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Date: Tue, 23 Jan 2024 12:53:07 +0100
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Subject: Re: [PATCH v1 01/11] arm/pgtable: define PFN_PTE_SHIFT on arm and
 arm64
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To: Christophe Leroy <christophe.leroy@csgroup.eu>,
 Ryan Roberts <ryan.roberts@arm.com>,
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Cc: "linux-mm@kvack.org" <linux-mm@kvack.org>,
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 "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
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From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <11a7232c-8623-4c2c-b980-a13645f3fe89@csgroup.eu>
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On 23.01.24 12:48, Christophe Leroy wrote:
> 
> 
> Le 23/01/2024 à 12:38, Ryan Roberts a écrit :
>> 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.
> 
> Right. There was a discussion about it without any conclusion:
> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20231024143604.16749-1-aneesh.kumar@linux.ibm.com/
> 
> As far as understand the simple increment is better on ppc/32 but worse
> in ppc/64.

Sounds like we're micro-optimizing for a specific compiler version 
output. Hurray.

-- 
Cheers,

David / dhildenb