linux-kernel - Re: [PATCH v1 01/11] arm/pgtable: define PFN_PTE

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Message-ID: <e0d9caab-39c7-446a-aeef-5d914d321c72@arm.com>
Date: Tue, 23 Jan 2024 11:38:30 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc: "linux-mm@...ck.org" <linux-mm@...ck.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Russell King <linux@...linux.org.uk>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Dinh Nguyen <dinguyen@...nel.org>, Michael Ellerman <mpe@...erman.id.au>,
 Nicholas Piggin <npiggin@...il.com>,
 "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Alexander Gordeev <agordeev@...ux.ibm.com>,
 Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, "David S. Miller"
 <davem@...emloft.net>,
 "linux-arm-kernel@...ts.infradead.org"
 <linux-arm-kernel@...ts.infradead.org>,
 "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
 "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
 "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
 "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>
Subject: Re: [PATCH v1 01/11] arm/pgtable: define PFN_PTE_SHIFT on arm and
 arm64

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.

>  
> 
>