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

ira.weiny
2020-04-22T21:22:12Z

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

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

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>

---
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 | 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.
+
+ 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.)
+
+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;
+
+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