On Sun, Apr 12, 2020 at 10:40:46PM -0700, ira.weiny@xxxxxxxxx wrote: > From: Ira Weiny <ira.weiny@xxxxxxxxx> > > Update the Usage section to reflect the new individual dax selection > functionality. Yum. :) > 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. I thought ext2 supports DAX? > +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. I think we can omit this last sentence for the formal documentation... :) > + > + > +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. "statx will never indicate that S_DAX is set 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. "However, in-core inode state (S_DAX) will continue to be overridden until the filesystem is remounted with dax=inode and the inode is evicted." ...since we don't currently evict inodes just because a remount occurred. :) --D > > > Implementation Tips for Block Driver Writers > -- > 2.25.1 >
