1Direct Access for files 2----------------------- 3 4Motivation 5---------- 6 7The page cache is usually used to buffer reads and writes to files. 8It is also used to provide the pages which are mapped into userspace 9by a call to mmap. 10 11For block devices that are memory-like, the page cache pages would be 12unnecessary copies of the original storage. The DAX code removes the 13extra copy by performing reads and writes directly to the storage device. 14For file mappings, the storage device is mapped directly into userspace. 15 16 17Usage 18----- 19 20If you have a block device which supports DAX, you can make a filesystem 21on it as usual. The DAX code currently only supports files with a block 22size equal to your kernel's PAGE_SIZE, so you may need to specify a block 23size when creating the filesystem. 24 25Currently 3 filesystems support DAX: ext2, ext4 and xfs. Enabling DAX on them 26is different. 27 28Enabling DAX on ext2 29----------------------------- 30 31When mounting the filesystem, use the "-o dax" option on the command line or 32add 'dax' to the options in /etc/fstab. This works to enable DAX on all files 33within the filesystem. It is equivalent to the '-o dax=always' behavior below. 34 35 36Enabling DAX on xfs and ext4 37---------------------------- 38 39Summary 40------- 41 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. 45 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. 49 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. 55 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 58 following hold: 59 60 "-o dax=inode" means "follow FS_XFLAG_DAX" and is the default. 61 62 "-o dax=never" means "never set S_DAX, ignore FS_XFLAG_DAX." 63 64 "-o dax=always" means "always set S_DAX ignore FS_XFLAG_DAX." 65 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. 69 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. 74 75 5. The S_DAX policy can be changed via: 76 77 a) Setting the parent directory FS_XFLAG_DAX as needed before files are 78 created 79 80 b) Setting the appropriate dax="foo" mount option 81 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. 85 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 89 needs to: 90 91 a) stop the application so there are no active references to the data set 92 the policy change will affect 93 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: 96 97 i. drop-caches 98 ii. a filesystem unmount and mount cycle 99 iii. a system reboot 100 101 102Details 103------- 104 105There are 2 per-file dax flags. One is a persistent inode setting (FS_XFLAG_DAX) 106and the other is a volatile flag indicating the active state of the feature 107(S_DAX). 108 109FS_XFLAG_DAX is preserved within the filesystem. This persistent config 110setting 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'. 112 113New files and directories automatically inherit FS_XFLAG_DAX from 114their parent directory _when_ _created_. Therefore, setting FS_XFLAG_DAX at 115directory creation time can be used to set a default behavior for an entire 116sub-tree. 117 118To clarify inheritance, here are 3 examples: 119 120Example A: 121 122mkdir -p a/b/c 123xfs_io -c 'chattr +x' a 124mkdir a/b/c/d 125mkdir a/e 126 127 dax: a,e 128 no dax: b,c,d 129 130Example B: 131 132mkdir a 133xfs_io -c 'chattr +x' a 134mkdir -p a/b/c/d 135 136 dax: a,b,c,d 137 no dax: 138 139Example C: 140 141mkdir -p a/b/c 142xfs_io -c 'chattr +x' c 143mkdir a/b/c/d 144 145 dax: c,d 146 no dax: a,b 147 148 149The current enabled state (S_DAX) is set when a file inode is instantiated in 150memory by the kernel. It is set based on the underlying media support, the 151value of FS_XFLAG_DAX and the filesystem's dax mount option. 152 153statx can be used to query S_DAX. NOTE that only regular files will ever have 154S_DAX set and therefore statx will never indicate that S_DAX is set on 155directories. 156 157Setting the FS_XFLAG_DAX flag (specifically or through inheritance) occurs even 158if the underlying media does not support dax and/or the filesystem is 159overridden with a mount option. 160 161 162 163Implementation Tips for Block Driver Writers 164-------------------------------------------- 165 166To support DAX in your block driver, implement the 'direct_access' 167block device operation. It is used to translate the sector number 168(expressed in units of 512-byte sectors) to a page frame number (pfn) 169that identifies the physical page for the memory. It also returns a 170kernel virtual address that can be used to access the memory. 171 172The direct_access method takes a 'size' parameter that indicates the 173number of bytes being requested. The function should return the number 174of bytes that can be contiguously accessed at that offset. It may also 175return a negative errno if an error occurs. 176 177In order to support this method, the storage must be byte-accessible by 178the CPU at all times. If your device uses paging techniques to expose 179a large amount of memory through a smaller window, then you cannot 180implement direct_access. Equally, if your device can occasionally 181stall the CPU for an extended period, you should also not attempt to 182implement direct_access. 183 184These 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 188 189 190Implementation Tips for Filesystem Writers 191------------------------------------------ 192 193Filesystem support consists of 194- adding support to mark inodes as being DAX by setting the S_DAX flag in 195 i_flags 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 207 208The iomap handlers for allocating blocks must make sure that allocated blocks 209are zeroed out and converted to written extents before being returned to avoid 210exposure of uninitialized data through mmap. 211 212These 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 216 217 218Handling Media Errors 219--------------------- 220 221The libnvdimm subsystem stores a record of known media error locations for 222each pmem block device (in gendisk->badblocks). If we fault at such location, 223or one with a latent error not yet discovered, the application can expect 224to receive a SIGBUS. Libnvdimm also allows clearing of these errors by simply 225writing the affected sectors (through the pmem driver, and if the underlying 226NVDIMM supports the clear_poison DSM defined by ACPI). 227 228Since DAX IO normally doesn't go through the driver/bio path, applications or 229sysadmins have an option to restore the lost data from a prior backup/inbuilt 230redundancy in the following ways: 231 2321. 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. 236 2372. 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). 240 241These are the two basic paths that allow DAX filesystems to continue operating 242in the presence of media errors. More robust error recovery mechanisms can be 243built on top of this in the future, for example, involving redundancy/mirroring 244provided at the block layer through DM, or additionally, at the filesystem 245level. These would have to rely on the above two tenets, that error clearing 246can happen either by sending an IO through the driver, or zeroing (also through 247the driver). 248 249 250Shortcomings 251------------ 252 253Even if the kernel or its modules are stored on a filesystem that supports 254DAX on a block device that supports DAX, they will still be copied into RAM. 255 256The DAX code does not work correctly on architectures which have virtually 257mapped caches such as ARM, MIPS and SPARC. 258 259Calling get_user_pages() on a range of user memory that has been mmaped 260from a DAX file will fail when there are no 'struct page' to describe 261those pages. This problem has been addressed in some device drivers 262by adding optional struct page support for pages under the control of 263the driver (see CONFIG_NVDIMM_PFN in drivers/nvdimm for an example of 264how to do this). In the non struct page cases O_DIRECT reads/writes to 265those memory ranges from a non-DAX file will fail (note that O_DIRECT 266reads/writes _of a DAX file_ do work, it is the memory that is being 267accessed that is key here). Other things that will not work in the 268non struct page case include RDMA, sendfile() and splice(). 269