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

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Date: Tue, 23 Jan 2024 12:53:07 +0100
From: David Hildenbrand <david@...hat.com>
To: Christophe Leroy <christophe.leroy@...roup.eu>,
 Ryan Roberts <ryan.roberts@....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

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