[PATCH 01/25] mm: Introduce struct folio

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

We have trouble keeping track of whether we've already called
compound_head() to ensure we're not operating on a tail page.  Further,
it's never clear whether we intend a struct page to refer to PAGE_SIZE
bytes or page_size(compound_head(page)).

Introduce a new type 'struct folio' that always refers to an entire
(possibly compound) page, and points to the head page (or base page).

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/mm.h       | 10 ++++++++++
 include/linux/mm_types.h | 17 +++++++++++++++++
 include/linux/pagemap.h  | 14 ++++++++++++++
 3 files changed, 41 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 5299b90a6c40..ed20fd0c6169 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -916,6 +916,11 @@ static inline unsigned int compound_order(struct page *page)
 	return page[1].compound_order;
 }
 
+static inline unsigned int folio_order(struct folio *folio)
+{
+	return compound_order(&folio->page);
+}
+
 static inline bool hpage_pincount_available(struct page *page)
 {
 	/*
@@ -967,6 +972,11 @@ static inline unsigned int page_shift(struct page *page)
 
 void free_compound_page(struct page *page);
 
+static inline unsigned long folio_nr_pages(struct folio *folio)
+{
+	return compound_nr(&folio->page);
+}
+
 #ifdef CONFIG_MMU
 /*
  * Do pte_mkwrite, but only if the vma says VM_WRITE.  We do this when
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 65df8abd90bd..d7e487d9998f 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -223,6 +223,23 @@ struct page {
 #endif
 } _struct_page_alignment;
 
+/*
+ * A struct folio is either a base (order-0) page or the head page of
+ * a compound page.
+ */
+struct folio {
+	struct page page;
+};
+
+static inline struct folio *page_folio(struct page *page)
+{
+	unsigned long head = READ_ONCE(page->compound_head);
+
+	if (unlikely(head & 1))
+		return (struct folio *)(head - 1);
+	return (struct folio *)page;
+}
+
 static inline atomic_t *compound_mapcount_ptr(struct page *page)
 {
 	return &page[1].compound_mapcount;
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index f18857c79478..b5af2c3719ab 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -437,6 +437,20 @@ 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_PAGE(index >= folio_nr_pages(folio), &folio->page);
+	return &folio->page + index;
+}
+
 /*
  * Given the page we found in the page cache, return the page corresponding
  * to this index in the file
-- 
2.29.2