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Date: Wed, 12 Jun 2024 23:52:54 -0700
From: Oliver Upton <oliver.upton@linux.dev>
To: Sean Christopherson <seanjc@google.com>
Cc: James Houghton <jthoughton@google.com>, Yu Zhao <yuzhao@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Ankit Agrawal <ankita@nvidia.com>,
	Axel Rasmussen <axelrasmussen@google.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	David Matlack <dmatlack@google.com>,
	David Rientjes <rientjes@google.com>,
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	Shaoqin Huang <shahuang@redhat.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
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	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [PATCH v5 4/9] mm: Add test_clear_young_fast_only MMU notifier
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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