Re: [PATCH 1/9] mm: introduce hw_pte_t for PTE table storage

On Thu, Aug 06, 2026 at 09:38:39AM +0100, Muhammad Usama Anjum wrote:
> pte_t is used both for logical PTE values and for entries stored in a PTE
> table, so pte_t * does not distinguish a pointer to a copied value from a
> pointer to table storage.
> 
> Introduce hw_pte_t as the generic name for a PTE table element. Define it
> as a macro alias of pte_t by default. When an architecture selects
> ARCH_HAS_HW_PTE_T, define it as a structure containing a pte_t instead.
> This preserves the representation while allowing converted architectures
> to enforce the distinction at compile time.
> 
> Keep the C type definitions behind an __ASSEMBLY__ check because
> architecture assembly sources can include this header indirectly. Include
> asm/page.h so consumers such as linux/vmalloc.h retain the page definitions
> they previously obtained from that header.
> 
> Signed-off-by: Muhammad Usama Anjum <[email protected]>
> ---
> Changes since RFC v1:
> - Add the ARCH_HAS_HW_PTE_T opt-in and generic wrapper definition.
> - Exclude the C type definitions from assembly sources.
> - Update the description for the new opt-in model.
> ---
>  MAINTAINERS                   |  1 +
>  include/linux/pgtable_types.h | 17 +++++++++++++++++
>  mm/Kconfig                    |  3 +++
>  3 files changed, 21 insertions(+)
>  create mode 100644 include/linux/pgtable_types.h
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index e9c8567308a75..7169bea968cf5 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -16982,6 +16982,7 @@ F:	include/linux/mmu_notifier.h
>  F:	include/linux/pagewalk.h
>  F:	include/linux/pgalloc.h
>  F:	include/linux/pgtable.h
> +F:	include/linux/pgtable_types.h
>  F:	include/linux/ptdump.h
>  F:	include/linux/vmpressure.h
>  F:	include/linux/vmstat.h
> diff --git a/include/linux/pgtable_types.h b/include/linux/pgtable_types.h
> new file mode 100644
> index 0000000000000..70c3edd00a01b
> --- /dev/null
> +++ b/include/linux/pgtable_types.h
> @@ -0,0 +1,17 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_PGTABLE_TYPES_H
> +#define _LINUX_PGTABLE_TYPES_H
> +
> +#include <asm/page.h>
> +
> +#ifndef __ASSEMBLY__
> +
> +#ifdef CONFIG_ARCH_HAS_HW_PTE_T
> +typedef struct { pte_t __pte; } hw_pte_t;

On s390 it fails to compile once we do typedef hw_pte_t *pgtable_t
in asm/page.h. m68k, powerpc and sparc may also have such problem.

The below declaration helps to resolve it using forward declaration
and without meddling with headers, though I do not like it much:

typedef struct __hw_pte_t { pte_t __pte; } hw_pte_t;

> +#else
> +#define hw_pte_t pte_t
> +#endif
> +
> +#endif /* !__ASSEMBLY__ */
> +
> +#endif /* _LINUX_PGTABLE_TYPES_H */
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 331daf7fcfab5..31ba9ebf4aafd 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1316,6 +1316,9 @@ comment "GUP_TEST needs to have DEBUG_FS enabled"
>  config GUP_GET_PXX_LOW_HIGH
>  	bool
>  
> +config ARCH_HAS_HW_PTE_T
> +	bool
> +
>  config DMAPOOL_TEST
>  	tristate "Enable a module to run time tests on dma_pool"
>  	depends on HAS_DMA
> -- 
> 2.47.3
>