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

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Message-ID: <aXN-PsEU8jk7LEwh@hal-station.localdomain>
Date: Fri, 23 Jan 2026 08:57:18 -0500
From: Hamza Mahfooz <someguy@...ective-light.com>
To: Sean Christopherson <seanjc@...gle.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 Thu, Jan 22, 2026 at 04:27:42PM -0800, Sean Christopherson wrote:
> 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.
> 

My testing mostly focused on correctness, though I did see the fault rate
go down on a long running VM that I used to host a web server (I only
only gave it around a gig of RAM, so it is on the more extreme end).

> 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.

for_each_valid_sp() already filters out obsolete pages, so we should be
good to go from that perspective.

> 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.

Sounds good to me, I put it in __kvm_mmu_get_shadow_page() since it seemed
clearer and it's currently the only caller of kvm_mmu_find_shadow_page().