From: Ira Weiny <ira.weiny@xxxxxxxxx> Update the Usage section to reflect the new individual dax selection functionality. Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx> --- 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 | 166 +++++++++++++++++++++++++++++- 1 file changed, 163 insertions(+), 3 deletions(-) diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt index 679729442fd2..af14c1b330a9 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 2 filesystems support DAX, ext4 and xfs. Enabling DAX on them is +different at this time. + +Enabling DAX on ext4 +-------------------- + +When mounting the filesystem, use the "-o dax" option on the command line or +add 'dax' to the options in /etc/fstab. + + +Enabling DAX on xfs +------------------- + +Summary +------- + + 1. There exists an in-kernel access mode flag S_DAX that is set when + file accesses go directly to persistent memory, bypassing the page + cache. Applications must call statx to discover the current S_DAX + state (STATX_ATTR_DAX). + + 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. + Basically, Formula-1 cars require a bit more care and feeding than your + averaged Toyota minivan, as it were. + + +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 always return false 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, file enabled +state, S_DAX, will continue to be the overridden until the file system is +remounted with dax=inode. Implementation Tips for Block Driver Writers -- 2.25.1
