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

[Muhammad Usama Anjum <usama.anjum@arm.com>]

On 09/08/2026 6:45 pm, Alexander Gordeev wrote:
> On Fri, Aug 07, 2026 at 04:24:00PM +0100, Muhammad Usama Anjum wrote:
>> On 07/08/2026 8:09 am, Alexander Gordeev wrote:
>>> 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 <usama.anjum@arm.com>
>>>> ---
>>>> 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;
>> Thank you for testing it out on s390.
>>
>> As __hw_pte_t isn't being used yet in this series, would s390 enablement
>> patches add __hw_pte_t to this definition?
> 
> I hope there is a better solution. As I noted m68k, powerpc and sparc
> may also be affected, so I would suggest to look into those as well.
> I would prefer s390 to use the generic one rather than circumvent a
> compile error in a custom way.
I've just checked all of these architectures by doing dirty conversion and
reached to same conclusion that __hw_pte_t must be defined like:
 
typedef struct __hw_pte_t { pte_t __pte; } hw_pte_t;

I'lll add __hw_pte_t to this series. (Initially on last email I'd thought
that the first user would add __hw_pte_t. But it seems sensible to add it
now)

-- 
Thanks,
Usama