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

Anshuman Khandual
2026-04-10T04:29:19Z

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@linux-foundation.org>
>> Cc: David Hildenbrand <david@kernel.org>
>> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>> Cc: Mike Rapoport <rppt@kernel.org>
>> Cc: linux-mm@kvack.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>> 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.