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Subject[PATCH V6 8/8] Documentation/dax: Update Usage section
From: Ira Weiny <>

Update the Usage section to reflect the new individual dax selection

Signed-off-by: Ira Weiny <>

Changes from V5:
Update to reflect the agreed upon semantics
Documentation/filesystems/dax.txt | 94 ++++++++++++++++++++++++++++++-
1 file changed, 91 insertions(+), 3 deletions(-)

diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt
index 679729442fd2..d84e8101cf8a 100644
--- a/Documentation/filesystems/dax.txt
+++ b/Documentation/filesystems/dax.txt
@@ -17,11 +17,99 @@ For file mappings, the storage device is mapped directly into userspace.

-If you have a block device which supports DAX, you can make a filesystem
+If you have a block device which supports DAX, you can make a file system
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 file system.
+Enabling DAX on an individual file basis (XFS)
+There are 2 per file dax flags. One is a physical inode setting (FS_XFLAG_DAX) and
+the other a currently enabled state (S_DAX).
+FS_XFLAG_DAX is maintained on individual file and directory inodes. It is
+preserved within the file system. This 'physical' config setting can be set on
+directories using an ioctl and/or an application such as "xfs_io -c 'chattr
+[-+]x'". Files and directories automatically inherit FS_XFLAG_DAX from their
+parent directory _when_ _created_. Therefore, setting FS_XFLAG_DAX at
+directory creation time can be used to set a default behavior for an entire
+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 enabled state (S_DAX) is set when a file inode is loaded based on
+the underlying media support and the file systems dax mount option setting. See
+statx can be used to query S_DAX. NOTE that a directory will never have S_DAX
+set and therefore statx will always return false. FS_XFLAG_DAX can be queried
+with ioctl or xfs_io on directories.
+NOTE: Setting FS_XFLAG_DAX on a directory is possible even if the underlying
+media does not support dax. Furthermore, files and directories will continue
+to inherit FS_XLFAG_DAX even if the underlying media does not support dax.
+overriding FS_XFLAG_DAX (the dax= mount option)
+The dax mount option is a tri-state option (never, always, iflag):
+ "-o dax=never" means "never set S_DAX, ignore FS_XFLAG_DAX"
+ "-o dax=always" means "always set S_DAX (at least on pmem), ignore FS_XFLAG_DAX"
+ "-o dax" by itself means "dax=always" to remain compatible with older
+ kernels
+ "-o dax=iflag" means "follow FS_XFLAG_DAX"
+The default state is 'iflag'. The following algorithm is used to determine the
+effective mode of the file S_DAX on a capable device.
+ if (dax_mount == "always")
+ S_DAX = true;
+ else if (dax_mount == "off"
+ S_DAX = false;
+Using the mount option does not change the physical configured state of
+individual files.
+NOTE: Setting FS_XFLAG_DAX on a directory is possible while the file system is
+mounted with the dax override. In addition, files and directories will inherit
+FS_XFLAG_DAX as normal while the file system is overriden. However, the file's
+enabled state will continue to be the mount option until remounted with

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