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

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Date: Tue, 13 Feb 2024 10:54:52 +0100
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....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 12.02.24 22:34, Ryan Roberts wrote:
> 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.

Oh, I say stupid things all the time. Please push back if you disagree. :)

[shrinking a patch set if possible and reasonable is often a good idea]

-- 
Cheers,

David / dhildenb