Using as data directory a partition mounted like

[root@mysql mysql]# cat /etc/fstab | grep data
UUID=63de9746-cda7-45f2-b397-7a25ca4ff8be /data ext4 defaults,noatime,nodiratime,noacl,data=journal 1 2

Will result in:

130209 12:37:54 InnoDB: Failed to set O_DIRECT on file ./a/b.ibd: CREATE: Invalid argument, continuing anyway
130209 12:37:54 InnoDB: O_DIRECT is known to result in 'Invalid argument' on Linux on tmpfs, see MySQL Bug#26662

For every InnoDB table (even for innodb_data)

130209 12:29:41 InnoDB: Failed to set O_DIRECT on file ./innodb_data: OPEN: Invalid argument, continuing anyway
130209 12:29:41 InnoDB: O_DIRECT is known to result in 'Invalid argument' on Linux on tmpfs, see MySQL Bug#26662