======================= 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 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().