linux-kernel - Re: [PATCH v5 03/25] mm: Make pte_next_pfn() a wrapper around pte_advance

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Message-ID: <cdf4e537-5b0d-45ba-8094-527127c791a6@arm.com>
Date: Mon, 12 Feb 2024 14:10:55 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Ard Biesheuvel <ardb@...nel.org>, Marc Zyngier <maz@...nel.org>,
 James Morse <james.morse@....com>, Andrey Ryabinin <ryabinin.a.a@...il.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Mark Rutland <mark.rutland@....com>,
 Kefeng Wang <wangkefeng.wang@...wei.com>, John Hubbard
 <jhubbard@...dia.com>, Zi Yan <ziy@...dia.com>,
 Barry Song <21cnbao@...il.com>, Alistair Popple <apopple@...dia.com>,
 Yang Shi <shy828301@...il.com>, Nicholas Piggin <npiggin@...il.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>
Cc: linux-arm-kernel@...ts.infradead.org, x86@...nel.org,
 linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 03/25] mm: Make pte_next_pfn() a wrapper around
 pte_advance_pfn()

On 12/02/2024 12:14, David Hildenbrand wrote:
> On 02.02.24 09:07, Ryan Roberts wrote:
>> The goal is to be able to advance a PTE by an arbitrary number of PFNs.
>> So introduce a new API that takes a nr param.
>>
>> We are going to remove pte_next_pfn() and replace it with
>> pte_advance_pfn(). As a first step, implement pte_next_pfn() as a
>> wrapper around pte_advance_pfn() so that we can incrementally switch the
>> architectures over. Once all arches are moved over, we will change all
>> the core-mm callers to call pte_advance_pfn() directly and remove the
>> wrapper.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>> ---
>>   include/linux/pgtable.h | 8 +++++++-
>>   1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>> index 5e7eaf8f2b97..815d92dcb96b 100644
>> --- a/include/linux/pgtable.h
>> +++ b/include/linux/pgtable.h
>> @@ -214,9 +214,15 @@ static inline int pmd_dirty(pmd_t pmd)
>>       #ifndef pte_next_pfn
>> +#ifndef pte_advance_pfn
>> +static inline pte_t pte_advance_pfn(pte_t pte, unsigned long nr)
>> +{
>> +    return __pte(pte_val(pte) + (nr << PFN_PTE_SHIFT));
>> +}
>> +#endif
>>   static inline pte_t pte_next_pfn(pte_t pte)
>>   {
>> -    return __pte(pte_val(pte) + (1UL << PFN_PTE_SHIFT));
>> +    return pte_advance_pfn(pte, 1);
>>   }
>>   #endif
>>   
> 
> I do wonder if we simply want to leave pte_next_pfn() around? Especially patch
> #4, #6 don't really benefit from the change? So are the other set_ptes()
> implementations.
> 
> That is, only convert all pte_next_pfn()->pte_advance_pfn(), and leave a
> pte_next_pfn() macro in place.
> 
> Any downsides to that? 

The downside is just having multiple functions that effectively do the same
thing. Personally I think its cleaner and easier to understand the code with
just one generic function which we pass 1 to it where we only want to advance by
1. In the end, there are only a couple of places where pte_advance_pfn(1) is
used, so doesn't really seem valuable to me to maintain a specialization.

Unless you feel strongly that we need to keep pte_next_pfn() then I'd prefer to
leave it as I've done in this series.

> This patch here would become:
> 
> #ifndef pte_advance_pfn
> static inline pte_t pte_advance_pfn(pte_t pte, unsigned long nr)
> {
>     return __pte(pte_val(pte) + (nr << PFN_PTE_SHIFT));
> }
> #endif
> 
> #ifndef pte_next_pfn
> #define pte_next_pfn(pte) pte_advance_pfn(pte, 1)
> #endif
> 
> As you convert the three arches, make them define pte_advance_pfn and udnefine
> pte_next_pfn. in the end, you can drop the #ifdef around pte_next_pfn here.
>