Re: [PATCH V10 04/11] Documentation/dax: Update Usage section

[Dave Chinner]

On Wed, Apr 22, 2020 at 02:20:55PM -0700, ira.weiny@xxxxxxxxx wrote:
> From: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> Update the Usage section to reflect the new individual dax selection
> functionality.
> 
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> ---
> 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 | 164 +++++++++++++++++++++++++++++-
>  1 file changed, 161 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt
> index 679729442fd2..553712c5054e 100644
> --- a/Documentation/filesystems/dax.txt
> +++ b/Documentation/filesystems/dax.txt
> @@ -17,11 +17,169 @@ 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 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.

This needs to make it clear that the inheritance behaviour of this
flag affects both newly created regular files and sub-directories,
but no other types of directory entries.

> +
> +    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.

This is item 3), and needs to state that it is specific to regular
files as DAX is not a method that can be used to access the
directory structure.

Also "at inode load time" doesn't really mean anything useful to
users. "when the inode is instantiated in memory by the kernel" is
what you really mean, and given that 5(c) talks about "the kernel
evicts the inode from memory", we really need to use consistent
terminology here so that it's clear to users that the behaviours are
related.

> +
> + 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".

"Legacy option that is an alias for "dax=always". This may be
deprecated and removed in future, so "dax=always" is the preferred
method for specifying this behaviour."

> +
> +    "-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.

This should be item 2, so that it is defined before it is referenced
by the current item 2 in the list.

> +
> + 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

	(a) Set the parent directory FS_XFLAG_DAX as needed before
	files are created; or

> +    (b) Have the administrator set an override via mount option; or

	(b) Have the administrator set the desired behaviour 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

That will almost never work by itself as the cached dentry will
still pin the inode. Suggesting it will lead to people implementing
dumb loops where they open/check/close/sleep because they....

> +        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.

.... don't have permissions to do either of these things...

Essentially, you may as well say "reboot the machine" at this point,
because it's effectively the same thing from a production workload
point of view...

Realistically, I'm not sure we should even say "programs must cause
eviction", because that's something they cannot do directly without
admin privileges nor is it something we want to occur randomly on
production machines during production. i.e. this is something that
should only be done in scheduled downtime by an administrator, not
attempted by applications because DAX isn't immediately available.
The admin is in charge here, not the "program".

> + 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.

I don't think this adds any value. You may as well just say "caveat
empor", but that's kinda implied by the fact a computer is
involved...

> +
> +
> +Details
> +-------
> +
> +There are 2 per-file dax flags.  One is a physical inode setting (FS_XFLAG_DAX)

s/physical/persistent/

> +and the other a currently enabled state (S_DAX).

the other is a volatile flag indicating the active state of the
feature (S_DAX)

> +
> +FS_XFLAG_DAX is maintained, on disk, on individual inodes.

This is implementation detail, not a requirement. The only
requirement is that it is stored persistently by the filesystem.

> 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

s/physical/persistent/

... can be set, cleared and/or queried use 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.  (Doing so on the
> +root directory acts to set a default for the entire file system.)

No need for () around the example, but regardless, I don't think
this is a well thought out example. i.e.  setting it on an existing
filesystem which already contains data will not affect the default
behaviour of existing subdirectories or files. IOWs, it only sets the
default behaviour when set on an -empty- filesystem because of the
inheritance characteristics of the flag...

> +The current enabled state (S_DAX) is set when a file inode is _loaded_ based on

instantiated in memoy by the kernel.

> +the underlying media support, the value of FS_XFLAG_DAX, and the file systems

no comma before "and".

> +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 never indicate that S_DAX is set 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"

This is just repeating what is in the definition section.

> +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, in-core inode
> +state (S_DAX) will continue to be overridden until the filesystem is remounted
> +with dax=inode and the inode is evicted.

Just put this last paragraph up in the "behavioural definitions" and
this whole section can go away.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx