[PATCH V11.1] Documentation/dax: Update Usage section

[ira . weiny]

From: Ira Weiny <ira.weiny@xxxxxxxxx>

Update the Usage section to reflect the new individual dax selection
functionality.

Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>

---
Changes from V11:
	Minor changes from Darrick

Changes from V10:
	Clarifications from Dave
	Add '-c' to xfs_io examples

Changes from V9:
	Fix missing ')'
	Fix trialing '"'

Changes from V8:
	Updates from Darrick

Changes from V7:
	Cleanups/clarifications from Darrick and Dan

Changes from V6:
	Update to allow setting FS_XFLAG_DAX any time.
	Update with list of behaviors from Darrick
	https://lore.kernel.org/lkml/20200409165927.GD6741@magnolia/

Changes from V5:
	Update to reflect the agreed upon semantics
	https://lore.kernel.org/lkml/20200405061945.GA94792@xxxxxxxxxxxxxxxxxxxxxxxxx/
---
 Documentation/filesystems/dax.txt | 142 +++++++++++++++++++++++++++++-
 1 file changed, 139 insertions(+), 3 deletions(-)

diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt
index 679729442fd2..dc1c1aa36cc2 100644
--- a/Documentation/filesystems/dax.txt
+++ b/Documentation/filesystems/dax.txt
@@ -17,11 +17,147 @@ For file mappings, the storage device is mapped directly into userspace.
 Usage
 -----
 
-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.
+
+Currently 3 filesystems support DAX: ext2, ext4 and xfs.  Enabling DAX on them
+is different.
+
+Enabling DAX on ext4 and ext2
+-----------------------------
+
+When mounting the filesystem, use the "-o dax" option on the command line or
+add 'dax' to the options in /etc/fstab.  This works to enable DAX on all files
+within the filesystem.  It is equivalent to the '-o dax=always' behavior below.
+
+
+Enabling DAX on xfs
+-------------------
+
+Summary
+-------
+
+ 1. There exists an in-kernel file access mode flag S_DAX that corresponds to
+    the statx flag STATX_ATTR_DAX.  See the manpage for statx(2) for details
+    about this access mode.
+
+ 2. There exists a persistent flag FS_XFLAG_DAX that can be applied to regular
+    files and directories. This advisory flag can be set or cleared at any
+    time, but doing so does not immediately affect the S_DAX state.
+
+ 3. If the persistent FS_XFLAG_DAX flag is set on a directory, this flag will
+    be inherited by all regular files and subdirectories that are subsequently
+    created in this directory. Files and subdirectories that exist at the time
+    this flag is set or cleared on the parent directory are not modified by
+    this modification of the parent directory.
+
+ 4. There exists dax mount options which can override FS_XFLAG_DAX in the
+    setting of the S_DAX flag.  Given underlying storage which supports DAX the
+    following hold:
+
+    "-o dax=inode"  means "follow FS_XFLAG_DAX" and is the default.
+
+    "-o dax=never"  means "never set S_DAX, ignore FS_XFLAG_DAX."
+
+    "-o dax=always" means "always set S_DAX ignore FS_XFLAG_DAX."
+
+    "-o dax"        is a legacy option which is an alias for "dax=always".
+		    This may be removed in the future so "-o dax=always" is
+		    the preferred method for specifying this behavior.
+
+    NOTE: Modifications to and the inheritance behavior of FS_XFLAG_DAX remain
+    the same even when the file system is mounted with a dax option.  However,
+    in-core inode state (S_DAX) will be overridden until the file system is
+    remounted with dax=inode and the inode is evicted from kernel memory.
+
+ 5. The S_DAX policy can be changed via:
+
+    a) Setting the parent directory FS_XFLAG_DAX as needed before files are
+       created
+
+    b) Setting the appropriate dax="foo" mount option
+
+    c) Changing the FS_XFLAG_DAX on existing regular files and directories.
+       This has runtime constraints and limitations that are described in 6)
+       below.
+
+ 6. When changing the S_DAX policy via toggling the persistent FS_XFLAG_DAX flag,
+    the change in behaviour for existing regular files may not occur
+    immediately.  If the change must take effect immediately, the administrator
+    needs to:
+
+    a) stop the application so there are no active references to the data set
+       the policy change will affect
+
+    b) evict the data set from kernel caches so it will be re-instantiated when
+       the application is restarted. This can be achieved by:
+
+       i. drop-caches
+       ii. a filesystem unmount and mount cycle
+       iii. a system reboot
+
+
+Details
+-------
+
+There are 2 per-file dax flags.  One is a persistent inode setting (FS_XFLAG_DAX)
+and the other is a volatile flag indicating the active state of the feature
+(S_DAX).
+
+FS_XFLAG_DAX is preserved within the file system.  This persistent config
+setting can be set, cleared and/or queried using the FS_IOC_FS[GS]ETXATTR ioctl
+(see ioctl_xfs_fsgetxattr(2)) or an utility such as 'xfs_io'.
+
+New 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.
+
+To clarify inheritance, here are 3 examples:
+
+Example A:
+
+mkdir -p a/b/c
+xfs_io -c '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 -c '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 -c '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 instantiated in
+memory by the kernel.  It is set based on the underlying media support, the
+value of FS_XFLAG_DAX and the file system's dax mount option.
+
+statx can be used to query S_DAX.  NOTE that only regular files will ever have
+S_DAX set and therefore statx will never indicate that S_DAX is set on
+directories.
+
+Setting the FS_XFLAG_DAX (specifically or through inheritance) occurs even if
+the underlying media does not support dax and/or the file system is overridden
+with a mount option.
+
 
 
 Implementation Tips for Block Driver Writers
-- 
2.25.1