On Mon, Apr 13, 2020 at 10:21:26PM -0700, Dan Williams wrote: > On Sun, Apr 12, 2020 at 10:41 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 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. > > + > > +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. > > I had reserved some quibbling with this wording, but now that this is > being proposed as documentation I'll let my quibbling fly. "dax" may > imply, but does not require persistent memory nor does it necessarily > "bypass page cache". For example on configurations that support dax, > but turn off MAP_SYNC (like virtio-pmem), a software flush is > required. Instead, if we're going to define "dax" here I'd prefer it > be a #include of the man page definition that is careful (IIRC) to > only talk about semantics and not backend implementation details. In > other words, dax is to page-cache as direct-io is to page cache, > effectively not there, but dig a bit deeper and you may find it. Uh, which manpage? Are you talking about the MAP_SYNC documentation? I don't rewording this to say "There exists an in-kernel access mode flag S_DAX that, when set, enables MAP_SYNC semantics. Refer to mmap(2) for more details about what that means." --D > > 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. > > + > > + > > +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. > > + > > +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. > > > > > > Implementation Tips for Block Driver Writers > > -- > > 2.25.1 > >
