linux-kernel - Re: [PATCH v5 02/12] mm: constify pagemap related test functions for improved const-correctness

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Message-ID: <925480c3-d0ff-4f24-ada0-966ad9a83080@suse.cz>
Date: Mon, 1 Sep 2025 17:47:54 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: 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 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.