mm/sparse: move sparse_init_one_section() to internal.h

While at it, convert the BUG_ON to a VM_WARN_ON_ONCE, avoid long lines,
and merge sparse_encode_mem_map() into its only caller
sparse_init_one_section().

Clarify the comment a bit, pointing at page_to_pfn().

[david@kernel.org: s/VM_WARN_ON/VM_WARN_ON_ONCE/]
  Link: https://lkml.kernel.org/r/6b04c1a1-74e7-42e8-8523-a40802e5dacc@kernel.org
Link: https://lkml.kernel.org/r/20260320-sparsemem_cleanups-v2-13-096addc8800d@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

3 files changed, 23 insertions, 25 deletions

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 7cf4a194aea2..ed335567d64e 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1988,7 +1988,7 @@ struct mem_section {
 	/*
 	 * This is, logically, a pointer to an array of struct
 	 * pages.  However, it is stored with some other magic.
-	 * (see sparse.c::sparse_init_one_section())
+	 * (see sparse_init_one_section())
 	 *
 	 * Additionally during early boot we encode node id of
 	 * the location of the section here to guide allocation.
diff --git a/mm/internal.h b/mm/internal.h
index 11b0c91b6d9d..e14f58527688 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -964,6 +964,28 @@ void memmap_init_range(unsigned long, int, unsigned long, unsigned long,
  */
 #ifdef CONFIG_SPARSEMEM
 void sparse_init(void);
+
+static inline void sparse_init_one_section(struct mem_section *ms,
+		unsigned long pnum, struct page *mem_map,
+		struct mem_section_usage *usage, unsigned long flags)
+{
+	unsigned long coded_mem_map;
+
+	BUILD_BUG_ON(SECTION_MAP_LAST_BIT > PFN_SECTION_SHIFT);
+
+	/*
+	 * We encode the start PFN of the section into the mem_map such that
+	 * page_to_pfn() on !CONFIG_SPARSEMEM_VMEMMAP can simply subtract it
+	 * from the page pointer to obtain the PFN.
+	 */
+	coded_mem_map = (unsigned long)(mem_map - section_nr_to_pfn(pnum));
+	VM_WARN_ON_ONCE(coded_mem_map & ~SECTION_MAP_MASK);
+
+	ms->section_mem_map &= ~SECTION_MAP_MASK;
+	ms->section_mem_map |= coded_mem_map;
+	ms->section_mem_map |= flags | SECTION_HAS_MEM_MAP;
+	ms->usage = usage;
+}
 #else
 static inline void sparse_init(void) {}
 #endif /* CONFIG_SPARSEMEM */
diff --git a/mm/sparse.c b/mm/sparse.c
index 5c9cad390282..ed5de1a25f04 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -256,30 +256,6 @@ static void __init memblocks_present(void)
 		memory_present(nid, start, end);
 }
 
-/*
- * Subtle, we encode the real pfn into the mem_map such that
- * the identity pfn - section_mem_map will return the actual
- * physical page frame number.
- */
-static unsigned long sparse_encode_mem_map(struct page *mem_map, unsigned long pnum)
-{
-	unsigned long coded_mem_map =
-		(unsigned long)(mem_map - (section_nr_to_pfn(pnum)));
-	BUILD_BUG_ON(SECTION_MAP_LAST_BIT > PFN_SECTION_SHIFT);
-	BUG_ON(coded_mem_map & ~SECTION_MAP_MASK);
-	return coded_mem_map;
-}
-
-static void __meminit sparse_init_one_section(struct mem_section *ms,
-		unsigned long pnum, struct page *mem_map,
-		struct mem_section_usage *usage, unsigned long flags)
-{
-	ms->section_mem_map &= ~SECTION_MAP_MASK;
-	ms->section_mem_map |= sparse_encode_mem_map(mem_map, pnum)
-		| SECTION_HAS_MEM_MAP | flags;
-	ms->usage = usage;
-}
-
 static unsigned long usemap_size(void)
 {
 	return BITS_TO_LONGS(SECTION_BLOCKFLAGS_BITS) * sizeof(unsigned long);