Documentation/filesystems/dax.rst
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.
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 5 filesystems support DAX: ext2, ext4, xfs, virtiofs and erofs.
Enabling DAX on them is different.
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.
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.
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.
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.
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.
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.
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.
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.
The semantic of DAX on virtiofs is basically equal to that on ext4 and xfs,
except that when '-o dax=inode' is specified, virtiofs client derives the hint
whether DAX shall be enabled or not from virtiofs server through FUSE protocol,
rather than the persistent FS_XFLAG_DAX flag. That is, whether DAX shall be
enabled or not is completely determined by virtiofs server, while virtiofs
server itself may deploy various algorithm making this decision, e.g. depending
on the persistent FS_XFLAG_DAX flag on the host.
It is still supported to set or clear persistent FS_XFLAG_DAX flag inside
guest, but it is not guaranteed that DAX will be enabled or disabled for
corresponding file then. Users inside guest still need to call statx(2) and
check the statx flag STATX_ATTR_DAX to see if DAX is enabled for this file.
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:
Filesystem support consists of:
DAX by setting the S_DAX flag in
i_flagsdax_iomap_rw() when inode has S_DAX flag setDAX 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.iomap_zero_range() passing appropriate iomap operations
instead of :c:func:block_truncate_page() for DAX filesThe 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/
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:
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.
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).
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
mmapped 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().