linux-kernel - Re: [PATCH v1] mm: convert folio

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Message-ID: <CABzRoyaWCxOZAvMO5ZAxk3eaf4mkSQwfUwYW0dXcR3yhjCMJhA@mail.gmail.com>
Date: Wed, 24 Sep 2025 11:11:42 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Andrew Morton <akpm@...ux-foundation.org>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH v1] mm: convert folio_page() back to a macro

On Wed, Sep 24, 2025 at 12:44 AM David Hildenbrand <david@...hat.com> wrote:
>
> In commit 73b3294b1152 ("mm: simplify folio_page() and folio_page_idx()")
> we converted folio_page() into a static inline function. However
> briefly afterwards in commit a847b17009ec ("mm: constify highmem related
> functions for improved const-correctness") we had to add some nasty
> const-away casting to make the compiler happy when checking const
> correctness.
>
> So let's just convert it back to a simple macro so the compiler can
> check const correctness properly. There is the alternative of
> using a _Generic() similar to page_folio(), but there is not a lot of
> benefit compared to just using a simple macro.
>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Mike Rapoport <rppt@...nel.org>
> Cc: Suren Baghdasaryan <surenb@...gle.com>
> Cc: Michal Hocko <mhocko@...e.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---

LGTM. Feel free to add:
Reviewed-by: Lance Yang <lance.yang@...ux.dev>

>  include/linux/page-flags.h | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 568011930e358..48e27768e7ba9 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -316,10 +316,7 @@ static __always_inline unsigned long _compound_head(const struct page *page)
>   * 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(const struct folio *folio, unsigned long n)
> -{
> -       return (struct page *)(&folio->page + n);
> -}
> +#define folio_page(folio, n)   (&(folio)->page + (n))
>
>  static __always_inline int PageTail(const struct page *page)
>  {
> --
> 2.51.0
>
>