ftrace: Allow stealing pages from pipe buffer

Use generic steal operation on pipe buffer to allow stealing
ring buffer's read page from pipe buffer.

Note that this could reduce the performance of splice on the
splice_write side operation without affinity setting.
Since the ring buffer's read pages are allocated on the
tracing-node, but the splice user does not always execute
splice write side operation on the same node. In this case,
the page will be accessed from the another node.
Thus, it is strongly recommended to assign the splicing
thread to corresponding node.
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

kernel/trace/trace.c

@@ -4195,12 +4195,6 @@ static void buffer_pipe_buf_release(struct pipe_inode_info *pipe,
 	buf->private = 0;
 }
 
-static int buffer_pipe_buf_steal(struct pipe_inode_info *pipe,
-				 struct pipe_buffer *buf)
-{
-	return 1;
-}
-
 static void buffer_pipe_buf_get(struct pipe_inode_info *pipe,
 				struct pipe_buffer *buf)
 {
@@ -4216,7 +4210,7 @@ static const struct pipe_buf_operations buffer_pipe_buf_ops = {
 	.unmap			= generic_pipe_buf_unmap,
 	.confirm		= generic_pipe_buf_confirm,
 	.release		= buffer_pipe_buf_release,
-	.steal			= buffer_pipe_buf_steal,
+	.steal			= generic_pipe_buf_steal,
 	.get			= buffer_pipe_buf_get,
 };