[PATCH v2 51/69] mm/sparse-vmemmap: Introduce vmemmap_nr_struct_pages()

[Muchun Song <songmuchun@bytedance.com>]

compound_nr_pages() exposes sparse vmemmap optimization details to the
core memory initialization code.

Introduce vmemmap_nr_struct_pages() to report how many struct pages are
actually allocated and need initialization for an optimized vmemmap
mapping. This gives memmap_init_zone_device() the information it needs
without depending on sparse-vmemmap internals.

With this helper in place, drop compound_nr_pages() and keep the
vmemmap-specific logic inside sparse-vmemmap code.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/internal.h | 11 ++++++++++-
 mm/mm_init.c  | 21 +--------------------
 mm/sparse.c   | 13 ++++++-------
 3 files changed, 17 insertions(+), 28 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 06022074ebcb..9597a703bc73 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -997,7 +997,16 @@ static inline void __section_mark_present(struct mem_section *ms,
 	ms->section_mem_map |= SECTION_MARKED_PRESENT;
 }
 
-int section_nr_vmemmap_pages(unsigned long pfn, unsigned long nr_pages);
+int vmemmap_nr_struct_pages(unsigned long pfn, unsigned long nr_pages);
+
+static inline int section_nr_vmemmap_pages(unsigned long pfn, unsigned long nr_pages)
+{
+	VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, PAGES_PER_SUBSECTION));
+	VM_WARN_ON_ONCE(nr_pages > PAGES_PER_SECTION);
+
+	return DIV_ROUND_UP(vmemmap_nr_struct_pages(pfn, nr_pages) * sizeof(struct page),
+			    PAGE_SIZE);
+}
 #else
 static inline void sparse_memblocks_present(void) {}
 static inline void sparse_init(void) {}
diff --git a/mm/mm_init.c b/mm/mm_init.c
index 9ff118e35641..4ea39392993b 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1062,25 +1062,6 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
 	}
 }
 
-/*
- * With compound page geometry and when struct pages are stored in ram most
- * tail pages are reused. Consequently, the amount of unique struct pages to
- * initialize is a lot smaller that the total amount of struct pages being
- * mapped. This is a paired / mild layering violation with explicit knowledge
- * of how the sparse_vmemmap internals handle compound pages in the lack
- * of an altmap.
- */
-static inline unsigned long compound_nr_pages(unsigned long pfn,
-					      struct dev_pagemap *pgmap)
-{
-	const struct mem_section *ms = __pfn_to_section(pfn);
-
-	if (!section_vmemmap_optimizable(ms))
-		return pgmap_vmemmap_nr(pgmap);
-
-	return VMEMMAP_RESERVE_NR * (PAGE_SIZE / sizeof(struct page));
-}
-
 static void __ref memmap_init_compound(struct page *head,
 				       unsigned long head_pfn,
 				       unsigned long zone_idx, int nid,
@@ -1145,7 +1126,7 @@ void __ref memmap_init_zone_device(struct zone *zone,
 			continue;
 
 		memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
-				     compound_nr_pages(pfn, pgmap));
+				     vmemmap_nr_struct_pages(pfn, pfns_per_compound));
 	}
 
 	pageblock_migratetype_init_range(start_pfn, nr_pages, MIGRATE_MOVABLE, false, false);
diff --git a/mm/sparse.c b/mm/sparse.c
index 3390cb82f114..f314b9babc4a 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -237,26 +237,25 @@ void __weak __meminit vmemmap_populate_print_last(void)
 {
 }
 
-int __meminit section_nr_vmemmap_pages(unsigned long pfn, unsigned long nr_pages)
+int __meminit vmemmap_nr_struct_pages(unsigned long pfn, unsigned long nr_pages)
 {
 	const unsigned int order = pfn_to_section_order(pfn);
 	const unsigned long pages_per_compound = 1UL << order;
 
-	VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, PAGES_PER_SUBSECTION));
-	VM_WARN_ON_ONCE(nr_pages > PAGES_PER_SECTION);
-
 	if (!order_vmemmap_optimizable(order))
-		return DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE);
+		return nr_pages;
 
 	if (order < PFN_SECTION_SHIFT) {
 		VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, pages_per_compound));
-		return OPTIMIZED_FOLIO_VMEMMAP_PAGES * nr_pages / pages_per_compound;
+		return OPTIMIZED_FOLIO_VMEMMAP_NR_STRUCT_PAGES * nr_pages / pages_per_compound;
 	}
 
 	VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, PAGES_PER_SECTION));
+	/* Ensure the requested range does not cross a compound page boundary. */
+	VM_WARN_ON_ONCE((pfn % pages_per_compound) + nr_pages > pages_per_compound);
 
 	if (IS_ALIGNED(pfn, pages_per_compound))
-		return OPTIMIZED_FOLIO_VMEMMAP_PAGES;
+		return OPTIMIZED_FOLIO_VMEMMAP_NR_STRUCT_PAGES;
 
 	return 0;
 }
-- 
2.54.0