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Date: Tue, 23 Jan 2024 11:38:30 +0000
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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: David Hildenbrand <david@redhat.com>,
 Christophe Leroy <christophe.leroy@csgroup.eu>,
 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Cc: "linux-mm@kvack.org" <linux-mm@kvack.org>,
 Andrew Morton <akpm@linux-foundation.org>,
 Matthew Wilcox <willy@infradead.org>, Russell King <linux@armlinux.org.uk>,
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 Dinh Nguyen <dinguyen@kernel.org>, Michael Ellerman <mpe@ellerman.id.au>,
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 "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
 "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
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 <palmer@dabbelt.com>, Albert Ou <aou@eecs.berkeley.edu>,
 Alexander Gordeev <agordeev@linux.ibm.com>,
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References: <20240122194200.381241-1-david@redhat.com>
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 <fdaeb9a5-d890-499a-92c8-d171df43ad01@arm.com>
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From: Ryan Roberts <ryan.roberts@arm.com>
In-Reply-To: <3a970289-a72f-418e-b43c-89f67f0d5283@redhat.com>
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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.

>  
> 
>