Subject: Re: [PATCH v3 05/13] mm: introduce CONFIG_ARCH_LAZY_MMU

From: Mike Rapoport <[email-masked]>
Date: Sat, 18 Oct 2025 12:52:55 +0300

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 <[email-masked]>
> ---

...

> @@ -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.