Re: [PATCH v2 3/6] ARM: drop CONFIG_HIGHPTE support

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Thu, Nov 12, 2026 at 12:25:25PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
>
> CONFIG_HIGHPTE was added in linux-2.6.32, a few years before 64-bit
> support. At the time it made sense, as the CONFIG_ARM_LPAE option allowed
> systems with 16GB of memory that made lowmem a particularly scarce
> resource, and the HIGHPTE implementation gave feature parity with 32-bit
> x86 and frv machines.
>
> Since Arm is the last architecture remaining that uses this, and almost
> no 32-bit machines support more than 4GB of RAM, the cost of continuing
> to maintain HIGHPTE seems unjustified, so remove it here to allow
> simplifying the generic page table handling.
>
> Link: https://lore.kernel.org/lkml/20241204103042.1904639-8-arnd@kernel.org/T/#u
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> I sent a patch to drop HIGHPTE support on x86 today, see
> https://lore.kernel.org/lkml/20241210144945.2325330-9-arnd@kernel.org/T/#u
>
> If that one gets merged, we can merge this one instead of the one
> that makes HIGHPTE depend on !PREEMPT_RT, but if we decide against
> the x86 change, then we probably don't want this one either.
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

With Mike's change applied, LGTM and:

Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  arch/arm/Kconfig               | 11 -----------
>  arch/arm/include/asm/pgalloc.h |  8 +-------
>  2 files changed, 1 insertion(+), 18 deletions(-)
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index fd01ccab9c12..fc66d52d847e 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -1225,17 +1225,6 @@ config HIGHMEM
>
>  	  If unsure, say n.
>
> -config HIGHPTE
> -	bool "Allocate 2nd-level pagetables from highmem" if EXPERT
> -	depends on HIGHMEM && !PREEMPT_RT
> -	default y
> -	help
> -	  The VM uses one page of physical memory for each page table.
> -	  For systems with a lot of processes, this can use a lot of
> -	  precious low memory, eventually leading to low memory being
> -	  consumed by page tables.  Setting this option will allow
> -	  user-space 2nd level page tables to reside in high memory.
> -
>  config ARM_PAN
>  	bool "Enable privileged no-access"
>  	depends on MMU
> diff --git a/arch/arm/include/asm/pgalloc.h b/arch/arm/include/asm/pgalloc.h
> index a17f01235c29..ef6cb3e6d179 100644
> --- a/arch/arm/include/asm/pgalloc.h
> +++ b/arch/arm/include/asm/pgalloc.h
> @@ -85,18 +85,12 @@ pte_alloc_one_kernel(struct mm_struct *mm)
>  	return pte;
>  }
>
> -#ifdef CONFIG_HIGHPTE
> -#define PGTABLE_HIGHMEM __GFP_HIGHMEM
> -#else
> -#define PGTABLE_HIGHMEM 0
> -#endif
> -
>  static inline pgtable_t
>  pte_alloc_one(struct mm_struct *mm)
>  {
>  	struct page *pte;
>
> -	pte = __pte_alloc_one(mm, GFP_PGTABLE_USER | PGTABLE_HIGHMEM);
> +	pte = __pte_alloc_one(mm, GFP_PGTABLE_USER);
>  	if (!pte)
>  		return NULL;
>  	if (!PageHighMem(pte))
> --
> 2.53.0
>

--
Cheers, Lorenzo