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

[David Hildenbrand (Arm)]

On 3/31/26 21:55, Andrew Morton 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 comment; such details risk falling out of date over time and may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.
> 
> Thanks.  So this can fix the build on some arch/kconfig combinations?
> 
> Do you think we should fix older kernels?
> 
>> Fixes: def9b71ee651 ("include/linux/mmzone.h: fix explanation of lower bits in the SPARSEMEM mem_map pointer")
> 
> Eight years ago so I'm going with "no".

IIUC, there is nothing broken. Could only be revealed by some new
architecture.

So no stable :)

-- 
Cheers,

David