pipe_write contains a wakeup storm, 2 writers that write into the same
fifo can wake each other up, and spend 100% cpu time with
wakeup/schedule, without making any progress.

The only regression I'm aware of is that

  $ dd if=/dev/zero | grep not_there

will fail due to OOM, because grep does something like

	for(;;) {
		rlen = read(fd, buf, len);
		if (rlen == len) {
			len *= 2;
			buf = realloc(buf, len);
		}
	}

if it operates on pipes, and due to the improved syscall merging, read
will always return the maximum possible amount of data. But that's a grep
bug, not a kernel problem.