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diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt deleted file mode 100644 index 8fdb78f3c6c9..000000000000 --- a/Documentation/filesystems/dax.txt +++ /dev/null @@ -1,268 +0,0 @@ -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 3 filesystems support DAX: ext2, ext4 and xfs. Enabling DAX on them -is different. - -Enabling DAX on ext2 ------------------------------ - -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". - This 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 in behaviour for existing regular files may not occur - immediately. If the change must take effect immediately, the administrator - needs to: - - a) stop the application so there are no active references to the data set - the policy change will affect - - b) evict the data set from kernel caches so it will be re-instantiated when - the application is restarted. This can be achieved by: - - i. drop-caches - ii. a filesystem unmount and mount cycle - iii. a system reboot - - -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: - -mkdir -p a/b/c -xfs_io -c 'chattr +x' a -mkdir a/b/c/d -mkdir a/e - - dax: a,e - no dax: b,c,d - -Example B: - -mkdir a -xfs_io -c 'chattr +x' a -mkdir -p a/b/c/d - - dax: a,b,c,d - no dax: - -Example C: - -mkdir -p a/b/c -xfs_io -c 'chattr +x' c -mkdir a/b/c/d - - 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. - - - -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 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 dax_iomap_fault() passing the appropriate - fault size and iomap operations. -- calling iomap_zero_range() passing appropriate iomap operations instead of - 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: -- ext2: see Documentation/filesystems/ext2.rst -- ext4: see Documentation/filesystems/ext4/ -- xfs: see Documentation/admin-guide/xfs.rst - - -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 get_user_pages() on a range of user memory that has been mmaped -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 that 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, sendfile() and splice(). |