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

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Date: Wed, 12 Jun 2024 23:52:54 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
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 at 12:49:49PM -0700, Sean Christopherson wrote:
> 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.

Yeah, I think I'm headed in the same direction after actually reading
the MM side of this, heh.

-- 
Thanks,
Oliver