Re: [PATCH] mm/sparse: keep mem_section_usage_size() internal

["David Hildenbrand (Arm)" <david@kernel.org>]

On 8/4/26 12:01, Muchun Song wrote:
> mem_section_usage_size() is only needed by sparsemem implementation code,
> but declaring it in mmzone.h makes the helper visible to all mmzone.h
> users.

Worth mentioning that

commit ae751d567baa08342e5e34b378b72a6f9b2cfada
Author: David Hildenbrand (Arm) <david@kernel.org>
Date:   Mon May 11 16:05:34 2026 +0200

    mm/bootmem_info: stop marking mem_section_usage as MIX_SECTION_INFO

Remove the last external user.

> 
> Move the helper to sparse.h so sparse.c and sparse-vmemmap.c can share
> it through the internal header. While doing so, calculate the allocation
> size with struct_size(), which ties the expression to the pageblock_flags
> trailing array instead of open-coding the struct header plus bitmap size.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  include/linux/mmzone.h |  1 -
>  mm/sparse.c            | 10 ----------
>  mm/sparse.h            |  6 ++++++
>  3 files changed, 6 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index a26c8b855222..86891163e0ef 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -2070,7 +2070,6 @@ static inline struct mem_section *__nr_to_section(unsigned long nr)
>  #endif
>  	return &mem_section[root][nr & SECTION_ROOT_MASK];
>  }
> -extern size_t mem_section_usage_size(void);
>  
>  /*
>   * We use the lower bits of the mem_map pointer to store a little bit of
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 704a9dec2b9a..46458e5300e2 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -213,16 +213,6 @@ static void __init memblocks_present(void)
>  		memory_present(nid, start, end);
>  }
>  
> -static unsigned long usemap_size(void)
> -{
> -	return BITS_TO_LONGS(SECTION_BLOCKFLAGS_BITS) * sizeof(unsigned long);
> -}
> -
> -size_t mem_section_usage_size(void)
> -{
> -	return sizeof(struct mem_section_usage) + usemap_size();
> -}
> -
>  #ifdef CONFIG_SPARSEMEM_VMEMMAP
>  unsigned long __init section_map_size(void)
>  {
> diff --git a/mm/sparse.h b/mm/sparse.h
> index 95aa031213f2..4960a17b706f 100644
> --- a/mm/sparse.h
> +++ b/mm/sparse.h
> @@ -47,6 +47,12 @@ static inline void __section_mark_present(struct mem_section *ms,
>  
>  	ms->section_mem_map |= SECTION_MARKED_PRESENT;
>  }
> +
> +static inline size_t mem_section_usage_size(void)
> +{
> +	return struct_size((struct mem_section_usage *)NULL, pageblock_flags,
> +			   BITS_TO_LONGS(SECTION_BLOCKFLAGS_BITS));
> +}

There is struct_size_t() that avoids the manual "NULL" trick.

-- 
Cheers,

David