[PATCH v2] mm/sparse: fix comment for section map alignment

[Muchun Song <songmuchun@bytedance.com>]

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 Such details risk falling out of date over time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.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
v1 -> v2:
 - Drop the actual BUILD_BUG_ON logic modification (keeping the simple
   comparison) and only simplify/clarify the mmzone.h comment.
 - Add explanation explicitly noting that a single section is always
   expected to cover full pages, per discussions with David Hildenbrand
   and Andrew Morton.
---
 include/linux/mmzone.h | 25 ++++++++++---------------
 1 file changed, 10 insertions(+), 15 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 7de42be81d4b..a071f1a0e242 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -2056,21 +2056,16 @@ 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,
-- 
2.20.1