From: Ira Weiny <[email protected]>
Update the Usage section to reflect the new individual dax selection
functionality.
Signed-off-by: Ira Weiny <[email protected]>
---
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/[email protected]/
---
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
+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.
+
+
+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.)
+
+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;
+
+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
On Wed, Apr 15, 2020 at 08:29:42AM -0700, Darrick J. Wong wrote:
> On Tue, Apr 14, 2020 at 11:45:23PM -0700, [email protected] wrote:
> > From: Ira Weiny <[email protected]>
> >
> > Update the Usage section to reflect the new individual dax selection
> > functionality.
> >
> > Signed-off-by: Ira Weiny <[email protected]>
> >
> > ---
> > 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/[email protected]/
> > ---
> > 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
> >