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  Such details risk falling out of date over time and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>

Author: songmuchun

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,