linux-kernel - Re: [PATCH v5 8/9] mm: multi-gen LRU: Have secondary MMUs participate in aging

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Message-ID: <ZmnGlpBR91TyI3Lt@google.com>
Date: Wed, 12 Jun 2024 09:02:30 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: James Houghton <jthoughton@...gle.com>
Cc: 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>, 
	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 v5 8/9] mm: multi-gen LRU: Have secondary MMUs participate
 in aging

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?