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

David Hildenbrand
2024-02-12T14:29:07Z

On 12.02.24 15:10, Ryan Roberts wrote:
> 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.

Well, not really functions, just a macro. Like we have set_pte_at()
translating to set_ptes().

Arguably, we have more callers of set_pte_at().

"Easier to understand", I don't know. :)

>
> 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.

Well, it makes you patch set shorter and there is less code churn.

So personally, I'd just leave pte_next_pfn() in there. But whatever you
prefer, not the end of the world.

--
Cheers,

David / dhildenb