[PATCH V7 9/9] 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 V6:
	Update to allow setting FS_XFLAG_DAX any time.
	Update with list of behaviors from Darrick
	20200409165927.GD6741@magnolia/">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 | 166 +++++++++++++++++++++++++++++-
 1 file changed, 163 insertions(+), 3 deletions(-)

diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt
index 679729442fd2..af14c1b330a9 100644
--- a/Documentation/filesystems/dax.txt
+++ b/Documentation/filesystems/dax.txt
@@ -17,11 +17,171 @@ 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 2 filesystems support DAX, ext4 and xfs.  Enabling DAX on them is
+different at this time.
+
+Enabling DAX on ext4
+--------------------
+
+When mounting the filesystem, use the "-o dax" option on the command line or
+add 'dax' to the options in /etc/fstab.
+
+
+Enabling DAX on xfs
+-------------------
+
+Summary
+-------
+
+ 1. There exists an in-kernel access mode flag S_DAX that is set when
+    file accesses go directly to persistent memory, bypassing the page
+    cache.  Applications must call statx to discover the current S_DAX
+    state (STATX_ATTR_DAX).
+
+ 2. There exists an advisory file inode flag FS_XFLAG_DAX that is
+    inherited from the parent directory FS_XFLAG_DAX inode flag at file
+    creation time.  This advisory flag can be set or cleared at any
+    time, but doing so does not immediately affect the S_DAX state.
+
+    Unless overridden by mount options (see (3)), if FS_XFLAG_DAX is set
+    and the fs is on pmem then it will enable S_DAX at inode load time;
+    if FS_XFLAG_DAX is not set, it will not enable S_DAX.
+
+ 3. There exists a dax= mount option.
+
+    "-o dax=never"  means "never set S_DAX, ignore FS_XFLAG_DAX."
+
+    "-o dax=always" means "always set S_DAX (at least on pmem),
+                    and ignore FS_XFLAG_DAX."
+
+    "-o dax"        is an alias for "dax=always".
+
+    "-o dax=inode"  means "follow FS_XFLAG_DAX" and is the default.
+
+ 4. There exists an advisory directory inode flag FS_XFLAG_DAX that can
+    be set or cleared at any time.  The flag state is inherited by any files or
+    subdirectories when they are created within that directory.
+
+ 5. Programs that require a specific file access mode (DAX or not DAX)
+    can do one of the following:
+
+    (a) Create files in directories that the FS_XFLAG_DAX flag set as
+        needed; or
+
+    (b) Have the administrator set an override via mount option; or
+
+    (c) Set or clear the file's FS_XFLAG_DAX flag as needed.  Programs
+        must then cause the kernel to evict the inode from memory.  This
+        can be done by:
+
+        i>  Closing the file and re-opening the file and using statx to
+            see if the fs has changed the S_DAX flag; and
+
+        ii> If the file still does not have the desired S_DAX access
+            mode, either unmount and remount the filesystem, or close
+            the file and use drop_caches.
+
+ 6. It is expected that users who want to squeeze every last bit of performance
+    out of the particular rough and tumble bits of their storage will also be
+    exposed to the difficulties of what happens when the operating system can't
+    totally virtualize those hardware capabilities.  DAX is such a feature.
+    Basically, Formula-1 cars require a bit more care and feeding than your
+    averaged Toyota minivan, as it were.
+
+
+Details
+-------
+
+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 disk, on individual 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 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, the value of FS_XFLAG_DAX, and the file systems
+dax mount option setting.  See below.
+
+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 on directories.
+
+NOTE: 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.
+
+
+Overriding FS_XFLAG_DAX (dax= mount option)
+-------------------------------------------
+
+There exists a dax mount option.  Using the mount option does not change the
+physical configured state of individual files but overrides the S_DAX operating
+state when inodes are loaded.
+
+Given underlying media support, the dax mount option is a tri-state option
+(never, always, inode) with the following meanings:
+
+   "-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" by itself means "dax=always" to remain compatible with older
+	         kernels
+   "-o dax=inode" means "follow FS_XFLAG_DAX"
+
+The default state is 'inode'.  Given underlying media support, the following
+algorithm is used to determine the effective mode of the file S_DAX on a
+capable device.
+
+	S_DAX = FS_XFLAG_DAX;
+
+	if (dax_mount == "always")
+		S_DAX = true;
+	else if (dax_mount == "off"
+		S_DAX = false;
+
+To reiterate: Setting, and inheritance, continues to affect FS_XFLAG_DAX even
+while the file system is mounted with a dax override.  However, file enabled
+state, S_DAX, will continue to be the overridden until the file system is
+remounted with dax=inode.
 
 
 Implementation Tips for Block Driver Writers
-- 
2.25.1