1 Direct Access for files
2 -----------------------
7 The page cache is usually used to buffer reads and writes to files.
8 It is also used to provide the pages which are mapped into userspace
11 For block devices that are memory-like, the page cache pages would be
12 unnecessary copies of the original storage. The DAX code removes the
13 extra copy by performing reads and writes directly to the storage device.
14 For file mappings, the storage device is mapped directly into userspace.
20 If you have a block device which supports DAX, you can make a filesystem
21 on it as usual. The DAX code currently only supports files with a block
22 size equal to your kernel's PAGE_SIZE, so you may need to specify a block
23 size when creating the filesystem.
25 Currently 3 filesystems support DAX: ext2, ext4 and xfs. Enabling DAX on them
28 Enabling DAX on ext4 and ext2
29 -----------------------------
31 When mounting the filesystem, use the "-o dax" option on the command line or
32 add 'dax' to the options in /etc/fstab. This works to enable DAX on all files
33 within the filesystem. It is equivalent to the '-o dax=always' behavior below.
42 1. There exists an in-kernel file access mode flag S_DAX that corresponds to
43 the statx flag STATX_ATTR_DAX. See the manpage for statx(2) for details
44 about this access mode.
46 2. There exists a persistent flag FS_XFLAG_DAX that can be applied to regular
47 files and directories. This advisory flag can be set or cleared at any
48 time, but doing so does not immediately affect the S_DAX state.
50 3. If the persistent FS_XFLAG_DAX flag is set on a directory, this flag will
51 be inherited by all regular files and subdirectories that are subsequently
52 created in this directory. Files and subdirectories that exist at the time
53 this flag is set or cleared on the parent directory are not modified by
54 this modification of the parent directory.
56 4. There exist dax mount options which can override FS_XFLAG_DAX in the
57 setting of the S_DAX flag. Given underlying storage which supports DAX the
60 "-o dax=inode" means "follow FS_XFLAG_DAX" and is the default.
62 "-o dax=never" means "never set S_DAX, ignore FS_XFLAG_DAX."
64 "-o dax=always" means "always set S_DAX ignore FS_XFLAG_DAX."
66 "-o dax" is a legacy option which is an alias for "dax=always".
67 This may be removed in the future so "-o dax=always" is
68 the preferred method for specifying this behavior.
70 NOTE: Modifications to and the inheritance behavior of FS_XFLAG_DAX remain
71 the same even when the filesystem is mounted with a dax option. However,
72 in-core inode state (S_DAX) will be overridden until the filesystem is
73 remounted with dax=inode and the inode is evicted from kernel memory.
75 5. The S_DAX policy can be changed via:
77 a) Setting the parent directory FS_XFLAG_DAX as needed before files are
80 b) Setting the appropriate dax="foo" mount option
82 c) Changing the FS_XFLAG_DAX flag on existing regular files and
83 directories. This has runtime constraints and limitations that are
84 described in 6) below.
86 6. When changing the S_DAX policy via toggling the persistent FS_XFLAG_DAX flag,
87 the change in behaviour for existing regular files may not occur
88 immediately. If the change must take effect immediately, the administrator
91 a) stop the application so there are no active references to the data set
92 the policy change will affect
94 b) evict the data set from kernel caches so it will be re-instantiated when
95 the application is restarted. This can be achieved by:
98 ii. a filesystem unmount and mount cycle
105 There are 2 per-file dax flags. One is a persistent inode setting (FS_XFLAG_DAX)
106 and the other is a volatile flag indicating the active state of the feature
109 FS_XFLAG_DAX is preserved within the filesystem. This persistent config
110 setting can be set, cleared and/or queried using the FS_IOC_FS[GS]ETXATTR ioctl
111 (see ioctl_xfs_fsgetxattr(2)) or an utility such as 'xfs_io'.
113 New files and directories automatically inherit FS_XFLAG_DAX from
114 their parent directory _when_ _created_. Therefore, setting FS_XFLAG_DAX at
115 directory creation time can be used to set a default behavior for an entire
118 To clarify inheritance, here are 3 examples:
123 xfs_io -c 'chattr +x' a
133 xfs_io -c 'chattr +x' a
142 xfs_io -c 'chattr +x' c
149 The current enabled state (S_DAX) is set when a file inode is instantiated in
150 memory by the kernel. It is set based on the underlying media support, the
151 value of FS_XFLAG_DAX and the filesystem's dax mount option.
153 statx can be used to query S_DAX. NOTE that only regular files will ever have
154 S_DAX set and therefore statx will never indicate that S_DAX is set on
157 Setting the FS_XFLAG_DAX flag (specifically or through inheritance) occurs even
158 if the underlying media does not support dax and/or the filesystem is
159 overridden with a mount option.
163 Implementation Tips for Block Driver Writers
164 --------------------------------------------
166 To support DAX in your block driver, implement the 'direct_access'
167 block device operation. It is used to translate the sector number
168 (expressed in units of 512-byte sectors) to a page frame number (pfn)
169 that identifies the physical page for the memory. It also returns a
170 kernel virtual address that can be used to access the memory.
172 The direct_access method takes a 'size' parameter that indicates the
173 number of bytes being requested. The function should return the number
174 of bytes that can be contiguously accessed at that offset. It may also
175 return a negative errno if an error occurs.
177 In order to support this method, the storage must be byte-accessible by
178 the CPU at all times. If your device uses paging techniques to expose
179 a large amount of memory through a smaller window, then you cannot
180 implement direct_access. Equally, if your device can occasionally
181 stall the CPU for an extended period, you should also not attempt to
182 implement direct_access.
184 These block devices may be used for inspiration:
185 - brd: RAM backed block device driver
186 - dcssblk: s390 dcss block device driver
187 - pmem: NVDIMM persistent memory driver
190 Implementation Tips for Filesystem Writers
191 ------------------------------------------
193 Filesystem support consists of
194 - adding support to mark inodes as being DAX by setting the S_DAX flag in
196 - implementing ->read_iter and ->write_iter operations which use dax_iomap_rw()
197 when inode has S_DAX flag set
198 - implementing an mmap file operation for DAX files which sets the
199 VM_MIXEDMAP and VM_HUGEPAGE flags on the VMA, and setting the vm_ops to
200 include handlers for fault, pmd_fault, page_mkwrite, pfn_mkwrite. These
201 handlers should probably call dax_iomap_fault() passing the appropriate
202 fault size and iomap operations.
203 - calling iomap_zero_range() passing appropriate iomap operations instead of
204 block_truncate_page() for DAX files
205 - ensuring that there is sufficient locking between reads, writes,
206 truncates and page faults
208 The iomap handlers for allocating blocks must make sure that allocated blocks
209 are zeroed out and converted to written extents before being returned to avoid
210 exposure of uninitialized data through mmap.
212 These filesystems may be used for inspiration:
213 - ext2: see Documentation/filesystems/ext2.rst
214 - ext4: see Documentation/filesystems/ext4/
215 - xfs: see Documentation/admin-guide/xfs.rst
218 Handling Media Errors
219 ---------------------
221 The libnvdimm subsystem stores a record of known media error locations for
222 each pmem block device (in gendisk->badblocks). If we fault at such location,
223 or one with a latent error not yet discovered, the application can expect
224 to receive a SIGBUS. Libnvdimm also allows clearing of these errors by simply
225 writing the affected sectors (through the pmem driver, and if the underlying
226 NVDIMM supports the clear_poison DSM defined by ACPI).
228 Since DAX IO normally doesn't go through the driver/bio path, applications or
229 sysadmins have an option to restore the lost data from a prior backup/inbuilt
230 redundancy in the following ways:
232 1. Delete the affected file, and restore from a backup (sysadmin route):
233 This will free the filesystem blocks that were being used by the file,
234 and the next time they're allocated, they will be zeroed first, which
235 happens through the driver, and will clear bad sectors.
237 2. Truncate or hole-punch the part of the file that has a bad-block (at least
238 an entire aligned sector has to be hole-punched, but not necessarily an
239 entire filesystem block).
241 These are the two basic paths that allow DAX filesystems to continue operating
242 in the presence of media errors. More robust error recovery mechanisms can be
243 built on top of this in the future, for example, involving redundancy/mirroring
244 provided at the block layer through DM, or additionally, at the filesystem
245 level. These would have to rely on the above two tenets, that error clearing
246 can happen either by sending an IO through the driver, or zeroing (also through
253 Even if the kernel or its modules are stored on a filesystem that supports
254 DAX on a block device that supports DAX, they will still be copied into RAM.
256 The DAX code does not work correctly on architectures which have virtually
257 mapped caches such as ARM, MIPS and SPARC.
259 Calling get_user_pages() on a range of user memory that has been mmaped
260 from a DAX file will fail when there are no 'struct page' to describe
261 those pages. This problem has been addressed in some device drivers
262 by adding optional struct page support for pages under the control of
263 the driver (see CONFIG_NVDIMM_PFN in drivers/nvdimm for an example of
264 how to do this). In the non struct page cases O_DIRECT reads/writes to
265 those memory ranges from a non-DAX file will fail (note that O_DIRECT
266 reads/writes _of a DAX file_ do work, it is the memory that is being
267 accessed that is key here). Other things that will not work in the
268 non struct page case include RDMA, sendfile() and splice().