308 lines
11 KiB
ReStructuredText
308 lines
11 KiB
ReStructuredText
=======================
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Direct Access for files
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=======================
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Motivation
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----------
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The page cache is usually used to buffer reads and writes to files.
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It is also used to provide the pages which are mapped into userspace
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by a call to mmap.
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For block devices that are memory-like, the page cache pages would be
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unnecessary copies of the original storage. The `DAX` code removes the
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extra copy by performing reads and writes directly to the storage device.
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For file mappings, the storage device is mapped directly into userspace.
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Usage
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-----
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If you have a block device which supports `DAX`, you can make a filesystem
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on it as usual. The `DAX` code currently only supports files with a block
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size equal to your kernel's `PAGE_SIZE`, so you may need to specify a block
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size when creating the filesystem.
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Currently 5 filesystems support `DAX`: ext2, ext4, xfs, virtiofs and erofs.
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Enabling `DAX` on them is different.
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Enabling DAX on ext2 and erofs
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------------------------------
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When mounting the filesystem, use the ``-o dax`` option on the command line or
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add 'dax' to the options in ``/etc/fstab``. This works to enable `DAX` on all files
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within the filesystem. It is equivalent to the ``-o dax=always`` behavior below.
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Enabling DAX on xfs and ext4
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----------------------------
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Summary
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-------
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1. There exists an in-kernel file access mode flag `S_DAX` that corresponds to
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the statx flag `STATX_ATTR_DAX`. See the manpage for statx(2) for details
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about this access mode.
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2. There exists a persistent flag `FS_XFLAG_DAX` that can be applied to regular
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files and directories. This advisory flag can be set or cleared at any
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time, but doing so does not immediately affect the `S_DAX` state.
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3. If the persistent `FS_XFLAG_DAX` flag is set on a directory, this flag will
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be inherited by all regular files and subdirectories that are subsequently
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created in this directory. Files and subdirectories that exist at the time
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this flag is set or cleared on the parent directory are not modified by
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this modification of the parent directory.
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4. There exist dax mount options which can override `FS_XFLAG_DAX` in the
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setting of the `S_DAX` flag. Given underlying storage which supports `DAX` the
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following hold:
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``-o dax=inode`` means "follow `FS_XFLAG_DAX`" and is the default.
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``-o dax=never`` means "never set `S_DAX`, ignore `FS_XFLAG_DAX`."
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``-o dax=always`` means "always set `S_DAX` ignore `FS_XFLAG_DAX`."
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``-o dax`` is a legacy option which is an alias for ``dax=always``.
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.. warning::
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The option ``-o dax`` may be removed in the future so ``-o dax=always`` is
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the preferred method for specifying this behavior.
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.. note::
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Modifications to and the inheritance behavior of `FS_XFLAG_DAX` remain
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the same even when the filesystem is mounted with a dax option. However,
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in-core inode state (`S_DAX`) will be overridden until the filesystem is
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remounted with dax=inode and the inode is evicted from kernel memory.
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5. The `S_DAX` policy can be changed via:
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a) Setting the parent directory `FS_XFLAG_DAX` as needed before files are
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created
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b) Setting the appropriate dax="foo" mount option
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c) Changing the `FS_XFLAG_DAX` flag on existing regular files and
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directories. This has runtime constraints and limitations that are
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described in 6) below.
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6. When changing the `S_DAX` policy via toggling the persistent `FS_XFLAG_DAX`
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flag, the change to existing regular files won't take effect until the
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files are closed by all processes.
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Details
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-------
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There are 2 per-file dax flags. One is a persistent inode setting (`FS_XFLAG_DAX`)
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and the other is a volatile flag indicating the active state of the feature
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(`S_DAX`).
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`FS_XFLAG_DAX` is preserved within the filesystem. This persistent config
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setting can be set, cleared and/or queried using the `FS_IOC_FS`[`GS`]`ETXATTR` ioctl
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(see ioctl_xfs_fsgetxattr(2)) or an utility such as 'xfs_io'.
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New files and directories automatically inherit `FS_XFLAG_DAX` from
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their parent directory **when created**. Therefore, setting `FS_XFLAG_DAX` at
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directory creation time can be used to set a default behavior for an entire
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sub-tree.
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To clarify inheritance, here are 3 examples:
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Example A:
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.. code-block:: shell
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mkdir -p a/b/c
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xfs_io -c 'chattr +x' a
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mkdir a/b/c/d
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mkdir a/e
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------[outcome]------
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dax: a,e
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no dax: b,c,d
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Example B:
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.. code-block:: shell
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mkdir a
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xfs_io -c 'chattr +x' a
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mkdir -p a/b/c/d
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------[outcome]------
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dax: a,b,c,d
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no dax:
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Example C:
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.. code-block:: shell
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mkdir -p a/b/c
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xfs_io -c 'chattr +x' c
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mkdir a/b/c/d
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------[outcome]------
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dax: c,d
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no dax: a,b
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The current enabled state (`S_DAX`) is set when a file inode is instantiated in
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memory by the kernel. It is set based on the underlying media support, the
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value of `FS_XFLAG_DAX` and the filesystem's dax mount option.
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statx can be used to query `S_DAX`.
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.. note::
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That only regular files will ever have `S_DAX` set and therefore statx
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will never indicate that `S_DAX` is set on directories.
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Setting the `FS_XFLAG_DAX` flag (specifically or through inheritance) occurs even
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if the underlying media does not support dax and/or the filesystem is
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overridden with a mount option.
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Enabling DAX on virtiofs
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----------------------------
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The semantic of DAX on virtiofs is basically equal to that on ext4 and xfs,
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except that when '-o dax=inode' is specified, virtiofs client derives the hint
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whether DAX shall be enabled or not from virtiofs server through FUSE protocol,
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rather than the persistent `FS_XFLAG_DAX` flag. That is, whether DAX shall be
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enabled or not is completely determined by virtiofs server, while virtiofs
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server itself may deploy various algorithm making this decision, e.g. depending
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on the persistent `FS_XFLAG_DAX` flag on the host.
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It is still supported to set or clear persistent `FS_XFLAG_DAX` flag inside
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guest, but it is not guaranteed that DAX will be enabled or disabled for
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corresponding file then. Users inside guest still need to call statx(2) and
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check the statx flag `STATX_ATTR_DAX` to see if DAX is enabled for this file.
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Implementation Tips for Block Driver Writers
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--------------------------------------------
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To support `DAX` in your block driver, implement the 'direct_access'
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block device operation. It is used to translate the sector number
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(expressed in units of 512-byte sectors) to a page frame number (pfn)
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that identifies the physical page for the memory. It also returns a
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kernel virtual address that can be used to access the memory.
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The direct_access method takes a 'size' parameter that indicates the
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number of bytes being requested. The function should return the number
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of bytes that can be contiguously accessed at that offset. It may also
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return a negative errno if an error occurs.
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In order to support this method, the storage must be byte-accessible by
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the CPU at all times. If your device uses paging techniques to expose
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a large amount of memory through a smaller window, then you cannot
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implement direct_access. Equally, if your device can occasionally
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stall the CPU for an extended period, you should also not attempt to
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implement direct_access.
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These block devices may be used for inspiration:
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- brd: RAM backed block device driver
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- dcssblk: s390 dcss block device driver
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- pmem: NVDIMM persistent memory driver
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Implementation Tips for Filesystem Writers
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------------------------------------------
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Filesystem support consists of:
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* Adding support to mark inodes as being `DAX` by setting the `S_DAX` flag in
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i_flags
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* Implementing ->read_iter and ->write_iter operations which use
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:c:func:`dax_iomap_rw()` when inode has `S_DAX` flag set
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* Implementing an mmap file operation for `DAX` files which sets the
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`VM_MIXEDMAP` and `VM_HUGEPAGE` flags on the `VMA`, and setting the vm_ops to
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include handlers for fault, pmd_fault, page_mkwrite, pfn_mkwrite. These
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handlers should probably call :c:func:`dax_iomap_fault()` passing the
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appropriate fault size and iomap operations.
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* Calling :c:func:`iomap_zero_range()` passing appropriate iomap operations
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instead of :c:func:`block_truncate_page()` for `DAX` files
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* Ensuring that there is sufficient locking between reads, writes,
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truncates and page faults
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The iomap handlers for allocating blocks must make sure that allocated blocks
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are zeroed out and converted to written extents before being returned to avoid
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exposure of uninitialized data through mmap.
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These filesystems may be used for inspiration:
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.. seealso::
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ext2: see Documentation/filesystems/ext2.rst
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.. seealso::
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xfs: see Documentation/admin-guide/xfs.rst
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.. seealso::
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ext4: see Documentation/filesystems/ext4/
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Handling Media Errors
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---------------------
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The libnvdimm subsystem stores a record of known media error locations for
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each pmem block device (in gendisk->badblocks). If we fault at such location,
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or one with a latent error not yet discovered, the application can expect
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to receive a `SIGBUS`. Libnvdimm also allows clearing of these errors by simply
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writing the affected sectors (through the pmem driver, and if the underlying
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NVDIMM supports the clear_poison DSM defined by ACPI).
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Since `DAX` IO normally doesn't go through the ``driver/bio`` path, applications or
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sysadmins have an option to restore the lost data from a prior ``backup/inbuilt``
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redundancy in the following ways:
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1. Delete the affected file, and restore from a backup (sysadmin route):
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This will free the filesystem blocks that were being used by the file,
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and the next time they're allocated, they will be zeroed first, which
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happens through the driver, and will clear bad sectors.
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2. Truncate or hole-punch the part of the file that has a bad-block (at least
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an entire aligned sector has to be hole-punched, but not necessarily an
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entire filesystem block).
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These are the two basic paths that allow `DAX` filesystems to continue operating
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in the presence of media errors. More robust error recovery mechanisms can be
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built on top of this in the future, for example, involving redundancy/mirroring
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provided at the block layer through DM, or additionally, at the filesystem
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level. These would have to rely on the above two tenets, that error clearing
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can happen either by sending an IO through the driver, or zeroing (also through
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the driver).
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Shortcomings
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------------
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Even if the kernel or its modules are stored on a filesystem that supports
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`DAX` on a block device that supports `DAX`, they will still be copied into RAM.
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The DAX code does not work correctly on architectures which have virtually
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mapped caches such as ARM, MIPS and SPARC.
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Calling :c:func:`get_user_pages()` on a range of user memory that has been
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mmapped from a `DAX` file will fail when there are no 'struct page' to describe
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those pages. This problem has been addressed in some device drivers
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by adding optional struct page support for pages under the control of
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the driver (see `CONFIG_NVDIMM_PFN` in ``drivers/nvdimm`` for an example of
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how to do this). In the non struct page cases `O_DIRECT` reads/writes to
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those memory ranges from a non-`DAX` file will fail
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.. note::
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`O_DIRECT` reads/writes _of a `DAX` file do work, it is the memory that
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is being accessed that is key here). Other things that will not work in
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the non struct page case include RDMA, :c:func:`sendfile()` and
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:c:func:`splice()`.
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