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Date:   Sun, 20 Oct 2019 08:59:30 -0700
Cc:     Ira Weiny <>,
        Alexander Viro <>,
        "Darrick J. Wong" <>,
        Dan Williams <>,
        Dave Chinner <>,
        Christoph Hellwig <>,
        "Theodore Y. Ts'o" <>, Jan Kara <>,,,
Subject: [PATCH 0/5] Enable per-file/directory DAX operations

From: Ira Weiny <>

At LSF/MM'19 [1] [2] we discussed applications that overestimate memory
consumption due to their inability to detect whether the kernel will
instantiate page cache for a file, and cases where a global dax enable via a
mount option is too coarse.

The following patch series enables selecting the use of DAX on individual files
and/or directories on xfs, and lays some groundwork to do so in ext4.  In this
scheme the dax mount option can be omitted to allow the per-file property to
take effect.

The insight at LSF/MM was to separate the per-mount or per-file "physical"
capability switch from an "effective" attribute for the file.

At LSF/MM we discussed the difficulties of switching the mode of a file with
active mappings / page cache. Rather than solve those races the decision was to
just limit mode flips to 0-length files.

Finally, the physical DAX flag inheritance is maintained from previous work on 
XFS but should be added for other file systems for consistence.


Cc: Alexander Viro <>
Cc: "Darrick J. Wong" <>
Cc: Dan Williams <>
Cc: Dave Chinner <>
Cc: Christoph Hellwig <>
Cc: "Theodore Y. Ts'o" <>
Cc: Jan Kara <>

Ira Weiny (5):
  fs/stat: Define DAX statx attribute
  fs/xfs: Isolate the physical DAX flag from effective
  fs/xfs: Separate functionality of xfs_inode_supports_dax()
  fs/xfs: Clean up DAX support check
  fs/xfs: Allow toggle of physical DAX flag

 fs/stat.c                 |  3 +++
 fs/xfs/xfs_ioctl.c        | 32 ++++++++++++++------------------
 fs/xfs/xfs_iops.c         | 36 ++++++++++++++++++++++++++++++------
 fs/xfs/xfs_iops.h         |  2 ++
 include/uapi/linux/stat.h |  1 +
 5 files changed, 50 insertions(+), 24 deletions(-)


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