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.  When mounting it, use the "-o dax"
  option on the command line or add 'dax' to the options in /etc/fstab.

  
  
  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:
  - axonram: Axon DDR2 device driver
  - brd: RAM backed block device driver
  - dcssblk: s390 dcss block device 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 the direct_IO address space operation, and calling
    dax_do_io() instead of blockdev_direct_IO() if S_DAX is 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 and page_mkwrite (which should
    probably call dax_fault(), dax_pmd_fault() and dax_mkwrite(), passing the
    appropriate get_block() callback)

  - calling dax_truncate_page() instead of block_truncate_page() for DAX files

  - calling dax_zero_page_range() instead of zero_user() for DAX files

  - ensuring that there is sufficient locking between reads, writes,
    truncates and page faults
  
  The get_block() callback passed to the DAX functions may return
  uninitialised extents.  If it does, it must ensure that simultaneous
  calls to get_block() (for example by a page-fault racing with a read()
  or a write()) work correctly.
  
  These filesystems may be used for inspiration:
  - ext2: the second extended filesystem, see Documentation/filesystems/ext2.txt

  - ext4: the fourth extended filesystem, see Documentation/filesystems/ext4.txt

  
  
  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 as there are no 'struct page' to describe
  those pages.  This problem is being worked on.  That means that 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 include RDMA, sendfile() and splice().