Re: [PATCH v5 03/25] mm: Make pte_next_pfn() a wrapper around pte_advance_pfn()

Ryan Roberts
2024-02-12T14:11:02Z

On 12/02/2024 12:14, David Hildenbrand wrote:
> On 02.02.24 09:07, Ryan Roberts wrote:
>> The goal is to be able to advance a PTE by an arbitrary number of PFNs.
>> So introduce a new API that takes a nr param.
>>
>> We are going to remove pte_next_pfn() and replace it with
>> pte_advance_pfn(). As a first step, implement pte_next_pfn() as a
>> wrapper around pte_advance_pfn() so that we can incrementally switch the
>> architectures over. Once all arches are moved over, we will change all
>> the core-mm callers to call pte_advance_pfn() directly and remove the
>> wrapper.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>>   include/linux/pgtable.h | 8 +++++++-
>>   1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>> index 5e7eaf8f2b97..815d92dcb96b 100644
>> --- a/include/linux/pgtable.h
>> +++ b/include/linux/pgtable.h
>> @@ -214,9 +214,15 @@ static inline int pmd_dirty(pmd_t pmd)
>>       #ifndef pte_next_pfn
>> +#ifndef pte_advance_pfn
>> +static inline pte_t pte_advance_pfn(pte_t pte, unsigned long nr)
>> +{
>> +    return __pte(pte_val(pte) + (nr << PFN_PTE_SHIFT));
>> +}
>> +#endif
>>   static inline pte_t pte_next_pfn(pte_t pte)
>>   {
>> -    return __pte(pte_val(pte) + (1UL << PFN_PTE_SHIFT));
>> +    return pte_advance_pfn(pte, 1);
>>   }
>>   #endif
>>
>
> I do wonder if we simply want to leave pte_next_pfn() around? Especially patch
> #4, #6 don't really benefit from the change? So are the other set_ptes()
> implementations.
>
> That is, only convert all pte_next_pfn()->pte_advance_pfn(), and leave a
> pte_next_pfn() macro in place.
>
> Any downsides to that?

The downside is just having multiple functions that effectively do the same
thing. Personally I think its cleaner and easier to understand the code with
just one generic function which we pass 1 to it where we only want to advance by
1. In the end, there are only a couple of places where pte_advance_pfn(1) is
used, so doesn't really seem valuable to me to maintain a specialization.

Unless you feel strongly that we need to keep pte_next_pfn() then I'd prefer to
leave it as I've done in this series.

> This patch here would become:
>
> #ifndef pte_advance_pfn
> static inline pte_t pte_advance_pfn(pte_t pte, unsigned long nr)
> {
>     return __pte(pte_val(pte) + (nr << PFN_PTE_SHIFT));
> }
> #endif
>
> #ifndef pte_next_pfn
> #define pte_next_pfn(pte) pte_advance_pfn(pte, 1)
> #endif
>
> As you convert the three arches, make them define pte_advance_pfn and udnefine
> pte_next_pfn. in the end, you can drop the #ifdef around pte_next_pfn here.
>