Re: [PATCH v1 01/11] arm/pgtable: define PFN_PTE_SHIFT on arm and arm64

[Aneesh Kumar K . V]

David Hildenbrand <david@xxxxxxxxxx> writes:

> On 23.01.24 12:38, Ryan Roberts wrote:
>> On 23/01/2024 11:31, David Hildenbrand wrote:
>>>>>
>>>>>> If high bits are used for
>>>>>> something else, then we might produce a garbage PTE on overflow, but that
>>>>>> shouldn't really matter I concluded for folio_pte_batch() purposes, we'd not
>>>>>> detect "belongs to this folio batch" either way.
>>>>>
>>>>> Exactly.
>>>>>
>>>>>>
>>>>>> Maybe it's likely cleaner to also have a custom pte_next_pfn() on ppc, I just
>>>>>> hope that we don't lose any other arbitrary PTE bits by doing the pte_pgprot().
>>>>>
>>>>> I don't see the need for ppc to implement pte_next_pfn().
>>>>
>>>> Agreed.
>>>
>>> So likely we should then do on top for powerpc (whitespace damage):
>>>
>>> diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
>>> index a04ae4449a025..549a440ed7f65 100644
>>> --- a/arch/powerpc/mm/pgtable.c
>>> +++ b/arch/powerpc/mm/pgtable.c
>>> @@ -220,10 +220,7 @@ void set_ptes(struct mm_struct *mm, unsigned long addr,
>>> pte_t *ptep,
>>>                          break;
>>>                  ptep++;
>>>                  addr += PAGE_SIZE;
>>> -               /*
>>> -                * increment the pfn.
>>> -                */
>>> -               pte = pfn_pte(pte_pfn(pte) + 1, pte_pgprot((pte)));
>>> +               pte = pte_next_pfn(pte);
>>>          }
>>>   }
>> 
>> Looks like commit 47b8def9358c ("powerpc/mm: Avoid calling
>> arch_enter/leave_lazy_mmu() in set_ptes") changed from doing the simple
>> increment to this more complex approach, but the log doesn't say why.
>
> @Aneesh, was that change on purpose?
>

Because we had a bug with the patch that introduced the change and that
line was confusing. The right thing should have been to add
pte_pfn_next() to make it clear. It was confusing because not all pte
format had pfn at PAGE_SHIFT offset (even though we did use the correct
PTE_RPN_SHIFT in this specific case). To make it simpler I ended up
switching that line to pte_pfn(pte) + 1 .

-aneesh