linux-kernel - Re: [PATCH] KVM: x86/mmu: move reused pages to the top of active_mmu

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Message-ID: <aXLAfjpWwNLNv7pP@google.com>
Date: Thu, 22 Jan 2026 16:27:42 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Hamza Mahfooz <someguy@...ective-light.com>
Cc: kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>, 
	Thomas Gleixner <tglx@...nel.org>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: move reused pages to the top of active_mmu_pages

On Tue, Jan 20, 2026, Hamza Mahfooz wrote:
> Move reused shadow pages to the head of active_mmu_pages in
> __kvm_mmu_get_shadow_page(). This will allow us to move towards more of
> a LRU approximation eviction strategy instead of just straight FIFO.

Does this actually have a (positive) impact on real-world workloads?  It seems
like an obvious improvment, but there's enough subtlely around active_mmu_pages
that I don't want to make any changes without a strong benefit.

Specifically, kvm_zap_obsolete_pages() has a hard dependency on the list being
FIFO.  We _might_ be ok if we make sure to filter out obsolete pages, but only
because of KVM's behavior of (a) only allowing two memslot generations at any
given time and (b) zapping all shadow pages from the old/obsolete generation
prior to kvm_zap_obsolete_pages() exiting.

But it most definitely makes me nervous.

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 3911ac9bddfd..929085d46dd7 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2327,6 +2327,16 @@ static struct kvm_mmu_page *kvm_mmu_find_shadow_page(struct kvm *kvm,
 
        if (collisions > kvm->stat.max_mmu_page_hash_collisions)
                kvm->stat.max_mmu_page_hash_collisions = collisions;
+
+       /*
+        * If a shadow page was found, move it to the head of the active pages
+        * as a rudimentary form of LRU-reclaim (KVM reclaims shadow pages from
+        * tail=>head if the VM hits the limit on the number of MMU pages).
+        * */
+       if (sp && !WARN_ON_ONCE(is_obsolete_sp(kvm, sp)) &&
+           !list_is_head(&sp->link, &kvm->arch.active_mmu_pages))
+               list_move(&sp->link, &kvm->arch.active_mmu_pages);
+
        return sp;
 }

> Signed-off-by: Hamza Mahfooz <someguy@...ective-light.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 02c450686b4a..2fe04e01863d 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2395,7 +2395,8 @@ static struct kvm_mmu_page *__kvm_mmu_get_shadow_page(struct kvm *kvm,
>  	if (!sp) {
>  		created = true;
>  		sp = kvm_mmu_alloc_shadow_page(kvm, caches, gfn, sp_list, role);
> -	}
> +	} else if (!list_is_head(&sp->link, &kvm->arch.active_mmu_pages))
> +		list_move(&sp->link, &kvm->arch.active_mmu_pages);

As alluded to above, I think I'd prefer to put this in kvm_mmu_find_shadow_page()?
Largely a moot point, but it seems like we'd want to move a page to the head of
the list if we look it up for any reason.