308 lines
11 KiB
ReStructuredText
308 lines
11 KiB
ReStructuredText
=======================
|
|
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 5 filesystems support `DAX`: ext2, ext4, xfs, virtiofs and erofs.
|
|
Enabling `DAX` on them is different.
|
|
|
|
Enabling DAX on ext2 and erofs
|
|
------------------------------
|
|
|
|
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.
|
|
|
|
|
|
Enabling DAX on virtiofs
|
|
----------------------------
|
|
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.
|
|
|
|
|
|
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()`.
|