On Tue, Apr 28, 2020 at 01:27:38PM -0700, Darrick J. Wong wrote: > On Mon, Apr 27, 2020 at 05:21:35PM -0700, ira.weiny@xxxxxxxxx wrote: > > From: Ira Weiny <ira.weiny@xxxxxxxxx> > > [snip] > > + > > + 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 > > Well, I'm at the level of minor edits: "...and subdirectories that..." Done. > > > + 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. > > "hold:" Dene. > > > + > > + "-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. > > We can also clear the flag at any time no matter the mount option state. > Perhaps: > > "NOTE: Modifications to and inheritance behavior of FS_XFLAG_DAX remain > the same even when the filesystem is mounted with a dax option." Done. > > > + 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: > > "The S_DAX policy". I don't want people to get confused. Done. > > > + > > + 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. > > "Setting", and "Changing" at the front of these three bullet points? > > Were you to put these together as full sentences, you'd want them to > read "The DAX policy can be changed via setting the parent directory > FS_XFLAG_DAX..." > Done. > > + > > + 6. When changing the DAX policy via toggling the persistent FS_XFLAG_DAX flag, > > "When changing the S_DAX policy..." Done. > > > + 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: > > "achieved" Done. > > > + > > + 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: > > "...inheritance, here are..." Done. > > > + > > +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. > > "...and the file system's dax mount option string." No. I don't think string is the right word here. It is the setting not the string which is controlling the behavior. How about: "...and the file system's dax mount option." > > > + > > +statx can be used to query S_DAX. NOTE that a directory will never have S_DAX > > "Note that only regular files will ever have S_DAX set..."? Done. Ira > > --D > > > +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 > >
