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Date:   Wed, 26 Feb 2020 21:24:42 -0800
Cc:     Ira Weiny <>,
        Alexander Viro <>,
        "Darrick J. Wong" <>,
        Dan Williams <>,
        Dave Chinner <>,
        Christoph Hellwig <>,
        "Theodore Y. Ts'o" <>, Jan Kara <>,,,
Subject: [PATCH V5 12/12] Documentation/dax: Update Usage section

From: Ira Weiny <>

Update the Usage section to reflect the new individual dax selection

Signed-off-by: Ira Weiny <>
 Documentation/filesystems/dax.txt | 84 ++++++++++++++++++++++++++++++-
 1 file changed, 82 insertions(+), 2 deletions(-)

diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt
index 679729442fd2..32e37c550f76 100644
--- a/Documentation/filesystems/dax.txt
+++ b/Documentation/filesystems/dax.txt
@@ -20,8 +20,88 @@ Usage
 If you have a block device which supports DAX, you can make a filesystem
 on it as usual.  The DAX code currently only supports files with a block
 size equal to your kernel's PAGE_SIZE, so you may need to specify a block
-size when creating the filesystem.  When mounting it, use the "-o dax"
-option on the command line or add 'dax' to the options in /etc/fstab.
+size when creating the filesystem.
+Enabling DAX on an individual file basis (XFS)
+There are 2 per file dax flags.  One is a physical configuration setting and
+the other a currently enabled state.
+The physical configuration setting is maintained on individual file and
+directory inodes.  It is preserved within the file system.  This 'physical'
+config setting can be set using an ioctl and/or an application such as "xfs_io
+-c 'chattr [-+]x'".  Files and directories automatically inherit their physical
+dax setting from their parent directory when created.  Therefore, setting the
+physical dax setting at directory creation time can be used to set a default
+behavior for that sub-tree.  Doing so on the root directory acts to set a
+default for the entire file system.
+To clarify inheritance here are 3 examples:
+Example A:
+mkdir -p a/b/c
+xfs_io 'chattr +x' a
+mkdir a/b/c/d
+mkdir a/e
+	dax: a,e
+	no dax: b,c,d
+Example B:
+mkdir a
+xfs_io 'chattr +x' a
+mkdir -p a/b/c/d
+	dax: a,b,c,d
+	no dax:
+Example C:
+mkdir -p a/b/c
+xfs_io 'chattr +x' c
+mkdir a/b/c/d
+	dax: c,d
+	no dax: a,b
+The current inode enabled state is set when a file inode is loaded and it is
+determined that the underlying media supports dax.
+statx can be used to query the file's current enabled state.  NOTE that a
+directory will never be operating in a dax state.  Therefore, the dax config
+state must be queried to see what config state a file or sub-directory will
+inherit from a directory.
+NOTE: Setting a file or directory's config state with xfs_io is possible even
+if the underlying media does not support dax.
+Enabling dax on a file system wide basis ('-o dax' mount option)
+The physical dax configuration of all files can be overridden using a mount
+option.  In summary:
+	(physical flag || mount option) && capable device == dax in effect
+	(  <xfs_io>    ||  <'-o dax'> ) && capable device == <statx dax true>
+To enable the mount override, use "-o dax" on the command line or add
+'dax' to the options in /etc/fstab
+Using the mount option does not change the physical configured state of
+individual files.  Therefore, remounting _without_ the mount option will allow
+the file system to set file's enabled state directly based on their config
+NOTE: Setting a file or directory's physical config state is possible while the
+file system is mounted with the dax override.  However, the file's enabled
+state will continue to be overridden and "dax enabled" until the mount option
+is removed and a remount performed.  At that point the file's physical config
+state dictates the enabled state.
 Implementation Tips for Block Driver Writers

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