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 V10: Clarifications from Dave Add '-c' to xfs_io examples 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 | 139 +++++++++++++++++++++++++++++- 1 file changed, 136 insertions(+), 3 deletions(-) diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt index 679729442fd2..409e4e83e46a 100644 --- a/Documentation/filesystems/dax.txt +++ b/Documentation/filesystems/dax.txt @@ -17,11 +17,144 @@ 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 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 sub directories 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 + 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: Setting and inheritance affect FS_XFLAG_DAX at all times 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 remounted with + dax=inode and the inode is evicted from kernel memory. + + 5. The DAX policy can be changed via: + + a) Set the parent directory FS_XFLAG_DAX as needed before files are created + + b) Set the appropriate dax="foo" mount option + + c) Change the FS_XFLAG_DAX on existing regular files and directories. This + has runtime constraints and limitations that are described in 6) below. + + 6. When changing the 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 acheived by: + + i. drop-caches + ii. a filesystem unmount and mount cycle + 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 +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 systems dax mount option setting. + +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. + +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. + Implementation Tips for Block Driver Writers -- 2.25.1
