linux-kernel - Re: [PATCH v3 05/13] mm: introduce CONFIG_ARCH_LAZY

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Message-ID: <aPNjd2dg3YN-TZKH@kernel.org>
Date: Sat, 18 Oct 2025 12:52:55 +0300
From: Mike Rapoport <rppt@...nel.org>
To: Kevin Brodsky <kevin.brodsky@....com>
Cc: linux-mm@...ck.org, 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>,
	"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>, 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 v3 05/13] mm: introduce CONFIG_ARCH_LAZY_MMU

On Wed, Oct 15, 2025 at 09:27:19AM +0100, Kevin Brodsky wrote:
> 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_LAZY_MMU 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_LAZY_MMU. 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>
> ---

...

> @@ -231,7 +231,7 @@ static inline int pmd_dirty(pmd_t pmd)
>   * held, but for kernel PTE updates, no lock is held). Nesting is not permitted
>   * and the mode cannot be used in interrupt context.
>   */
> -#ifndef __HAVE_ARCH_ENTER_LAZY_MMU_MODE
> +#ifndef CONFIG_ARCH_LAZY_MMU
>  static inline void arch_enter_lazy_mmu_mode(void) {}
>  static inline void arch_leave_lazy_mmu_mode(void) {}
>  static inline void arch_flush_lazy_mmu_mode(void) {}
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 0e26f4fc8717..2fdcb42ca1a1 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1372,6 +1372,9 @@ config PT_RECLAIM
>  config FIND_NORMAL_PAGE
>  	def_bool n
>  
> +config ARCH_LAZY_MMU
> +	bool
> +

I think a better name would be ARCH_HAS_LAZY_MMU and the config option fits
better to arch/Kconfig.

>  source "mm/damon/Kconfig"
>  
>  endmenu
> -- 
> 2.47.0
> 

-- 
Sincerely yours,
Mike.