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

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Message-ID: <3bc05163-2671-4239-ae5a-b9207fdca459@arm.com>
Date: Mon, 12 Feb 2024 21:34:53 +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 14:29, David Hildenbrand wrote:
> On 12.02.24 15:10, Ryan Roberts wrote:
>> 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.
> 
> Well, not really functions, just a macro. Like we have set_pte_at() translating
> to set_ptes().
> 
> Arguably, we have more callers of set_pte_at().
> 
> "Easier to understand", I don't know. :)
> 
>>
>> 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.
> 
> Well, it makes you patch set shorter and there is less code churn.
> 
> So personally, I'd just leave pte_next_pfn() in there. But whatever you prefer,
> not the end of the world.

I thought about this a bit more and remembered that I'm the apprentice so I've
changed it as you suggested.