On Wed, Apr 15, 2020 at 08:29:42AM -0700, Darrick J. Wong wrote: > On Tue, Apr 14, 2020 at 11:45:23PM -0700, 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 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 | 166 +++++++++++++++++++++++++++++- > > 1 file changed, 163 insertions(+), 3 deletions(-) > > > > diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt > > index 679729442fd2..893820c53f49 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 3 filesystems support DAX, ext2, ext4 and xfs. Enabling DAX on them > > "...support DAX: ext2, ext4..." > > Please put a colon after "DAX" since it's not part of the list. > > > +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 > > +with the exception that the STATX_ATTR_DAX flag is not supported, nor needed, > > +as it is always true. > > STATX_ATTR_DAX isn't supported? I thought ext[24] set S_DAX, so the > statx flag should work the same as it does on xfs? yea it will work... > > I also wonder if it's worth mentioning that in the long run ext4 will > match the xfs semantics, but maybe that's better left for the ext4 rfc > series. That was my thought. Leave ext4 for now. > > > + > > + > > +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 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. > > + > > + > > +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.) > > Urk, I guess I need to push that patch to make mkfs.xfs do this. ;) I think you should be able to set FS_XFLAG_DAX after mkfs.xfs. But yea some people might like the option... :-D > > > +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 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; > > The logic in this pseudocode doesn't match the order that's in > xfs_inode_enable_dax. I struggled with this a bit. > I think the outcome is the same, but it's easier > to verify that if the statements are in roughly the same order. > > if dax=never: > S_DAX = false > elif the file system and media don't both support DAX: The above text does say "Given underlying media support". Because I wanted to make that separate. > S_DAX = false > elif dax=always: > S_DAX = true > else: > S_DAX = inode flag status The above text is trying to convey the "override" nature of the flags. Where the code is trying to be a bit more efficient. Ira > > --D > > > + > > +To reiterate: Setting, and inheritance, continues to affect FS_XFLAG_DAX even > > +while the file system is mounted with a dax override. 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." > > > > > > Implementation Tips for Block Driver Writers > > -- > > 2.25.1 > >
