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

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Message-ID: <1ac5be9f-fc05-463f-9512-74922acf8980@arm.com>
Date: Thu, 6 Nov 2025 10:33:24 +0000
From: Kevin Brodsky <kevin.brodsky@....com>
To: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, 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

On 05/11/2025 04:40, Ritesh Harjani (IBM) wrote:
> 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  |
>     -----------------------

That's an interesting idea but I'm not sure it really makes sense for
lazy MMU? AFAIU these arch-support.txt files are meant to help identify
which generic features an arch has support for. Lazy MMU isn't really a
feature though, in the sense that what it does is entirely defined by
the arch. This patch does introduce a generic layer, but ultimately it
remains a collection of arch hooks.

> 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>

Thanks for the review!

- Kevin