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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 Such details risk falling out of date over time and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 <songmuchun@bytedance.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Petr Tesarik <ptesarik@suse.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

authored by

Muchun Song and committed by
Andrew Morton 77c368f0 df620ec4

+10 -15
+10 -15
include/linux/mmzone.h
··· 2068 2068 extern size_t mem_section_usage_size(void); 2069 2069 2070 2070 /* 2071 - * We use the lower bits of the mem_map pointer to store 2072 - * a little bit of information. The pointer is calculated 2073 - * as mem_map - section_nr_to_pfn(pnum). The result is 2074 - * aligned to the minimum alignment of the two values: 2075 - * 1. All mem_map arrays are page-aligned. 2076 - * 2. section_nr_to_pfn() always clears PFN_SECTION_SHIFT 2077 - * lowest bits. PFN_SECTION_SHIFT is arch-specific 2078 - * (equal SECTION_SIZE_BITS - PAGE_SHIFT), and the 2079 - * worst combination is powerpc with 256k pages, 2080 - * which results in PFN_SECTION_SHIFT equal 6. 2081 - * To sum it up, at least 6 bits are available on all architectures. 2082 - * However, we can exceed 6 bits on some other architectures except 2083 - * powerpc (e.g. 15 bits are available on x86_64, 13 bits are available 2084 - * with the worst case of 64K pages on arm64) if we make sure the 2085 - * exceeded bit is not applicable to powerpc. 2071 + * We use the lower bits of the mem_map pointer to store a little bit of 2072 + * information. The pointer is calculated as mem_map - section_nr_to_pfn(). 2073 + * The result is aligned to the minimum alignment of the two values: 2074 + * 2075 + * 1. All mem_map arrays are page-aligned. 2076 + * 2. section_nr_to_pfn() always clears PFN_SECTION_SHIFT lowest bits. 2077 + * 2078 + * We always expect a single section to cover full pages. Therefore, 2079 + * we can safely assume that PFN_SECTION_SHIFT is large enough to 2080 + * accommodate SECTION_MAP_LAST_BIT. We use BUILD_BUG_ON() to ensure this. 2086 2081 */ 2087 2082 enum { 2088 2083 SECTION_MARKED_PRESENT_BIT,