Re: [RFC V1 02/16] mm: Add read-write accessors for vm_page_prot

[Anshuman Khandual]

On 09/04/26 4:07 PM, Mike Rapoport wrote:
> Hi Anshuman,
> 
> On Tue, Feb 24, 2026 at 10:41:39AM +0530, Anshuman Khandual wrote:
>> Currently vma->vm_page_prot is safely read from and written to, without any
>> locks with READ_ONCE() and WRITE_ONCE(). But with introduction of D128 page
>> tables on arm64 platform, vm_page_prot grows to 128 bits which can't safely
>> be handled with READ_ONCE() and WRITE_ONCE().
>>
>> Add read and write accessors for vm_page_prot like pgprot_read/write_once()
>> which any platform can override when required, although still defaulting as
>> READ_ONCE() and WRITE_ONCE(), thus preserving the functionality for others.
>>
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Cc: David Hildenbrand <david@xxxxxxxxxx>
>> Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
>> Cc: Mike Rapoport <rppt@xxxxxxxxxx>
>> Cc: linux-mm@xxxxxxxxx
>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>> ---
>>  include/linux/pgtable.h | 14 ++++++++++++++
>>  mm/huge_memory.c        |  4 ++--
>>  mm/memory.c             |  2 +-
>>  mm/migrate.c            |  2 +-
>>  mm/mmap.c               |  2 +-
>>  5 files changed, 19 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>> index da17139a1279..8858b8b03a02 100644
>> --- a/include/linux/pgtable.h
>> +++ b/include/linux/pgtable.h
>> @@ -495,6 +495,20 @@ static inline pgd_t pgdp_get(pgd_t *pgdp)
>>  }
>>  #endif
>>  
>> +#ifndef pgprot_read_once
>> +static inline pgprot_t pgprot_read_once(pgprot_t *prot)
> 
> I don't think we need _once in the helper name. Presence of the helper
> already implies that pointer should not be just dereferenced from one side
> and that using the helper will do The Right Thing from the other side.


Makes sense - will drop __once from the helper name.