1======================= 2Direct Access for files 3======================= 4 5Motivation 6---------- 7 8The page cache is usually used to buffer reads and writes to files. 9It is also used to provide the pages which are mapped into userspace 10by a call to mmap. 11 12For block devices that are memory-like, the page cache pages would be 13unnecessary copies of the original storage. The `DAX` code removes the 14extra copy by performing reads and writes directly to the storage device. 15For file mappings, the storage device is mapped directly into userspace. 16 17 18Usage 19----- 20 21If you have a block device which supports `DAX`, you can make a filesystem 22on it as usual. The `DAX` code currently only supports files with a block 23size equal to your kernel's `PAGE_SIZE`, so you may need to specify a block 24size when creating the filesystem. 25 26Currently 5 filesystems support `DAX`: ext2, ext4, xfs, virtiofs and erofs. 27Enabling `DAX` on them is different. 28 29Enabling DAX on ext2 and erofs 30------------------------------ 31 32When mounting the filesystem, use the ``-o dax`` option on the command line or 33add 'dax' to the options in ``/etc/fstab``. This works to enable `DAX` on all files 34within the filesystem. It is equivalent to the ``-o dax=always`` behavior below. 35 36 37Enabling DAX on xfs and ext4 38---------------------------- 39 40Summary 41------- 42 43 1. There exists an in-kernel file access mode flag `S_DAX` that corresponds to 44 the statx flag `STATX_ATTR_DAX`. See the manpage for statx(2) for details 45 about this access mode. 46 47 2. There exists a persistent flag `FS_XFLAG_DAX` that can be applied to regular 48 files and directories. This advisory flag can be set or cleared at any 49 time, but doing so does not immediately affect the `S_DAX` state. 50 51 3. If the persistent `FS_XFLAG_DAX` flag is set on a directory, this flag will 52 be inherited by all regular files and subdirectories that are subsequently 53 created in this directory. Files and subdirectories that exist at the time 54 this flag is set or cleared on the parent directory are not modified by 55 this modification of the parent directory. 56 57 4. There exist dax mount options which can override `FS_XFLAG_DAX` in the 58 setting of the `S_DAX` flag. Given underlying storage which supports `DAX` the 59 following hold: 60 61 ``-o dax=inode`` means "follow `FS_XFLAG_DAX`" and is the default. 62 63 ``-o dax=never`` means "never set `S_DAX`, ignore `FS_XFLAG_DAX`." 64 65 ``-o dax=always`` means "always set `S_DAX` ignore `FS_XFLAG_DAX`." 66 67 ``-o dax`` is a legacy option which is an alias for ``dax=always``. 68 69 .. warning:: 70 71 The option ``-o dax`` may be removed in the future so ``-o dax=always`` is 72 the preferred method for specifying this behavior. 73 74 .. note:: 75 76 Modifications to and the inheritance behavior of `FS_XFLAG_DAX` remain 77 the same even when the filesystem is mounted with a dax option. However, 78 in-core inode state (`S_DAX`) will be overridden until the filesystem is 79 remounted with dax=inode and the inode is evicted from kernel memory. 80 81 5. The `S_DAX` policy can be changed via: 82 83 a) Setting the parent directory `FS_XFLAG_DAX` as needed before files are 84 created 85 86 b) Setting the appropriate dax="foo" mount option 87 88 c) Changing the `FS_XFLAG_DAX` flag on existing regular files and 89 directories. This has runtime constraints and limitations that are 90 described in 6) below. 91 92 6. When changing the `S_DAX` policy via toggling the persistent `FS_XFLAG_DAX` 93 flag, the change to existing regular files won't take effect until the 94 files are closed by all processes. 95 96 97Details 98------- 99 100There are 2 per-file dax flags. One is a persistent inode setting (`FS_XFLAG_DAX`) 101and the other is a volatile flag indicating the active state of the feature 102(`S_DAX`). 103 104`FS_XFLAG_DAX` is preserved within the filesystem. This persistent config 105setting can be set, cleared and/or queried using the `FS_IOC_FS`[`GS`]`ETXATTR` ioctl 106(see ioctl_xfs_fsgetxattr(2)) or an utility such as 'xfs_io'. 107 108New files and directories automatically inherit `FS_XFLAG_DAX` from 109their parent directory **when created**. Therefore, setting `FS_XFLAG_DAX` at 110directory creation time can be used to set a default behavior for an entire 111sub-tree. 112 113To clarify inheritance, here are 3 examples: 114 115Example A: 116 117.. code-block:: shell 118 119 mkdir -p a/b/c 120 xfs_io -c 'chattr +x' a 121 mkdir a/b/c/d 122 mkdir a/e 123 124 ------[outcome]------ 125 126 dax: a,e 127 no dax: b,c,d 128 129Example B: 130 131.. code-block:: shell 132 133 mkdir a 134 xfs_io -c 'chattr +x' a 135 mkdir -p a/b/c/d 136 137 ------[outcome]------ 138 139 dax: a,b,c,d 140 no dax: 141 142Example C: 143 144.. code-block:: shell 145 146 mkdir -p a/b/c 147 xfs_io -c 'chattr +x' c 148 mkdir a/b/c/d 149 150 ------[outcome]------ 151 152 dax: c,d 153 no dax: a,b 154 155The current enabled state (`S_DAX`) is set when a file inode is instantiated in 156memory by the kernel. It is set based on the underlying media support, the 157value of `FS_XFLAG_DAX` and the filesystem's dax mount option. 158 159statx can be used to query `S_DAX`. 160 161.. note:: 162 163 That only regular files will ever have `S_DAX` set and therefore statx 164 will never indicate that `S_DAX` is set on directories. 165 166Setting the `FS_XFLAG_DAX` flag (specifically or through inheritance) occurs even 167if the underlying media does not support dax and/or the filesystem is 168overridden with a mount option. 169 170 171Enabling DAX on virtiofs 172---------------------------- 173The semantic of DAX on virtiofs is basically equal to that on ext4 and xfs, 174except that when '-o dax=inode' is specified, virtiofs client derives the hint 175whether DAX shall be enabled or not from virtiofs server through FUSE protocol, 176rather than the persistent `FS_XFLAG_DAX` flag. That is, whether DAX shall be 177enabled or not is completely determined by virtiofs server, while virtiofs 178server itself may deploy various algorithm making this decision, e.g. depending 179on the persistent `FS_XFLAG_DAX` flag on the host. 180 181It is still supported to set or clear persistent `FS_XFLAG_DAX` flag inside 182guest, but it is not guaranteed that DAX will be enabled or disabled for 183corresponding file then. Users inside guest still need to call statx(2) and 184check the statx flag `STATX_ATTR_DAX` to see if DAX is enabled for this file. 185 186 187Implementation Tips for Block Driver Writers 188-------------------------------------------- 189 190To support `DAX` in your block driver, implement the 'direct_access' 191block device operation. It is used to translate the sector number 192(expressed in units of 512-byte sectors) to a page frame number (pfn) 193that identifies the physical page for the memory. It also returns a 194kernel virtual address that can be used to access the memory. 195 196The direct_access method takes a 'size' parameter that indicates the 197number of bytes being requested. The function should return the number 198of bytes that can be contiguously accessed at that offset. It may also 199return a negative errno if an error occurs. 200 201In order to support this method, the storage must be byte-accessible by 202the CPU at all times. If your device uses paging techniques to expose 203a large amount of memory through a smaller window, then you cannot 204implement direct_access. Equally, if your device can occasionally 205stall the CPU for an extended period, you should also not attempt to 206implement direct_access. 207 208These block devices may be used for inspiration: 209- brd: RAM backed block device driver 210- dcssblk: s390 dcss block device driver 211- pmem: NVDIMM persistent memory driver 212 213 214Implementation Tips for Filesystem Writers 215------------------------------------------ 216 217Filesystem support consists of: 218 219* Adding support to mark inodes as being `DAX` by setting the `S_DAX` flag in 220 i_flags 221* Implementing ->read_iter and ->write_iter operations which use 222 :c:func:`dax_iomap_rw()` when inode has `S_DAX` flag set 223* Implementing an mmap file operation for `DAX` files which sets the 224 `VM_MIXEDMAP` and `VM_HUGEPAGE` flags on the `VMA`, and setting the vm_ops to 225 include handlers for fault, pmd_fault, page_mkwrite, pfn_mkwrite. These 226 handlers should probably call :c:func:`dax_iomap_fault()` passing the 227 appropriate fault size and iomap operations. 228* Calling :c:func:`iomap_zero_range()` passing appropriate iomap operations 229 instead of :c:func:`block_truncate_page()` for `DAX` files 230* Ensuring that there is sufficient locking between reads, writes, 231 truncates and page faults 232 233The iomap handlers for allocating blocks must make sure that allocated blocks 234are zeroed out and converted to written extents before being returned to avoid 235exposure of uninitialized data through mmap. 236 237These filesystems may be used for inspiration: 238 239.. seealso:: 240 241 ext2: see Documentation/filesystems/ext2.rst 242 243.. seealso:: 244 245 xfs: see Documentation/admin-guide/xfs.rst 246 247.. seealso:: 248 249 ext4: see Documentation/filesystems/ext4/ 250 251 252Handling Media Errors 253--------------------- 254 255The libnvdimm subsystem stores a record of known media error locations for 256each pmem block device (in gendisk->badblocks). If we fault at such location, 257or one with a latent error not yet discovered, the application can expect 258to receive a `SIGBUS`. Libnvdimm also allows clearing of these errors by simply 259writing the affected sectors (through the pmem driver, and if the underlying 260NVDIMM supports the clear_poison DSM defined by ACPI). 261 262Since `DAX` IO normally doesn't go through the ``driver/bio`` path, applications or 263sysadmins have an option to restore the lost data from a prior ``backup/inbuilt`` 264redundancy in the following ways: 265 2661. Delete the affected file, and restore from a backup (sysadmin route): 267 This will free the filesystem blocks that were being used by the file, 268 and the next time they're allocated, they will be zeroed first, which 269 happens through the driver, and will clear bad sectors. 270 2712. Truncate or hole-punch the part of the file that has a bad-block (at least 272 an entire aligned sector has to be hole-punched, but not necessarily an 273 entire filesystem block). 274 275These are the two basic paths that allow `DAX` filesystems to continue operating 276in the presence of media errors. More robust error recovery mechanisms can be 277built on top of this in the future, for example, involving redundancy/mirroring 278provided at the block layer through DM, or additionally, at the filesystem 279level. These would have to rely on the above two tenets, that error clearing 280can happen either by sending an IO through the driver, or zeroing (also through 281the driver). 282 283 284Shortcomings 285------------ 286 287Even if the kernel or its modules are stored on a filesystem that supports 288`DAX` on a block device that supports `DAX`, they will still be copied into RAM. 289 290The DAX code does not work correctly on architectures which have virtually 291mapped caches such as ARM, MIPS and SPARC. 292 293Calling :c:func:`get_user_pages()` on a range of user memory that has been 294mmaped from a `DAX` file will fail when there are no 'struct page' to describe 295those pages. This problem has been addressed in some device drivers 296by adding optional struct page support for pages under the control of 297the driver (see `CONFIG_NVDIMM_PFN` in ``drivers/nvdimm`` for an example of 298how to do this). In the non struct page cases `O_DIRECT` reads/writes to 299those memory ranges from a non-`DAX` file will fail 300 301 302.. note:: 303 304 `O_DIRECT` reads/writes _of a `DAX` file do work, it is the memory that 305 is being accessed that is key here). Other things that will not work in 306 the non struct page case include RDMA, :c:func:`sendfile()` and 307 :c:func:`splice()`. 308