Message-ID: <001833dc-ee02-4bd3-8a37-820d0cd56be0@redhat.com>
Date: Mon, 1 Sep 2025 17:52:47 +0200
From: David Hildenbrand <david@...hat.com>
To: Vlastimil Babka <vbabka@...e.cz>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Max Kellermann <max.kellermann@...os.com>
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Subject: Re: [PATCH v5 02/12] mm: constify pagemap related test functions for
 improved const-correctness

On 01.09.25 17:47, Vlastimil Babka wrote:
> On 9/1/25 17:14, Lorenzo Stoakes wrote:
>> On Mon, Sep 01, 2025 at 04:50:50PM +0200, Max Kellermann wrote:
>>> On Mon, Sep 1, 2025 at 4:25 PM Lorenzo Stoakes
>>> <lorenzo.stoakes@...cle.com> wrote:
>>>> 1. (most useful) Const pointer (const <type> *<param>) means that the dereffed
>>>>     value is const, so *<param> = <val> or <param>-><field> = <val> are prohibited.
>>>
>>> Only this was what my initial patch was about.
>>
>> Right agreed then.
>>
>>>
>>>> 2. (less useful) We can't modify the actual pointer value either, so
>>>>     e.g. <param> = <new param> is prohibited.
>>>
>>> This wasn't my idea, it was Andrew Morton's idea, supported by Yuanchu Xie:
>>>   https://lore.kernel.org/lkml/CAJj2-QHVC0QW_4X95LLAnM=1g6apH==-OXZu65SVeBj0tSUcBg@mail.gmail.com/
>>
>> Andrew said:
>>
>> "Not that I'm suggesting that someone go in and make this change."
>>
>> And Yuanchu said:
>>
>> "Longer function readability would benefit from that, but it's IMO infeasible to
>> do so everywhere."
>>
>> (he also mentions it'd be good if gcc could wran on it).
>>
>> So this isn't quite true actually.
> 
> I understood it the same, that it would be nice if gcc treated incoming
> params (i.e. pointers, not pointed-to values) as const and warn otherwise,
> but not suggesting we should start doing that manually.
> 
> I personally agree that adding those extra "const" is of little value and
> makes the function definition lines longer and harder to read and so would
> rather not add those.
> 
> I mean we could first collectively decide (and that's not a review
> half-suggesting we do them) that we want them, and document that, but AFAIK
> that's not the case yet. While there's already an agreement that const
> pointed-to values is a good thing and nobody objects that.

Yeah, and discussed elsewhere in this series, it would also have to be 
clarified how to deal with the *const" (or const values in general) with 
function declaration vs. definition. I don't think we really have 
written-down rule for that yet.

-- 
Cheers

David / dhildenb

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