On Tue, Apr 14, 2020 at 11:45:23PM -0700, [email protected] wrote:
> From: Ira Weiny <[email protected]>
>
> Update the Usage section to reflect the new individual dax selection
> functionality.
>
> Signed-off-by: Ira Weiny <[email protected]>
>
> ---
> 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/[email protected]/
> ---
> 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..893820c53f49 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 3 filesystems support DAX, ext2, ext4 and xfs. Enabling DAX on them
"...support DAX: ext2, ext4..."
Please put a colon after "DAX" since it's not part of the list.
> +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
> +with the exception that the STATX_ATTR_DAX flag is not supported, nor needed,
> +as it is always true.
STATX_ATTR_DAX isn't supported? I thought ext[24] set S_DAX, so the
statx flag should work the same as it does on xfs?
I also wonder if it's worth mentioning that in the long run ext4 will
match the xfs semantics, but maybe that's better left for the ext4 rfc
series.
> +
> +
> +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.
> +
> + 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.
> +
> +
> +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.)
Urk, I guess I need to push that patch to make mkfs.xfs do this. ;)
> +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 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"
> +
> +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;
The logic in this pseudocode doesn't match the order that's in
xfs_inode_enable_dax. I think the outcome is the same, but it's easier
to verify that if the statements are in roughly the same order.
if dax=never:
S_DAX = false
elif the file system and media don't both support DAX:
S_DAX = false
elif dax=always:
S_DAX = true
else:
S_DAX = inode flag status
--D
> +
> +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."
>
>
> Implementation Tips for Block Driver Writers
> --
> 2.25.1
>