Subject: Re: linux-next: build warning after merge of the mm-unstable tree

From: Andrew Morton <[email-masked]>
Date: Wed, 3 Sep 2025 19:59:47 -0700

On Thu, 4 Sep 2025 12:14:38 +1000 Stephen Rothwell <[email-masked]> wrote:

> After merging the mm-unstable tree, today's linux-next build (arm
> multi_v7_defconfig) produced this warning:
> 
> In file included from include/linux/highmem.h:14,
>                  from include/linux/bvec.h:10,
>                  from include/linux/blk_types.h:10,
>                  from include/linux/writeback.h:13,
>                  from include/linux/memcontrol.h:23,
>                  from include/linux/swap.h:9,
>                  from include/linux/suspend.h:5,
>                  from arch/arm/mach-highbank/pm.c:9:
> include/linux/highmem-internal.h: In function 'kmap_local_folio':
> include/linux/highmem-internal.h:86:46: warning: passing argument 1 of 'folio_page' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
>    86 |         const struct page *page = folio_page(folio, offset / PAGE_SIZE);
>       |                                              ^~~~~
> In file included from include/linux/mmzone.h:23,
>                  from include/linux/swap.h:7:
> include/linux/page-flags.h:319:53: note: expected 'struct folio *' but argument is of type 'const struct folio *'
>   319 | static inline struct page *folio_page(struct folio *folio, unsigned long n)

Yeah, thanks, folio_page() is a problem.  I'll do the below nasty for
now, but nasty.

--- a/include/linux/page-flags.h~mm-constify-highmem-related-functions-for-improved-const-correctness-fix
+++ a/include/linux/page-flags.h
@@ -316,9 +316,9 @@ static __always_inline unsigned long _co
  * check that the page number lies within @folio; the caller is presumed
  * to have a reference to the page.
  */
-static inline struct page *folio_page(struct folio *folio, unsigned long n)
+static inline struct page *folio_page(const struct folio *folio, unsigned long n)
 {
-	return &folio->page + n;
+	return (struct page *)(&folio->page + n);
 }
 
 static __always_inline int PageTail(const struct page *page)
_