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    Subject[PATCH V11 04/11] Documentation/dax: Update Usage section
    Date
    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 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 | 139 +++++++++++++++++++++++++++++-
    1 file changed, 136 insertions(+), 3 deletions(-)

    diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt
    index 679729442fd2..409e4e83e46a 100644
    --- a/Documentation/filesystems/dax.txt
    +++ b/Documentation/filesystems/dax.txt
    @@ -17,11 +17,144 @@ 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 sub directories 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: Setting and inheritance affect FS_XFLAG_DAX at all times 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 DAX policy can be changed via:
    +
    + a) Set the parent directory FS_XFLAG_DAX as needed before files are created
    +
    + b) Set the appropriate dax="foo" mount option
    +
    + c) Change 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 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 acheived 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 systems dax mount option setting.
    +
    +statx can be used to query S_DAX. NOTE that a directory will never 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
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