Improved DAX Support Lands In Linux 5.8 - Initially Benefiting XFS + EXT4

The improved DAX code led by Intel has landed in the Linux 5.8 kernel with EXT4 and XFS being the initial file-systems to make use of this improved direct access mode.

DAX is the direct access of files backed by persistent memory (such as Intel Optane DC Persistent Memory) without needing to be copied via the page cache. In avoiding the page cache with the DAX mode, it means avoiding an extra copy for reads/writes to the storage device and allows mapping the storage device directly (and efficiently) into user-space.

This improved DAX code led by Intel allows for the direct access mode to be enabled on a per-file/directory (inode) basis rather than just being flipped on/off for the entire file-system. EXT4 and XFS with Linux 5.8 are making use of the new DAX code with this kernel cycle. Via statx() user-space can query to find out the direct access state of a particular file.

The improved DAX behavior was sent in on Tuesday. Darrick Wong noted, "After many years of LKML-wrangling about how to enable programs to query and influence the file data access mode (DAX) when a filesystem resides on storage devices such as persistent memory, Ira Weiny has emerged with a proposed set of standard behaviors that has not been shot down by anyone! We're more or less standardizing on the current XFS behavior and adapting ext4 to do the same."

Darrick also on Tuesday sent in the XFS changes for Linux 5.8. That pull has the new DAX tri-state mount option along with various low-level code refactoring in preparing for future features. There are also bug fixes, scalability improvements, and other work as part of that PR.