docs: convert dax.txt to rst

Change the file extension and add the rst constructs to integrate this
doc to the documentation infrastructure and take advantage of rst
features.

Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente@gmail.com>
Link: https://lore.kernel.org/r/20210531130515.10309-1-igormtorrente@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 291 insertions, 0 deletions

diff --git a/Documentation/filesystems/dax.rst b/Documentation/filesystems/dax.rst
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+=======================
+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``.
+
+    .. warning::
+
+      The option ``-o dax`` 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 to existing regular files won't take effect until the
+    files are closed by all processes.
+
+
+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:
+
+.. code-block:: shell
+
+  mkdir -p a/b/c
+  xfs_io -c 'chattr +x' a
+  mkdir a/b/c/d
+  mkdir a/e
+
+  ------[outcome]------
+
+  dax: a,e
+  no dax: b,c,d
+
+Example B:
+
+.. code-block:: shell
+
+  mkdir a
+  xfs_io -c 'chattr +x' a
+  mkdir -p a/b/c/d
+
+  ------[outcome]------
+
+  dax: a,b,c,d
+  no dax:
+
+Example C:
+
+.. code-block:: shell
+
+  mkdir -p a/b/c
+  xfs_io -c 'chattr +x' c
+  mkdir a/b/c/d
+
+  ------[outcome]------
+
+  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
+  :c:func:`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 :c:func:`dax_iomap_fault()` passing the
+  appropriate fault size and iomap operations.
+* Calling :c:func:`iomap_zero_range()` passing appropriate iomap operations
+  instead of :c:func:`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:
+
+.. seealso::
+
+  ext2: see Documentation/filesystems/ext2.rst
+
+.. seealso::
+
+  xfs:  see Documentation/admin-guide/xfs.rst
+
+.. seealso::
+
+  ext4: see Documentation/filesystems/ext4/
+
+
+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 :c:func:`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::
+
+  `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, :c:func:`sendfile()` and
+  :c:func:`splice()`.