readahead: readahead page allocations are OK to fail

Pass __GFP_NORETRY|__GFP_NOWARN for readahead page allocations.

readahead page allocations are completely optional.  They are OK to fail
and in particular shall not trigger OOM on themselves.

Reported-by: Dave Young <hidave.darkstar@gmail.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/readahead.c b/mm/readahead.c
index 2c0cc48..867f9dd 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -180,7 +180,7 @@
 		if (page)
 			continue;
 
-		page = page_cache_alloc_cold(mapping);
+		page = page_cache_alloc_readahead(mapping);
 		if (!page)
 			break;
 		page->index = page_offset;