Direct Access for files
-----------------------

Motivation
----------

The page cache is usually used to buffer reads and writes to files.
It is also used to provide the pages which are mapped into userspace
by a call to mmap.

For block devices that are memory-like, the page cache pages would be
unnecessary copies of the original storage.  The DAX code removes the
extra copy by performing reads and writes directly to the storage device.
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
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.

Currently 3 filesystems support DAX: ext2, ext4 and xfs.  Enabling DAX on them
is different.

Enabling DAX on 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.


Enabling DAX on xfs and ext4
----------------------------

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 a persistent flag FS_XFLAG_DAX that can be applied to regular
    files and directories. This advisory flag can be set or cleared at any
    time, but doing so does not immediately affect the S_DAX state.

 3. If the persistent FS_XFLAG_DAX flag is set on a directory, this flag will
    be inherited by all regular files and subdirectories that are subsequently
    created in this directory. Files and subdirectories that exist at the time
    this flag is set or cleared on the parent directory are not modified by
    this modification of the parent directory.

 4. There exist dax mount options which can override FS_XFLAG_DAX in the
    setting of the S_DAX flag.  Given underlying storage which supports DAX the
    following hold:

    "-o dax=inode"  means "follow FS_XFLAG_DAX" and is the default.

    "-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"        is a legacy option which is an alias for "dax=always".
		    This may be removed in the future so "-o dax=always" is
		    the preferred method for specifying this behavior.

    NOTE: Modifications to and the inheritance behavior of FS_XFLAG_DAX remain
    the same even when the filesystem is mounted with a dax option.  However,
    in-core inode state (S_DAX) will be overridden until the filesystem is
    remounted with dax=inode and the inode is evicted from kernel memory.

 5. The S_DAX policy can be changed via:

    a) Setting the parent directory FS_XFLAG_DAX as needed before files are
       created

    b) Setting the appropriate dax="foo" mount option

    c) Changing the FS_XFLAG_DAX flag on existing regular files and
       directories.  This has runtime constraints and limitations that are
       described in 6) below.

 6. When changing the S_DAX policy via toggling the persistent FS_XFLAG_DAX flag,
    the change in behaviour for existing regular files may not occur
    immediately.  If the change must take effect immediately, the administrator
    needs to:

    a) stop the application so there are no active references to the data set
       the policy change will affect

    b) evict the data set from kernel caches so it will be re-instantiated when
       the application is restarted. This can be achieved by:

       i. drop-caches
       ii. a filesystem unmount and mount cycle
       iii. a system reboot


Details
-------

There are 2 per-file dax flags.  One is a persistent inode setting (FS_XFLAG_DAX)
and the other is a volatile flag indicating the active state of the feature
(S_DAX).

FS_XFLAG_DAX is preserved within the filesystem.  This persistent config
setting can be set, cleared and/or queried using the FS_IOC_FS[GS]ETXATTR ioctl
(see ioctl_xfs_fsgetxattr(2)) or an utility such as 'xfs_io'.

New 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.

To clarify inheritance, here are 3 examples:

Example A:

mkdir -p a/b/c
xfs_io -c '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 -c '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 -c '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 instantiated in
memory by the kernel.  It is set based on the underlying media support, the
value of FS_XFLAG_DAX and the filesystem's dax mount option.

statx can be used to query S_DAX.  NOTE that only regular files will ever have
S_DAX set and therefore statx will never indicate that S_DAX is set on
directories.

Setting the FS_XFLAG_DAX flag (specifically or through inheritance) occurs even
if the underlying media does not support dax and/or the filesystem is
overridden with a mount option.


Implementation Tips for Block Driver Writers
--------------------------------------------

To support DAX in your block driver, implement the 'direct_access'
block device operation.  It is used to translate the sector number
(expressed in units of 512-byte sectors) to a page frame number (pfn)
that identifies the physical page for the memory.  It also returns a
kernel virtual address that can be used to access the memory.

The direct_access method takes a 'size' parameter that indicates the
number of bytes being requested.  The function should return the number
of bytes that can be contiguously accessed at that offset.  It may also
return a negative errno if an error occurs.

In order to support this method, the storage must be byte-accessible by
the CPU at all times.  If your device uses paging techniques to expose
a large amount of memory through a smaller window, then you cannot
implement direct_access.  Equally, if your device can occasionally
stall the CPU for an extended period, you should also not attempt to
implement direct_access.

These block devices may be used for inspiration:
- brd: RAM backed block device driver
- dcssblk: s390 dcss block device driver
- pmem: NVDIMM persistent memory driver


Implementation Tips for Filesystem Writers
------------------------------------------

Filesystem support consists of
- adding support to mark inodes as being DAX by setting the S_DAX flag in
  i_flags
- implementing ->read_iter and ->write_iter operations which use dax_iomap_rw()
  when inode has S_DAX flag set
- implementing an mmap file operation for DAX files which sets the
  VM_MIXEDMAP and VM_HUGEPAGE flags on the VMA, and setting the vm_ops to
  include handlers for fault, pmd_fault, page_mkwrite, pfn_mkwrite. These
  handlers should probably call dax_iomap_fault() passing the appropriate
  fault size and iomap operations.
- calling iomap_zero_range() passing appropriate iomap operations instead of
  block_truncate_page() for DAX files
- ensuring that there is sufficient locking between reads, writes,
  truncates and page faults

The iomap handlers for allocating blocks must make sure that allocated blocks
are zeroed out and converted to written extents before being returned to avoid
exposure of uninitialized data through mmap.

These filesystems may be used for inspiration:
- ext2: see Documentation/filesystems/ext2.rst
- ext4: see Documentation/filesystems/ext4/
- xfs:  see Documentation/admin-guide/xfs.rst


Handling Media Errors
---------------------

The libnvdimm subsystem stores a record of known media error locations for
each pmem block device (in gendisk->badblocks). If we fault at such location,
or one with a latent error not yet discovered, the application can expect
to receive a SIGBUS. Libnvdimm also allows clearing of these errors by simply
writing the affected sectors (through the pmem driver, and if the underlying
NVDIMM supports the clear_poison DSM defined by ACPI).

Since DAX IO normally doesn't go through the driver/bio path, applications or
sysadmins have an option to restore the lost data from a prior backup/inbuilt
redundancy in the following ways:

1. Delete the affected file, and restore from a backup (sysadmin route):
   This will free the filesystem blocks that were being used by the file,
   and the next time they're allocated, they will be zeroed first, which
   happens through the driver, and will clear bad sectors.

2. Truncate or hole-punch the part of the file that has a bad-block (at least
   an entire aligned sector has to be hole-punched, but not necessarily an
   entire filesystem block).

These are the two basic paths that allow DAX filesystems to continue operating
in the presence of media errors. More robust error recovery mechanisms can be
built on top of this in the future, for example, involving redundancy/mirroring
provided at the block layer through DM, or additionally, at the filesystem
level. These would have to rely on the above two tenets, that error clearing
can happen either by sending an IO through the driver, or zeroing (also through
the driver).


Shortcomings
------------

Even if the kernel or its modules are stored on a filesystem that supports
DAX on a block device that supports DAX, they will still be copied into RAM.

The DAX code does not work correctly on architectures which have virtually
mapped caches such as ARM, MIPS and SPARC.

Calling get_user_pages() on a range of user memory that has been mmaped
from a DAX file will fail when there are no 'struct page' to describe
those pages.  This problem has been addressed in some device drivers
by adding optional struct page support for pages under the control of
the driver (see CONFIG_NVDIMM_PFN in drivers/nvdimm for an example of
how to do this). In the non struct page cases O_DIRECT reads/writes to
those memory ranges from a non-DAX file will fail (note that O_DIRECT
reads/writes _of a DAX file_ do work, it is the memory that is being
accessed that is key here).  Other things that will not work in the
non struct page case include RDMA, sendfile() and splice().