linux-kernel - Re: [PATCH v4 05/12] mm: introduce CONFIG_ARCH_HAS_LAZY_MMU

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Message-ID: <87o6ph3xdq.ritesh.list@gmail.com>
Date: Wed, 05 Nov 2025 10:10:33 +0530
From: Ritesh Harjani (IBM) <ritesh.list@...il.com>
To: Kevin Brodsky <kevin.brodsky@....com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, Kevin Brodsky <kevin.brodsky@....com>, 
	Alexander Gordeev <agordeev@...ux.ibm.com>, Andreas Larsson <andreas@...sler.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Boris Ostrovsky <boris.ostrovsky@...cle.com>, 
	Borislav Petkov <bp@...en8.de>, Catalin Marinas <catalin.marinas@....com>, 
	Christophe Leroy <christophe.leroy@...roup.eu>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	David Hildenbrand <david@...hat.com>, "David S. Miller" <davem@...emloft.net>, 
	David Woodhouse <dwmw2@...radead.org>, "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>, 
	Jann Horn <jannh@...gle.com>, Juergen Gross <jgross@...e.com>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Madhavan Srinivasan <maddy@...ux.ibm.com>, Michael Ellerman <mpe@...erman.id.au>, Michal Hocko <mhocko@...e.com>, 
	Mike Rapoport <rppt@...nel.org>, Nicholas Piggin <npiggin@...il.com>, 
	Peter Zijlstra <peterz@...radead.org>, Ryan Roberts <ryan.roberts@....com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Thomas Gleixner <tglx@...utronix.de>, Vlastimil Babka <vbabka@...e.cz>, 
	Will Deacon <will@...nel.org>, Yeoreum Yun <yeoreum.yun@....com>, 
	linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org, 
	sparclinux@...r.kernel.org, xen-devel@...ts.xenproject.org, x86@...nel.org
Subject: Re: [PATCH v4 05/12] mm: introduce CONFIG_ARCH_HAS_LAZY_MMU_MODE

Kevin Brodsky <kevin.brodsky@....com> writes:

> Architectures currently opt in for implementing lazy_mmu helpers by
> defining __HAVE_ARCH_ENTER_LAZY_MMU_MODE.
>
> In preparation for introducing a generic lazy_mmu layer that will
> require storage in task_struct, let's switch to a cleaner approach:
> instead of defining a macro, select a CONFIG option.
>
> This patch introduces CONFIG_ARCH_HAS_LAZY_MMU_MODE and has each
> arch select it when it implements lazy_mmu helpers.
> __HAVE_ARCH_ENTER_LAZY_MMU_MODE is removed and <linux/pgtable.h>
> relies on the new CONFIG instead.
>
> On x86, lazy_mmu helpers are only implemented if PARAVIRT_XXL is
> selected. This creates some complications in arch/x86/boot/, because
> a few files manually undefine PARAVIRT* options. As a result
> <asm/paravirt.h> does not define the lazy_mmu helpers, but this
> breaks the build as <linux/pgtable.h> only defines them if
> !CONFIG_ARCH_HAS_LAZY_MMU_MODE. There does not seem to be a clean
> way out of this - let's just undefine that new CONFIG too.
>
> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
> ---
>  arch/arm64/Kconfig                                 | 1 +
>  arch/arm64/include/asm/pgtable.h                   | 1 -
>  arch/powerpc/include/asm/book3s/64/tlbflush-hash.h | 2 --
>  arch/powerpc/platforms/Kconfig.cputype             | 1 +
>  arch/sparc/Kconfig                                 | 1 +
>  arch/sparc/include/asm/tlbflush_64.h               | 2 --
>  arch/x86/Kconfig                                   | 1 +
>  arch/x86/boot/compressed/misc.h                    | 1 +
>  arch/x86/boot/startup/sme.c                        | 1 +
>  arch/x86/include/asm/paravirt.h                    | 1 -
>  include/linux/pgtable.h                            | 2 +-
>  mm/Kconfig                                         | 3 +++
>  12 files changed, 10 insertions(+), 7 deletions(-)

Maybe we can add this to ... ?

Documentation/features/vm/lazy_mmu/arch-support.txt

#
# Feature name:          lazy_mmu mode
#         Kconfig:       ARCH_HAS_LAZY_MMU_MODE
#         description:   arch supports arch_{enter|flush|leave}_lazy_mmu_mode()
#
    -----------------------
    |         arch |status|
    -----------------------
    |       arm64: |  ok  |
    |     powerpc: |  ok  |
    |       sparc: |  ok  |
    |         x86: |  ok  |
    -----------------------


As for this patch, the changes are mostly straight forward around the
configs part. This looks good to me. Please feel free to add: 

Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>

-ritesh