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

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 9/17/24 15:58, David Hildenbrand wrote:
> 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().

I will update the commit message with some thing like this.

mm: Use ptep_get() in set_pte_safe()

This converts PTE accesses in set_pte_safe() via ptep_get() helper which
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 as some possible race
conditions.

> 
> 
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>> Cc: "Mike Rapoport (IBM)" <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 | 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.

Which is not necessary ? Converting de-references to ptep_get() OR caching
the page table read value in a local variable ? ptep_get() conversion also
serves the purpose providing an opportunity for platform to override.

> 
> 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.
> 
set_pte/pmd_safe() can be moved to x86 platform - as that is currently the
sole user for these helpers. But because set_pgd_safe() gets used in riscv
platform, just wondering would it be worth moving only the pte/pmd helpers
but not the pgd one ?