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

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Message-ID: <cc72d070-4225-4256-9745-320b7bb669d8@lucifer.local>
Date: Mon, 1 Sep 2025 16:55:25 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Vlastimil Babka <vbabka@...e.cz>,
        Max Kellermann <max.kellermann@...os.com>, akpm@...ux-foundation.org,
        axelrasmussen@...gle.com, yuanchu@...gle.com, willy@...radead.org,
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Subject: Re: [PATCH v5 02/12] mm: constify pagemap related test functions for
 improved const-correctness

On Mon, Sep 01, 2025 at 05:52:47PM +0200, David Hildenbrand wrote:
> 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.

For this series the consensus is clear that we should eliminate these and revert
to const pointed-to values (1) only.

We can determine how we do this in future re: const actual pointers (2), but
this series isn't the place.

So Max - can you respin with the (2) const-ification removed please.

Thanks.