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

Randy Dunlap
2020-04-29T02:21:26Z

On 4/28/20 3:21 PM, ira.weiny@intel.com wrote:
> 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 '"'

trailing

>
> 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

Why "file system" in the first paragraph when "filesystem" is used here and below?

> +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

exist

> + 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

"file system" (2 times above)

> + 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.

FS_XFLAGS_DAX flag on

> + 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

filesystem

> + 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

file system

> +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 FS_XFLAG_DAX flag

> +the underlying media does not support dax and/or the file system is overridden

file system

Just be consistent, please.

> +with a mount option.
> +
>
>
> Implementation Tips for Block Driver Writers
>

thanks.
--
~Randy