This is yuk. It's just to amke Randy happy for now.

Cc: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/readahead.c

@@ -548,6 +548,7 @@ page_cache_async_readahead(struct address_space *mapping,
 	/* do read-ahead */
 	ondemand_readahead(mapping, ra, filp, true, offset, req_size);
 
+#ifdef CONFIG_BLOCK
 	/*
 	 * Normally the current page is !uptodate and lock_page() will be
 	 * immediately called to implicitly unplug the device. However this
@@ -557,5 +558,6 @@ page_cache_async_readahead(struct address_space *mapping,
 	 */
 	if (PageUptodate(page))
 		blk_run_backing_dev(mapping->backing_dev_info, NULL);
+#endif
 }
 EXPORT_SYMBOL_GPL(page_cache_async_readahead);