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

[Muhammad Usama Anjum <usama.anjum@xxxxxxx>]

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@xxxxxxx>
>> ---
>> 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?

This could have been avoided if each architecture defined its own hw_pte_t.
But for now we are keeping the generic definition of 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
>>

-- 
Thanks,
Usama