по той же ссылке на lwn.net написано:

>[оверквотинг удален]
>in the ext4 filesystem. If your storage is visible as regular memory to the host CPU, DAX
>enables the system to do execute-in-place of files stored there. In other words, when
>using DAX, you bypass the page cache and virtual-memory subsystem completely. For
>applications that use mmap(), this means a true zero-copy approach, and for code that
>uses the read() system call (or equivalent) you will have only one copy of the data. DAX
>was originally designed for fast flash-like storage that shows up as memory to the CPU;
>but in a virtual-machine environment, this type of storage is easy to emulate. All we
>need to do on the host is map the disk image file into the guest's physical memory, and
>use a small device driver in the guest kernel that exposes this memory region to the
>kernel as a DAX-ready block device.

На всякий случай еще раз повторю:

> For applications that use mmap(), this means a true zero-copy approach, and for code
> that uses the read() system call (or equivalent) you will have only one copy of the data.