linux-kernel - Re: [PATCH v6 05/11] mm: Add fast_only bool to test_young and clear

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Message-ID: <CADrL8HXFK=1cUS+0Z5k048U4rzpTNL634f57VtJ7TD_umrbNiA@mail.gmail.com>
Date: Wed, 7 Aug 2024 08:02:26 -0700
From: James Houghton <jthoughton@...gle.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: David Hildenbrand <david@...hat.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>, 
	Oliver Upton <oliver.upton@...ux.dev>, Raghavendra Rao Ananta <rananta@...gle.com>, 
	Ryan Roberts <ryan.roberts@....com>, Sean Christopherson <seanjc@...gle.com>, 
	Shaoqin Huang <shahuang@...hat.com>, Suzuki K Poulose <suzuki.poulose@....com>, 
	Wei Xu <weixugc@...gle.com>, Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>, 
	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 v6 05/11] mm: Add fast_only bool to test_young and
 clear_young MMU notifiers

On Tue, Aug 6, 2024 at 10:23 AM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> On Thu, Aug 01, 2024 at 04:13:40PM -0700, James Houghton wrote:
> > --- a/include/linux/mmu_notifier.h
> > +++ b/include/linux/mmu_notifier.h
> > @@ -106,6 +106,18 @@ struct mmu_notifier_ops {
> >          * clear_young is a lightweight version of clear_flush_young. Like the
> >          * latter, it is supposed to test-and-clear the young/accessed bitflag
> >          * in the secondary pte, but it may omit flushing the secondary tlb.
> > +        *
> > +        * The fast_only parameter indicates that this call should not block,
> > +        * and this function should not cause other MMU notifier calls to
> > +        * block. Usually this means that the implementation should be
> > +        * lockless.
> > +        *
> > +        * When called with fast_only, this notifier will be a no-op unless
> > +        * has_fast_aging is set on the struct mmu_notifier.
>
> If you add a has_fast_aging I wonder if it is better to introduce new
> ops instead? The semantics are a bit easier to explain that way

v5 implemented these with a new op[1]. *Just* having the new op is
kind of problematic -- we have yet another op to do something very
similar to what already exists. We are left with two options:
consolidate everything into a single notifier[2] or add a new
parameter to test/clear_young()[3]. The latter, implemented in this
v6, is somewhat simpler to implement (fewer LoC, reduces some
duplication in KVM), though it does indeed make the explanation for
test/clear_young() slightly more complex. I don't feel very strongly,
but unless you do, I think I just ought to stick with how the v6 does
it. :)

Thanks Jason!

[1]: https://lore.kernel.org/linux-mm/20240611002145.2078921-5-jthoughton@google.com/
[2]: https://lore.kernel.org/linux-mm/CADrL8HVHcKSW3hiHzKTit07gzo36jtCZCnM9ZpueyifgNdGggw@mail.gmail.com/
[3]: https://lore.kernel.org/linux-mm/CADrL8HXhGFWwHt728Bg15x1YxJmS=WD8z=KJc_Koaah=OvHDwg@mail.gmail.com/