linux-kernel - Re: [PATCH] mm: add `const` to lots of pointer parameters

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Message-Id: <20250827144832.87d2f1692fe61325628710f4@linux-foundation.org>
Date: Wed, 27 Aug 2025 14:48:32 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Max Kellermann <max.kellermann@...os.com>
Cc: david@...hat.com, lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com,
 vbabka@...e.cz, rppt@...nel.org, surenb@...gle.com, mhocko@...e.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: add `const` to lots of pointer parameters

On Wed, 27 Aug 2025 21:22:33 +0200 Max Kellermann <max.kellermann@...os.com> wrote:

> For improved const-correctness.

OK...

>
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -537,7 +537,7 @@ struct address_space {
>  /*
>   * Returns true if any of the pages in the mapping are marked with the tag.
>   */
> -static inline bool mapping_tagged(struct address_space *mapping, xa_mark_t tag)
> +static inline bool mapping_tagged(const struct address_space *mapping, xa_mark_t tag)
>  {
>  	return xa_marked(&mapping->i_pages, tag);
>  }

I'd actually be in favor of making all incoming args const (C should
have made this the default).

Because modifying an incoming arg is just obnoxious.  That value should
be viewed as part of the calling environment and should not be altered.

Try modifying a lengthy function and wanting to get at an incoming arg
only to find that something in the preceding 100 lines has gone and
messed with it.  Or forget to check fr this and get a nasty surprise when
testing.

Not that I'm suggesting that someone go in and make this change.

On the other hand, it would be neat if gcc had an option to warn when
someone does this.  I bet it would be simple to add.