Re: [PATCH V2 3/7] mm: Use ptep_get() for accessing PTE entries

[David Hildenbrand]

On 17.09.24 09:31, Anshuman Khandual wrote:
> Convert PTE accesses via ptep_get() helper that defaults as READ_ONCE() but
> also provides the platform an opportunity to override when required. This
> stores read page table entry value in a local variable which can be used in
> multiple instances there after. This helps in avoiding multiple memory load
> operations as well possible race conditions.

Please make it clearer in the subject+description that this really only 
involves set_pte_safe().


> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> Cc: "Mike Rapoport (IBM)" <rppt@xxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---
>   include/linux/pgtable.h | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 2a6a3cccfc36..547eeae8c43f 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1060,7 +1060,8 @@ static inline int pgd_same(pgd_t pgd_a, pgd_t pgd_b)
>    */
>   #define set_pte_safe(ptep, pte) \
>   ({ \
> -	WARN_ON_ONCE(pte_present(*ptep) && !pte_same(*ptep, pte)); \
> +	pte_t __old = ptep_get(ptep); \
> +	WARN_ON_ONCE(pte_present(__old) && !pte_same(__old, pte)); \
>   	set_pte(ptep, pte); \
>   })
>   

I don't think this is necessary. PTE present cannot flip concurrently, 
that's the whole reason of the "safe" part after all.

Can we just move these weird set_pte/pmd_safe() stuff to x86 init code 
and be done with it? Then it's also clear *where* it is getting used and 
for which reason.

--
Cheers,

David / dhildenb