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

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Date: Tue, 23 Jan 2024 11:48:43 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Ryan Roberts <ryan.roberts@....com>, David Hildenbrand <david@...hat.com>,
	"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



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.

Christophe