mm/filemap: Add readahead_folio()

The pointers stored in the page cache are folios, by definition.
This change comes with a behaviour change -- callers of readahead_folio()
are no longer required to put the page reference themselves. This matches
how readpage works, rather than matching how readpages used to work.

Signed-off-by: Matthew Wilcox (Oracle) <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Reviewed-by: David Howells <[e-mail]>
Acked-by: Vlastimil Babka <[e-mail]>

include/linux/pagemap.h

···
 	page_cache_async_ra(&ractl, page, req_count);
 }
 
+static inline struct folio *__readahead_folio(struct readahead_control *ractl)
+{
+	struct folio *folio;
+
+	BUG_ON(ractl->_batch_count > ractl->_nr_pages);
+	ractl->_nr_pages -= ractl->_batch_count;
+	ractl->_index += ractl->_batch_count;
+
+	if (!ractl->_nr_pages) {
+		ractl->_batch_count = 0;
+		return NULL;
+	}
+
+	folio = xa_load(&ractl->mapping->i_pages, ractl->_index);
+	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
+	ractl->_batch_count = folio_nr_pages(folio);
+
+	return folio;
+}
+
 /**
  * readahead_page - Get the next page to read.
- * @rac: The current readahead request.
+ * @ractl: The current readahead request.
  *
  * Context: The page is locked and has an elevated refcount.  The caller
  * should decreases the refcount once the page has been submitted for I/O
  * and unlock the page once all I/O to that page has completed.
  * Return: A pointer to the next page, or %NULL if we are done.
  */
-static inline struct page *readahead_page(struct readahead_control *rac)
+static inline struct page *readahead_page(struct readahead_control *ractl)
 {
-	struct page *page;
+	struct folio *folio = __readahead_folio(ractl);
 
-	BUG_ON(rac->_batch_count > rac->_nr_pages);
-	rac->_nr_pages -= rac->_batch_count;
-	rac->_index += rac->_batch_count;
+	return &folio->page;
+}
 
-	if (!rac->_nr_pages) {
-		rac->_batch_count = 0;
-		return NULL;
-	}
+/**
+ * readahead_folio - Get the next folio to read.
+ * @ractl: The current readahead request.
+ *
+ * Context: The folio is locked.  The caller should unlock the folio once
+ * all I/O to that folio has completed.
+ * Return: A pointer to the next folio, or %NULL if we are done.
+ */
+static inline struct folio *readahead_folio(struct readahead_control *ractl)
+{
+	struct folio *folio = __readahead_folio(ractl);
 
-	page = xa_load(&rac->mapping->i_pages, rac->_index);
-	VM_BUG_ON_PAGE(!PageLocked(page), page);
-	rac->_batch_count = thp_nr_pages(page);
-
-	return page;
+	if (folio)
+		folio_put(folio);
+	return folio;
 }
 
 static inline unsigned int __readahead_batch(struct readahead_control *rac,