[PATCH v2] mm/sparse: keep mem_section_usage_size() internal

[PATCH v2] mm/sparse: keep mem_section_usage_size() internal

[Muchun Song]

mem_section_usage_size() is only needed by sparsemem implementation code
after commit ae751d567baa ("mm/bootmem_info: stop marking mem_section_usage
as MIX_SECTION_INFO"), so keeping the declaration in mmzone.h now exposes
the helper to all mmzone.h users for no reason.

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_t(), 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>
---
v2:
- Mention that ae751d567baa removed the last external user (David)
- Use struct_size_t() for the mem_section_usage size calculation (David)
---
 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..3b744667a7e6 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_t(struct mem_section_usage, pageblock_flags,
+			     BITS_TO_LONGS(SECTION_BLOCKFLAGS_BITS));
+}
 #else
 static inline void sparse_init(void) {}
 #endif /* CONFIG_SPARSEMEM */
-- 
2.54.0

Re: [PATCH v2] mm/sparse: keep mem_section_usage_size() internal

[Mike Rapoport]

On Wed, Aug 05, 2026 at 10:25:36AM +0800, Muchun Song wrote:
> mem_section_usage_size() is only needed by sparsemem implementation code
> after commit ae751d567baa ("mm/bootmem_info: stop marking mem_section_usage
> as MIX_SECTION_INFO"), so keeping the declaration in mmzone.h now exposes
> the helper to all mmzone.h users for no reason.
> 
> 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_t(), 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>

Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
> v2:
> - Mention that ae751d567baa removed the last external user (David)
> - Use struct_size_t() for the mem_section_usage size calculation (David)
> ---
>  include/linux/mmzone.h |  1 -
>  mm/sparse.c            | 10 ----------
>  mm/sparse.h            |  6 ++++++
>  3 files changed, 6 insertions(+), 11 deletions(-)

-- 
Sincerely yours,
Mike.

Re: [PATCH v2] mm/sparse: keep mem_section_usage_size() internal

[David Hildenbrand (Arm)]

On 8/5/26 04:25, Muchun Song wrote:
> mem_section_usage_size() is only needed by sparsemem implementation code
> after commit ae751d567baa ("mm/bootmem_info: stop marking mem_section_usage
> as MIX_SECTION_INFO"), so keeping the declaration in mmzone.h now exposes
> the helper to all mmzone.h users for no reason.
> 
> 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_t(), 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>
> ---

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David