2. I really wasn't sure about this change honestly, but it was my
> > attempt to incorporate feedback like this[3] from v4. I'll consolidate
> > the callbacks like you had in v2.
> 
> My strong preference is to have the callers expectations of the
> secondary MMU be explicit. Having ->${BLAH}_fast_only() makes this
> abundantly clear both at the callsite and in the implementation.

Partially agreed.  We don't need a dedicated mmu_notifier API to achieve that
for the callsites, e.g. ptep_clear_young_notify() passes fast_only=false, and a
new ptep_clear_young_notify_fast_only() does the obvious.

On the back end, odds are very good KVM is going to squish the "fast" and "slow"
paths back into a common helper, so IMO having dedicated fast_only() APIs for the
mmu_notifier hooks doesn't add much value in the end.

I'm not opposed to dedicated hooks, but I after poking around a bit, I suspect
that passing a fast_only flag will end up being less cleaner for all parties.