+ /*
+ * To minimise LRU disruption, the caller can indicate that it only
+ * wants to isolate pages it will be able to operate on without
+ * blocking - clean pages for the most part.
+ *
+ * ISOLATE_CLEAN means that only clean pages should be isolated. This
+ * is used by reclaim when it is cannot write to backing storage
+ *
+ * ISOLATE_ASYNC_MIGRATE is used to indicate that it only wants to pages
+ * that it is possible to migrate without blocking
+ */
+ if (mode & (ISOLATE_CLEAN|ISOLATE_ASYNC_MIGRATE)) {
+ /* All the caller can do on PageWriteback is block */
+ if (PageWriteback(page))
+ return ret;
+
+ if (PageDirty(page)) {
+ struct address_space *mapping;
+
+ /* ISOLATE_CLEAN means only clean pages */
+ if (mode & ISOLATE_CLEAN)
+ return ret;
+
+ /*
+ * Only pages without mappings or that have a
+ * ->migratepage callback are possible to migrate
+ * without blocking
+ */
+ mapping = page_mapping(page);
+ if (mapping && !mapping->a_ops->migratepage)
+ return ret;
+ }
+ }