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

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Date: Wed, 24 Jan 2024 11:15:03 +0530
From: Aneesh Kumar K.V <aneesh.kumar@...nel.org>
To: David Hildenbrand <david@...hat.com>,
	Ryan Roberts <ryan.roberts@....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>,
	"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

David Hildenbrand <david@...hat.com> writes:

> On 23.01.24 12:38, Ryan Roberts wrote:
>> 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.
>
> @Aneesh, was that change on purpose?
>

Because we had a bug with the patch that introduced the change and that
line was confusing. The right thing should have been to add
pte_pfn_next() to make it clear. It was confusing because not all pte
format had pfn at PAGE_SHIFT offset (even though we did use the correct
PTE_RPN_SHIFT in this specific case). To make it simpler I ended up
switching that line to pte_pfn(pte) + 1 .

-aneesh