mm: convert head_subpages_mapcount() into folio_nr_pages_mapped()

Calling this 'mapcount' is confusing since mapcount is usually the number
of times something is mapped; instead this is the number of mapped pages. 
It's also better to enforce that this is a folio rather than a head page.

Move folio_nr_pages_mapped() into mm/internal.h since this is not
something we want device drivers or filesystems poking at.  Get rid of
folio_subpages_mapcount_ptr() and use folio->_nr_pages_mapped directly.

Link: https://lkml.kernel.org/r/20230111142915.1001531-3-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm.h

@@ -843,24 +843,6 @@ static inline int head_compound_mapcount(struct page *head)
 	return atomic_read(compound_mapcount_ptr(head)) + 1;
 }
 
-/*
- * If a 16GB hugetlb page were mapped by PTEs of all of its 4kB sub-pages,
- * its subpages_mapcount would be 0x400000: choose the COMPOUND_MAPPED bit
- * above that range, instead of 2*(PMD_SIZE/PAGE_SIZE).  Hugetlb currently
- * leaves subpages_mapcount at 0, but avoid surprise if it participates later.
- */
-#define COMPOUND_MAPPED	0x800000
-#define SUBPAGES_MAPPED	(COMPOUND_MAPPED - 1)
-
-/*
- * Number of sub-pages mapped by PTE, does not include compound mapcount.
- * Must be called only on head of compound page.
- */
-static inline int head_subpages_mapcount(struct page *head)
-{
-	return atomic_read(subpages_mapcount_ptr(head)) & SUBPAGES_MAPPED;
-}
-
 /*
  * The atomic page->_mapcount, starts from -1: so that transitions
  * both from it and to it can be tracked, using atomic_inc_and_test
@@ -920,9 +902,9 @@ static inline bool folio_large_is_mapped(struct folio *folio)
 {
 	/*
 	 * Reading folio_mapcount_ptr() below could be omitted if hugetlb
-	 * participated in incrementing subpages_mapcount when compound mapped.
+	 * participated in incrementing nr_pages_mapped when compound mapped.
 	 */
-	return atomic_read(folio_subpages_mapcount_ptr(folio)) > 0 ||
+	return atomic_read(&folio->_nr_pages_mapped) > 0 ||
 		atomic_read(folio_mapcount_ptr(folio)) >= 0;
 }
 

include/linux/mm_types.h

@@ -307,7 +307,7 @@ static inline struct page *encoded_page_ptr(struct encoded_page *page)
  * @_folio_dtor: Which destructor to use for this folio.
  * @_folio_order: Do not use directly, call folio_order().
  * @_compound_mapcount: Do not use directly, call folio_entire_mapcount().
- * @_subpages_mapcount: Do not use directly, call folio_mapcount().
+ * @_nr_pages_mapped: Do not use directly, call folio_mapcount().
  * @_pincount: Do not use directly, call folio_maybe_dma_pinned().
  * @_folio_nr_pages: Do not use directly, call folio_nr_pages().
  * @_flags_2: For alignment.  Do not use.
@@ -361,7 +361,7 @@ struct folio {
 			unsigned char _folio_dtor;
 			unsigned char _folio_order;
 			atomic_t _compound_mapcount;
-			atomic_t _subpages_mapcount;
+			atomic_t _nr_pages_mapped;
 			atomic_t _pincount;
 #ifdef CONFIG_64BIT
 			unsigned int _folio_nr_pages;
@@ -404,7 +404,7 @@ FOLIO_MATCH(compound_head, _head_1);
 FOLIO_MATCH(compound_dtor, _folio_dtor);
 FOLIO_MATCH(compound_order, _folio_order);
 FOLIO_MATCH(compound_mapcount, _compound_mapcount);
-FOLIO_MATCH(subpages_mapcount, _subpages_mapcount);
+FOLIO_MATCH(subpages_mapcount, _nr_pages_mapped);
 FOLIO_MATCH(compound_pincount, _pincount);
 #ifdef CONFIG_64BIT
 FOLIO_MATCH(compound_nr, _folio_nr_pages);
@@ -427,12 +427,6 @@ static inline atomic_t *folio_mapcount_ptr(struct folio *folio)
 	return &tail->compound_mapcount;
 }
 
-static inline atomic_t *folio_subpages_mapcount_ptr(struct folio *folio)
-{
-	struct page *tail = &folio->page + 1;
-	return &tail->subpages_mapcount;
-}
-
 static inline atomic_t *compound_mapcount_ptr(struct page *page)
 {
 	return &page[1].compound_mapcount;

mm/debug.c

@@ -94,10 +94,10 @@ static void __dump_page(struct page *page)
 			page, page_ref_count(head), mapcount, mapping,
 			page_to_pgoff(page), page_to_pfn(page));
 	if (compound) {
-		pr_warn("head:%p order:%u compound_mapcount:%d subpages_mapcount:%d pincount:%d\n",
+		pr_warn("head:%p order:%u compound_mapcount:%d nr_pages_mapped:%d pincount:%d\n",
 				head, compound_order(head),
 				head_compound_mapcount(head),
-				head_subpages_mapcount(head),
+				folio_nr_pages_mapped(folio),
 				atomic_read(&folio->_pincount));
 	}
 

mm/hugetlb.c

@@ -1475,7 +1475,7 @@ static void __destroy_compound_gigantic_folio(struct folio *folio,
 	struct page *p;
 
 	atomic_set(folio_mapcount_ptr(folio), 0);
-	atomic_set(folio_subpages_mapcount_ptr(folio), 0);
+	atomic_set(&folio->_nr_pages_mapped, 0);
 	atomic_set(&folio->_pincount, 0);
 
 	for (i = 1; i < nr_pages; i++) {
@@ -1997,7 +1997,7 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
 			set_compound_head(p, &folio->page);
 	}
 	atomic_set(folio_mapcount_ptr(folio), -1);
-	atomic_set(folio_subpages_mapcount_ptr(folio), 0);
+	atomic_set(&folio->_nr_pages_mapped, 0);
 	atomic_set(&folio->_pincount, 0);
 	return true;
 

mm/internal.h

@@ -52,6 +52,24 @@ struct folio_batch;
 
 void page_writeback_init(void);
 
+/*
+ * If a 16GB hugetlb folio were mapped by PTEs of all of its 4kB pages,
+ * its nr_pages_mapped would be 0x400000: choose the COMPOUND_MAPPED bit
+ * above that range, instead of 2*(PMD_SIZE/PAGE_SIZE).  Hugetlb currently
+ * leaves nr_pages_mapped at 0, but avoid surprise if it participates later.
+ */
+#define COMPOUND_MAPPED		0x800000
+#define FOLIO_PAGES_MAPPED	(COMPOUND_MAPPED - 1)
+
+/*
+ * How many individual pages have an elevated _mapcount.  Excludes
+ * the folio's entire_mapcount.
+ */
+static inline int folio_nr_pages_mapped(struct folio *folio)
+{
+	return atomic_read(&folio->_nr_pages_mapped) & FOLIO_PAGES_MAPPED;
+}
+
 static inline void *folio_raw_mapping(struct folio *folio)
 {
 	unsigned long mapping = (unsigned long)folio->mapping;

mm/rmap.c

@@ -1080,12 +1080,13 @@ int pfn_mkclean_range(unsigned long pfn, unsigned long nr_pages, pgoff_t pgoff,
 
 int total_compound_mapcount(struct page *head)
 {
+	struct folio *folio = (struct folio *)head;
 	int mapcount = head_compound_mapcount(head);
 	int nr_subpages;
 	int i;
 
 	/* In the common case, avoid the loop when no subpages mapped by PTE */
-	if (head_subpages_mapcount(head) == 0)
+	if (folio_nr_pages_mapped(folio) == 0)
 		return mapcount;
 	/*
 	 * Add all the PTE mappings of those subpages mapped by PTE.
@@ -1233,7 +1234,7 @@ void page_add_anon_rmap(struct page *page,
 			nr = atomic_add_return_relaxed(COMPOUND_MAPPED, mapped);
 			if (likely(nr < COMPOUND_MAPPED + COMPOUND_MAPPED)) {
 				nr_pmdmapped = thp_nr_pages(page);
-				nr = nr_pmdmapped - (nr & SUBPAGES_MAPPED);
+				nr = nr_pmdmapped - (nr & FOLIO_PAGES_MAPPED);
 				/* Raced ahead of a remove and another add? */
 				if (unlikely(nr < 0))
 					nr = 0;
@@ -1337,7 +1338,7 @@ void page_add_file_rmap(struct page *page,
 			nr = atomic_add_return_relaxed(COMPOUND_MAPPED, mapped);
 			if (likely(nr < COMPOUND_MAPPED + COMPOUND_MAPPED)) {
 				nr_pmdmapped = thp_nr_pages(page);
-				nr = nr_pmdmapped - (nr & SUBPAGES_MAPPED);
+				nr = nr_pmdmapped - (nr & FOLIO_PAGES_MAPPED);
 				/* Raced ahead of a remove and another add? */
 				if (unlikely(nr < 0))
 					nr = 0;
@@ -1399,7 +1400,7 @@ void page_remove_rmap(struct page *page,
 			nr = atomic_sub_return_relaxed(COMPOUND_MAPPED, mapped);
 			if (likely(nr < COMPOUND_MAPPED)) {
 				nr_pmdmapped = thp_nr_pages(page);
-				nr = nr_pmdmapped - (nr & SUBPAGES_MAPPED);
+				nr = nr_pmdmapped - (nr & FOLIO_PAGES_MAPPED);
 				/* Raced ahead of another remove and an add? */
 				if (unlikely(nr < 0))
 					nr = 0;