Re: [PATCH v2 03/10] filemap: Remove checks in mapping_set_folio_order_range()

[Usama Arif <usama.arif@linux.dev>]

On Thu,  9 Jul 2026 19:47:31 +0100 "Matthew Wilcox (Oracle)" <willy@infradead.org> wrote:

> These checks make it impossible to use mapping_set_folio_order_range()
> in hugetlb.  They add very little value and can simply be removed.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  include/linux/pagemap.h | 21 +++++----------------
>  1 file changed, 5 insertions(+), 16 deletions(-)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 2c3718d592d6..d0c9aecaa4ac 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -405,29 +405,18 @@ static inline size_t mapping_max_folio_size_supported(void)
>   * @min: Minimum folio order (between 0-MAX_PAGECACHE_ORDER inclusive).
>   * @max: Maximum folio order (between @min-MAX_PAGECACHE_ORDER inclusive).
>   *
> - * The filesystem should call this function in its inode constructor to
> - * indicate which base size (min) and maximum size (max) of folio the VFS
> - * can use to cache the contents of the file.  This should only be used
> - * if the filesystem needs special handling of folio sizes (ie there is
> - * something the core cannot know).
> + * The filesystem should call this function in its inode constructor
> + * to indicate which size folios can be used to cache the contents of
> + * the inode.  This should only be used if the filesystem needs special
> + * handling of folio sizes (ie there is something the core cannot know).
>   * Do not tune it based on, eg, i_size.
>   *
>   * Context: This should not be called while the inode is active as it
>   * is non-atomic.
>   */
>  static inline void mapping_set_folio_order_range(struct address_space *mapping,
> -						 unsigned int min,
> -						 unsigned int max)
> +		unsigned int min, unsigned int max)
>  {
> -	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
> -		return;
> -
> -	if (min > MAX_PAGECACHE_ORDER)
> -		min = MAX_PAGECACHE_ORDER;
> -
> -	if (max > MAX_PAGECACHE_ORDER)
> -		max = MAX_PAGECACHE_ORDER;
> -

This one is a bit interesting, This has been useful to me in the past when I
was trying to [1] use mapping_max_folio_order() for force_thp_readahead order.
On arm64 with 64K base PAGE_SIZE HPAGE_PMD_ORDER is 13 (512MB), while
existing MAX_PAGECACHE_ORDER was smaller and the cap is what prevents generic
page-cache code from advertising that unsupported 512MB order due to
MAX_XAS_ORDER.

[1] https://lore.kernel.org/all/20260601102205.3985788-3-usama.arif@linux.dev/

>  	if (max < min)
>  		max = min;
>  
> -- 
> 2.47.3
> 
>