Re: [PATCH V7 9/9] Documentation/dax: Update Usage section

[Darrick J. Wong]

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
> >         20200409165927.GD6741@magnolia/">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
> >