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

ira.weiny
2020-04-28T22:21:53Z

From: Ira Weiny <ira.weiny@intel.com>

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

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

---
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@iweiny-DESK2.sc.intel.com/
---
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