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

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Andrew Morton <akpm@linux-foundation.org> [250827 17:48]:
> On Wed, 27 Aug 2025 21:22:33 +0200 Max Kellermann <max.kellermann@ionos.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.

But what of the lazy programmer that just uses the variable at hand
instead of declaring another one?

I also wonder how this would affect inlining choices?