mm: Add folio_evictable()

This is the folio equivalent of page_evictable().  Unfortunately, it's
different from !folio_test_unevictable(), but I think it's used in places
where you have to be a VM expert and can reasonably be expected to know
the difference.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Vlastimil Babka <vbabka@suse.cz>

mm/internal.h

@@ -72,17 +72,28 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
 		pgoff_t end, struct pagevec *pvec, pgoff_t *indices);
 
 /**
- * page_evictable - test whether a page is evictable
+ * folio_evictable - Test whether a folio is evictable.
- * @page: the page to test
+ * @folio: The folio to test.
  *
- * Test whether page is evictable--i.e., should be placed on active/inactive
+ * Test whether @folio is evictable -- i.e., should be placed on
- * lists vs unevictable list.
+ * active/inactive lists vs unevictable list.
- *
- * Reasons page might not be evictable:
- * (1) page's mapping marked unevictable
- * (2) page is part of an mlocked VMA
  *
+ * Reasons folio might not be evictable:
+ * 1. folio's mapping marked unevictable
+ * 2. One of the pages in the folio is part of an mlocked VMA
  */
+static inline bool folio_evictable(struct folio *folio)
+{
+	bool ret;
+
+	/* Prevent address_space of inode and swap cache from being freed */
+	rcu_read_lock();
+	ret = !mapping_unevictable(folio_mapping(folio)) &&
+			!folio_test_mlocked(folio);
+	rcu_read_unlock();
+	return ret;
+}
+
 static inline bool page_evictable(struct page *page)
 {
 	bool ret;