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

[Muchun Song]

> On Apr 1, 2026, at 04:07, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> On Tue, 31 Mar 2026 19:30:23 +0800 Muchun Song <songmuchun@xxxxxxxxxxxxx> 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 comment; such details risk falling out of date over time and may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")

Got it. I see it.

> 
> I did the obvious thing:

Thank you for your work for me.

> 
> 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!)