1 ======================= 2 Direct Access for files 3 ======================= 4 5 Motivation 6 ---------- 7 8 The page cache is usually used to buffer reads and writes to files. 9 It is also used to provide the pages which are mapped into userspace 10 by a call to mmap. 11 12 For block devices that are memory-like, the page cache pages would be 13 unnecessary copies of the original storage. The `DAX` code removes the 14 extra copy by performing reads and writes directly to the storage device. 15 For file mappings, the storage device is mapped directly into userspace. 16 17 18 Usage 19 ----- 20 21 If you have a block device which supports `DAX`, you can make a filesystem 22 on it as usual. The `DAX` code currently only supports files with a block 23 size equal to your kernel's `PAGE_SIZE`, so you may need to specify a block 24 size when creating the filesystem. 25 26 Currently 5 filesystems support `DAX`: ext2, ext4, xfs, virtiofs and erofs. 27 Enabling `DAX` on them is different. 28 29 Enabling DAX on ext2 and erofs 30 ------------------------------ 31 32 When mounting the filesystem, use the ``-o dax`` option on the command line or 33 add 'dax' to the options in ``/etc/fstab``. This works to enable `DAX` on all files 34 within the filesystem. It is equivalent to the ``-o dax=always`` behavior below. 35 36 37 Enabling DAX on xfs and ext4 38 ---------------------------- 39 40 Summary 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 97 Details 98 ------- 99 100 There are 2 per-file dax flags. One is a persistent inode setting (`FS_XFLAG_DAX`) 101 and 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 105 setting 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'. 107 108 New files and directories automatically inherit `FS_XFLAG_DAX` from 109 their parent directory **when created**. Therefore, setting `FS_XFLAG_DAX` at 110 directory creation time can be used to set a default behavior for an entire 111 sub-tree. 112 113 To clarify inheritance, here are 3 examples: 114 115 Example 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 128 129 Example 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: 141 142 Example 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 154 155 The current enabled state (`S_DAX`) is set when a file inode is instantiated in 156 memory by the kernel. It is set based on the underlying media support, the 157 value of `FS_XFLAG_DAX` and the filesystem's dax mount option. 158 159 statx 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. 165 166 Setting the `FS_XFLAG_DAX` flag (specifically or through inheritance) occurs even 167 if the underlying media does not support dax and/or the filesystem is 168 overridden with a mount option. 169 170 171 Enabling DAX on virtiofs 172 ---------------------------- 173 The semantic of DAX on virtiofs is basically equal to that on ext4 and xfs, 174 except that when '-o dax=inode' is specified, virtiofs client derives the hint 175 whether DAX shall be enabled or not from virtiofs server through FUSE protocol, 176 rather than the persistent `FS_XFLAG_DAX` flag. That is, whether DAX shall be 177 enabled or not is completely determined by virtiofs server, while virtiofs 178 server itself may deploy various algorithm making this decision, e.g. depending 179 on the persistent `FS_XFLAG_DAX` flag on the host. 180 181 It is still supported to set or clear persistent `FS_XFLAG_DAX` flag inside 182 guest, but it is not guaranteed that DAX will be enabled or disabled for 183 corresponding file then. Users inside guest still need to call statx(2) and 184 check the statx flag `STATX_ATTR_DAX` to see if DAX is enabled for this file. 185 186 187 Implementation Tips for Block Driver Writers 188 -------------------------------------------- 189 190 To support `DAX` in your block driver, implement the 'direct_access' 191 block device operation. It is used to translate the sector number 192 (expressed in units of 512-byte sectors) to a page frame number (pfn) 193 that identifies the physical page for the memory. It also returns a 194 kernel virtual address that can be used to access the memory. 195 196 The direct_access method takes a 'size' parameter that indicates the 197 number of bytes being requested. The function should return the number 198 of bytes that can be contiguously accessed at that offset. It may also 199 return a negative errno if an error occurs. 200 201 In order to support this method, the storage must be byte-accessible by 202 the CPU at all times. If your device uses paging techniques to expose 203 a large amount of memory through a smaller window, then you cannot 204 implement direct_access. Equally, if your device can occasionally 205 stall the CPU for an extended period, you should also not attempt to 206 implement direct_access. 207 208 These 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 212 213 214 Implementation Tips for Filesystem Writers 215 ------------------------------------------ 216 217 Filesystem 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 232 233 The iomap handlers for allocating blocks must make sure that allocated blocks 234 are zeroed out and converted to written extents before being returned to avoid 235 exposure of uninitialized data through mmap. 236 237 These 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/ 250 251 252 Handling Media Errors 253 --------------------- 254 255 The libnvdimm subsystem stores a record of known media error locations for 256 each pmem block device (in gendisk->badblocks). If we fault at such location, 257 or one with a latent error not yet discovered, the application can expect 258 to receive a `SIGBUS`. Libnvdimm also allows clearing of these errors by simply 259 writing the affected sectors (through the pmem driver, and if the underlying 260 NVDIMM supports the clear_poison DSM defined by ACPI). 261 262 Since `DAX` IO normally doesn't go through the ``driver/bio`` path, applications or 263 sysadmins have an option to restore the lost data from a prior ``backup/inbuilt`` 264 redundancy in the following ways: 265 266 1. 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. 270 271 2. 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). 274 275 These are the two basic paths that allow `DAX` filesystems to continue operating 276 in the presence of media errors. More robust error recovery mechanisms can be 277 built on top of this in the future, for example, involving redundancy/mirroring 278 provided at the block layer through DM, or additionally, at the filesystem 279 level. These would have to rely on the above two tenets, that error clearing 280 can happen either by sending an IO through the driver, or zeroing (also through 281 the driver). 282 283 284 Shortcomings 285 ------------ 286 287 Even 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. 289 290 The DAX code does not work correctly on architectures which have virtually 291 mapped caches such as ARM, MIPS and SPARC. 292 293 Calling :c:func:`get_user_pages()` on a range of user memory that has been 294 mmapped from a `DAX` file will fail when there are no 'struct page' to describe 295 those pages. This problem has been addressed in some device drivers 296 by adding optional struct page support for pages under the control of 297 the driver (see `CONFIG_NVDIMM_PFN` in ``drivers/nvdimm`` for an example of 298 how to do this). In the non struct page cases `O_DIRECT` reads/writes to 299 those 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()`.
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