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 V5: Update to reflect the agreed upon semantics https://lore.kernel.org/lkml/20200405061945.GA94792@xxxxxxxxxxxxxxxxxxxxxxxxx/ --- Documentation/filesystems/dax.txt | 94 ++++++++++++++++++++++++++++++- 1 file changed, 91 insertions(+), 3 deletions(-) diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt index 679729442fd2..d84e8101cf8a 100644 --- a/Documentation/filesystems/dax.txt +++ b/Documentation/filesystems/dax.txt @@ -17,11 +17,99 @@ 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. + +Enabling DAX on an individual file basis (XFS) +---------------------------------------------- + +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 individual file and directory inodes. It is +preserved within the file system. This 'physical' config setting can be set on +directories 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 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. FS_XFLAG_DAX can be queried +with ioctl or xfs_io on directories. + +NOTE: Setting FS_XFLAG_DAX on a directory is possible even if the underlying +media does not support dax. Furthermore, files and directories will continue +to inherit FS_XLFAG_DAX even if the underlying media does not support dax. + + +overriding FS_XFLAG_DAX (the dax= mount option) +----------------------------------------------- + +The dax mount option is a tri-state option (never, always, iflag): + + "-o dax=never" means "never set S_DAX, ignore FS_XFLAG_DAX" + "-o dax=always" means "always set S_DAX (at least on pmem), ignore FS_XFLAG_DAX" + "-o dax" by itself means "dax=always" to remain compatible with older + kernels + "-o dax=iflag" means "follow FS_XFLAG_DAX" + +The default state is 'iflag'. 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; + +Using the mount option does not change the physical configured state of +individual files. + +NOTE: Setting FS_XFLAG_DAX on a directory is possible while the file system is +mounted with the dax override. In addition, files and directories will inherit +FS_XFLAG_DAX as normal while the file system is overriden. However, the file's +enabled state will continue to be the mount option until remounted with +dax=iflag. Implementation Tips for Block Driver Writers -- 2.25.1
