[PATCH v3 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       | 26 ++++++++++++++++++++++++++
 include/linux/mm_types.h | 17 +++++++++++++++++
 2 files changed, 43 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 2d6e715ab8ea..f20504017adf 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -924,6 +924,11 @@ 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)
 {
 	/*
@@ -975,6 +980,26 @@ 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);
+}
+
+static inline struct folio *next_folio(struct folio *folio)
+{
+	return folio + folio_nr_pages(folio);
+}
+
+static inline unsigned int folio_shift(struct folio *folio)
+{
+	return PAGE_SHIFT + folio_order(folio);
+}
+
+static inline size_t folio_size(struct folio *folio)
+{
+	return PAGE_SIZE << folio_order(folio);
+}
+
 #ifdef CONFIG_MMU
 /*
  * Do pte_mkwrite, but only if the vma says VM_WRITE.  We do this when
@@ -1618,6 +1643,7 @@ extern void pagefault_out_of_memory(void);
 
 #define offset_in_page(p)	((unsigned long)(p) & ~PAGE_MASK)
 #define offset_in_thp(page, p)	((unsigned long)(p) & (thp_size(page) - 1))
+#define offset_in_folio(folio, p) ((unsigned long)(p) & (folio_size(folio) - 1))
 
 /*
  * Flags passed to show_mem() and show_free_areas() to suppress output in
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 07d9acb5b19c..875dc6cd6ad2 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;
-- 
2.29.2