From: Muchun Song Date: Thu, 2 Apr 2026 10:23:20 +0000 (+0800) Subject: mm/sparse: fix comment for section map alignment X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=77c368f057e17b59b23899a1907ee9d4f4d7a532;p=thirdparty%2Fkernel%2Flinux.git mm/sparse: fix comment for section map alignment The comment in mmzone.h currently details exhaustive per-architecture bit-width lists and explains alignment using min(PAGE_SHIFT, PFN_SECTION_SHIFT). Such details risk falling out of date over time and may inadvertently be left un-updated. We always expect a single section to cover full pages. Therefore, we can safely assume that PFN_SECTION_SHIFT is large enough to accommodate SECTION_MAP_LAST_BIT. We use BUILD_BUG_ON() to ensure this. Update the comment to accurately reflect this consensus, making it clear that we rely on a single section covering full pages. Link: https://lore.kernel.org/20260402102320.3617578-1-songmuchun@bytedance.com Signed-off-by: Muchun Song Acked-by: David Hildenbrand (Arm) Cc: Liam Howlett Cc: Lorenzo Stoakes (Oracle) Cc: Michal Hocko Cc: Mike Rapoport Cc: Petr Tesarik Cc: Suren Baghdasaryan Signed-off-by: Andrew Morton --- diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index 20f920dede65..07f501a62d67 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -2068,21 +2068,16 @@ static inline struct mem_section *__nr_to_section(unsigned long nr) 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. + * + * We always expect a single section to cover full pages. Therefore, + * we can safely assume that PFN_SECTION_SHIFT is large enough to + * accommodate SECTION_MAP_LAST_BIT. We use BUILD_BUG_ON() to ensure this. */ enum { SECTION_MARKED_PRESENT_BIT,