+ /*
+ * The number of dirty pages determines if a zone is marked
+ * reclaim_congested which affects wait_iff_congested. kswapd
+ * will stall and start writing pages if the tail of the LRU
+ * is all dirty unqueued pages.
+ */
+ page_check_dirty_writeback(page, &dirty, &writeback);
+ if (dirty || writeback)
+ nr_dirty++;
+
+ if (dirty && !writeback)
+ nr_unqueued_dirty++;
+
+ /*
+ * Treat this page as congested if the underlying BDI is or if
+ * pages are cycling through the LRU so quickly that the
+ * pages marked for immediate reclaim are making it to the
+ * end of the LRU a second time.
+ */
+ mapping = page_mapping(page);
+ if ((mapping && bdi_write_congested(mapping->backing_dev_info)) ||
+ (writeback && PageReclaim(page)))
+ nr_congested++;
+
+ /*
+ * If a page at the tail of the LRU is under writeback, there
+ * are three cases to consider.
+ *
+ * 1) If reclaim is encountering an excessive number of pages
+ * under writeback and this page is both under writeback and
+ * PageReclaim then it indicates that pages are being queued
+ * for IO but are being recycled through the LRU before the
+ * IO can complete. Waiting on the page itself risks an
+ * indefinite stall if it is impossible to writeback the
+ * page due to IO error or disconnected storage so instead
+ * note that the LRU is being scanned too quickly and the
+ * caller can stall after page list has been processed.
+ *
+ * 2) Global reclaim encounters a page, memcg encounters a
+ * page that is not marked for immediate reclaim or
+ * the caller does not have __GFP_IO. In this case mark
+ * the page for immediate reclaim and continue scanning.
+ *
+ * __GFP_IO is checked because a loop driver thread might
+ * enter reclaim, and deadlock if it waits on a page for
+ * which it is needed to do the write (loop masks off
+ * __GFP_IO|__GFP_FS for this reason); but more thought
+ * would probably show more reasons.
+ *
+ * Don't require __GFP_FS, since we're not going into the
+ * FS, just waiting on its writeback completion. Worryingly,
+ * ext4 gfs2 and xfs allocate pages with
+ * grab_cache_page_write_begin(,,AOP_FLAG_NOFS), so testing
+ * may_enter_fs here is liable to OOM on them.
+ *
+ * 3) memcg encounters a page that is not already marked
+ * PageReclaim. memcg does not have any dirty pages
+ * throttling so we could easily OOM just because too many
+ * pages are in writeback and there is nothing else to
+ * reclaim. Wait for the writeback to complete.
+ */