Re: [PATCH] mm/sparse: fix BUILD_BUG_ON check for section map alignment

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 31 Mar 2026 19:30:23 +0800 Muchun Song <songmuchun@bytedance.com> wrote:

> The comment in mmzone.h states that the alignment requirement
> is the minimum of PAGE_SHIFT and PFN_SECTION_SHIFT. However, the
> pointer arithmetic (mem_map - section_nr_to_pfn()) results in
> a byte offset scaled by sizeof(struct page). Thus, the actual
> alignment provided by the second term is PFN_SECTION_SHIFT +
> __ffs(sizeof(struct page)).
> 
> Update the compile-time check and the mmzone.h comment to
> accurately reflect this mathematically guaranteed alignment by
> taking the minimum of PAGE_SHIFT and PFN_SECTION_SHIFT +
> __ffs(sizeof(struct page)). This avoids the issue of the check
> being overly restrictive on architectures like powerpc where
> PFN_SECTION_SHIFT alone is very small (e.g., 6).
> 
> Also, remove the exhaustive per-architecture bit-width list from the
> comment; such details risk falling out of date over time and may
> inadvertently be left un-updated, while the existing BUILD_BUG_ON
> provides sufficient compile-time verification of the constraint.
> 
> No runtime impact so far: SECTION_MAP_LAST_BIT happens to fit within
> the smaller limit on all existing architectures.
> 
> ...
>
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -269,7 +269,8 @@ static unsigned long sparse_encode_mem_map(struct page *mem_map, unsigned long p
>  {
>  	unsigned long coded_mem_map =
>  		(unsigned long)(mem_map - (section_nr_to_pfn(pnum)));
> -	BUILD_BUG_ON(SECTION_MAP_LAST_BIT > PFN_SECTION_SHIFT);
> +	BUILD_BUG_ON(SECTION_MAP_LAST_BIT > min(PFN_SECTION_SHIFT + __ffs(sizeof(struct page)),
> +						PAGE_SHIFT));
>  	BUG_ON(coded_mem_map & ~SECTION_MAP_MASK);
>  	return coded_mem_map;
>  }

In mm-stable this was moved into mm/internal.h's new
sparse_init_one_section().  By David's 6a2f8fb8ed2d ("mm/sparse: move
sparse_init_one_section() to internal.h")

I did the obvious thing:

 include/linux/mmzone.h |   24 +++++++++---------------
 mm/internal.h          |    3 ++-
 2 files changed, 11 insertions(+), 16 deletions(-)

--- a/include/linux/mmzone.h~mm-sparse-fix-build_bug_on-check-for-section-map-alignment
+++ a/include/linux/mmzone.h
@@ -2068,21 +2068,15 @@ static inline struct mem_section *__nr_t
 extern size_t mem_section_usage_size(void);
 
 /*
- * We use the lower bits of the mem_map pointer to store
- * a little bit of information.  The pointer is calculated
- * as mem_map - section_nr_to_pfn(pnum).  The result is
- * aligned to the minimum alignment of the two values:
- *   1. All mem_map arrays are page-aligned.
- *   2. section_nr_to_pfn() always clears PFN_SECTION_SHIFT
- *      lowest bits.  PFN_SECTION_SHIFT is arch-specific
- *      (equal SECTION_SIZE_BITS - PAGE_SHIFT), and the
- *      worst combination is powerpc with 256k pages,
- *      which results in PFN_SECTION_SHIFT equal 6.
- * To sum it up, at least 6 bits are available on all architectures.
- * However, we can exceed 6 bits on some other architectures except
- * powerpc (e.g. 15 bits are available on x86_64, 13 bits are available
- * with the worst case of 64K pages on arm64) if we make sure the
- * exceeded bit is not applicable to powerpc.
+ * We use the lower bits of the mem_map pointer to store a little bit of
+ * information. The pointer is calculated as mem_map - section_nr_to_pfn().
+ * The result is aligned to the minimum alignment of the two values:
+ *
+ * 1. All mem_map arrays are page-aligned.
+ * 2. section_nr_to_pfn() always clears PFN_SECTION_SHIFT lowest bits. Because
+ *    it is subtracted from a struct page pointer, the offset is scaled by
+ *    sizeof(struct page). This provides an alignment of PFN_SECTION_SHIFT +
+ *    __ffs(sizeof(struct page)).
  */
 enum {
 	SECTION_MARKED_PRESENT_BIT,
--- a/mm/internal.h~mm-sparse-fix-build_bug_on-check-for-section-map-alignment
+++ a/mm/internal.h
@@ -972,7 +972,8 @@ static inline void sparse_init_one_secti
 {
 	unsigned long coded_mem_map;
 
-	BUILD_BUG_ON(SECTION_MAP_LAST_BIT > PFN_SECTION_SHIFT);
+	BUILD_BUG_ON(SECTION_MAP_LAST_BIT > min(PFN_SECTION_SHIFT + __ffs(sizeof(struct page)),
+						PAGE_SHIFT));
 
 	/*
 	 * We encode the start PFN of the section into the mem_map such that
_

(boy that's an eyesore on an 80-col xterm!)