On 4/28/20 3:21 PM, ira.weiny@xxxxxxxxx wrote: > 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 V11: > Minor changes from Darrick > > Changes from V10: > Clarifications from Dave > Add '-c' to xfs_io examples > > Changes from V9: > Fix missing ')' > Fix trialing '"' trailing > > 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 | 142 +++++++++++++++++++++++++++++- > 1 file changed, 139 insertions(+), 3 deletions(-) > > diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt > index 679729442fd2..dc1c1aa36cc2 100644 > --- a/Documentation/filesystems/dax.txt > +++ b/Documentation/filesystems/dax.txt > @@ -17,11 +17,147 @@ 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 Why "file system" in the first paragraph when "filesystem" is used here and below? > +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 subdirectories 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 exist > + 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: Modifications to and the inheritance behavior of FS_XFLAG_DAX remain > + the same 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 "file system" (2 times above) > + remounted with dax=inode and the inode is evicted from kernel memory. > + > + 5. The S_DAX policy can be changed via: > + > + a) Setting the parent directory FS_XFLAG_DAX as needed before files are > + created > + > + b) Setting the appropriate dax="foo" mount option > + > + c) Changing the FS_XFLAG_DAX on existing regular files and directories. FS_XFLAGS_DAX flag on > + This has runtime constraints and limitations that are described in 6) > + below. > + > + 6. When changing the S_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 achieved by: > + > + i. drop-caches > + ii. a filesystem unmount and mount cycle filesystem > + 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 file system > +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 system's dax mount option. > + > +statx can be used to query S_DAX. NOTE that only regular files will ever 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 FS_XFLAG_DAX flag > +the underlying media does not support dax and/or the file system is overridden file system Just be consistent, please. > +with a mount option. > + > > > Implementation Tips for Block Driver Writers > thanks. -- ~Randy
