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

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Message-ID: <20240806172349.GQ676757@ziepe.ca>
Date: Tue, 6 Aug 2024 14:23:49 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: James Houghton <jthoughton@...gle.com>
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 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

Jason