[PATCH v2 09/27] mm: Add folio_index, folio_page and folio_contains

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

folio_index() is the equivalent of page_index() for folios.  folio_page()
finds the page in a folio for a page cache index.  folio_contains()
tells you whether a folio contains a particular page cache index.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/pagemap.h | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index a739ada01d27..c27b74c63b5e 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -447,6 +447,29 @@ static inline bool thp_contains(struct page *head, pgoff_t index)
 	return page_index(head) == (index & ~(thp_nr_pages(head) - 1UL));
 }
 
+static inline pgoff_t folio_index(struct folio *folio)
+{
+        if (unlikely(FolioSwapCache(folio)))
+                return __page_file_index(&folio->page);
+        return folio->page.index;
+}
+
+static inline struct page *folio_page(struct folio *folio, pgoff_t index)
+{
+	index -= folio_index(folio);
+	VM_BUG_ON_FOLIO(index >= folio_nr_pages(folio), folio);
+	return &folio->page + index;
+}
+
+/* Does this folio contain this index? */
+static inline bool folio_contains(struct folio *folio, pgoff_t index)
+{
+	/* HugeTLBfs indexes the page cache in units of hpage_size */
+	if (PageHuge(&folio->page))
+		return folio->page.index == index;
+	return index - folio_index(folio) < folio_nr_pages(folio);
+}
+
 /*
  * Given the page we found in the page cache, return the page corresponding
  * to this index in the file
-- 
2.29.2