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.  When mounting it, use the "-o dax"
 24option on the command line or add 'dax' to the options in /etc/fstab.
 25
 26
 27Implementation Tips for Block Driver Writers
 28--------------------------------------------
 29
 30To support DAX in your block driver, implement the 'direct_access'
 31block device operation.  It is used to translate the sector number
 32(expressed in units of 512-byte sectors) to a page frame number (pfn)
 33that identifies the physical page for the memory.  It also returns a
 34kernel virtual address that can be used to access the memory.
 35
 36The direct_access method takes a 'size' parameter that indicates the
 37number of bytes being requested.  The function should return the number
 38of bytes that can be contiguously accessed at that offset.  It may also
 39return a negative errno if an error occurs.
 40
 41In order to support this method, the storage must be byte-accessible by
 42the CPU at all times.  If your device uses paging techniques to expose
 43a large amount of memory through a smaller window, then you cannot
 44implement direct_access.  Equally, if your device can occasionally
 45stall the CPU for an extended period, you should also not attempt to
 46implement direct_access.
 47
 48These block devices may be used for inspiration:
 49- axonram: Axon DDR2 device driver
 50- brd: RAM backed block device driver
 51- dcssblk: s390 dcss block device driver
 52- pmem: NVDIMM persistent memory driver
 53
 54
 55Implementation Tips for Filesystem Writers
 56------------------------------------------
 57
 58Filesystem support consists of
 59- adding support to mark inodes as being DAX by setting the S_DAX flag in
 60  i_flags
 61- implementing ->read_iter and ->write_iter operations which use dax_iomap_rw()
 62  when inode has S_DAX flag set
 63- implementing an mmap file operation for DAX files which sets the
 64  VM_MIXEDMAP and VM_HUGEPAGE flags on the VMA, and setting the vm_ops to
 65  include handlers for fault, pmd_fault, page_mkwrite, pfn_mkwrite. These
 66  handlers should probably call dax_iomap_fault() (for fault and page_mkwrite
 67  handlers), dax_iomap_pmd_fault(), dax_pfn_mkwrite() passing the appropriate
 68  iomap operations.
 69- calling iomap_zero_range() passing appropriate iomap operations instead of
 70  block_truncate_page() for DAX files
 71- ensuring that there is sufficient locking between reads, writes,
 72  truncates and page faults
 73
 74The iomap handlers for allocating blocks must make sure that allocated blocks
 75are zeroed out and converted to written extents before being returned to avoid
 76exposure of uninitialized data through mmap.
 77
 78These filesystems may be used for inspiration:
 79- ext2: see Documentation/filesystems/ext2.txt
 80- ext4: see Documentation/filesystems/ext4.txt
 81- xfs:  see Documentation/filesystems/xfs.txt
 82
 83
 84Handling Media Errors
 85---------------------
 86
 87The libnvdimm subsystem stores a record of known media error locations for
 88each pmem block device (in gendisk->badblocks). If we fault at such location,
 89or one with a latent error not yet discovered, the application can expect
 90to receive a SIGBUS. Libnvdimm also allows clearing of these errors by simply
 91writing the affected sectors (through the pmem driver, and if the underlying
 92NVDIMM supports the clear_poison DSM defined by ACPI).
 93
 94Since DAX IO normally doesn't go through the driver/bio path, applications or
 95sysadmins have an option to restore the lost data from a prior backup/inbuilt
 96redundancy in the following ways:
 97
 981. Delete the affected file, and restore from a backup (sysadmin route):
 99   This will free the file system blocks that were being used by the file,
100   and the next time they're allocated, they will be zeroed first, which
101   happens through the driver, and will clear bad sectors.
102
1032. Truncate or hole-punch the part of the file that has a bad-block (at least
104   an entire aligned sector has to be hole-punched, but not necessarily an
105   entire filesystem block).
106
107These are the two basic paths that allow DAX filesystems to continue operating
108in the presence of media errors. More robust error recovery mechanisms can be
109built on top of this in the future, for example, involving redundancy/mirroring
110provided at the block layer through DM, or additionally, at the filesystem
111level. These would have to rely on the above two tenets, that error clearing
112can happen either by sending an IO through the driver, or zeroing (also through
113the driver).
114
115
116Shortcomings
117------------
118
119Even if the kernel or its modules are stored on a filesystem that supports
120DAX on a block device that supports DAX, they will still be copied into RAM.
121
122The DAX code does not work correctly on architectures which have virtually
123mapped caches such as ARM, MIPS and SPARC.
124
125Calling get_user_pages() on a range of user memory that has been mmaped
126from a DAX file will fail when there are no 'struct page' to describe
127those pages.  This problem has been addressed in some device drivers
128by adding optional struct page support for pages under the control of
129the driver (see CONFIG_NVDIMM_PFN in drivers/nvdimm for an example of
130how to do this). In the non struct page cases O_DIRECT reads/writes to
131those memory ranges from a non-DAX file will fail (note that O_DIRECT
132reads/writes _of a DAX file_ do work, it is the memory that is being
133accessed that is key here).  Other things that will not work in the
134non struct page case include RDMA, sendfile() and splice().