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Date: Wed, 12 Jun 2024 09:02:30 -0700
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Subject: Re: [PATCH v5 8/9] mm: multi-gen LRU: Have secondary MMUs participate
 in aging
From: Sean Christopherson <seanjc@google.com>
To: James Houghton <jthoughton@google.com>
Cc: 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>, James Morse <james.morse@arm.com>, 
	Jonathan Corbet <corbet@lwn.net>, Marc Zyngier <maz@kernel.org>, Oliver Upton <oliver.upton@linux.dev>, 
	Raghavendra Rao Ananta <rananta@google.com>, Ryan Roberts <ryan.roberts@arm.com>, 
	Shaoqin Huang <shahuang@redhat.com>, Suzuki K Poulose <suzuki.poulose@arm.com>, 
	Wei Xu <weixugc@google.com>, Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>, 
	Zenghui Yu <yuzenghui@huawei.com>, kvmarm@lists.linux.dev, kvm@vger.kernel.org, 
	linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, 
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
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On Tue, Jun 11, 2024, James Houghton wrote:
> diff --git a/mm/rmap.c b/mm/rmap.c
> index e8fc5ecb59b2..24a3ff639919 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -870,13 +870,10 @@ static bool folio_referenced_one(struct folio *folio,
>  			continue;
>  		}
>  
> -		if (pvmw.pte) {
> -			if (lru_gen_enabled() &&
> -			    pte_young(ptep_get(pvmw.pte))) {
> -				lru_gen_look_around(&pvmw);
> +		if (lru_gen_enabled() && pvmw.pte) {
> +			if (lru_gen_look_around(&pvmw))
>  				referenced++;
> -			}
> -
> +		} else if (pvmw.pte) {
>  			if (ptep_clear_flush_young_notify(vma, address,
>  						pvmw.pte))
>  				referenced++;

Random question not really related to KVM/secondary MMU participation.  AFAICT,
the MGLRU approach doesn't flush TLBs after aging pages.  How does MGLRU mitigate
false negatives on pxx_young() due to the CPU not setting Accessed bits because
of stale TLB entries?