Execute-in-place for file mappings
  ----------------------------------
  
  Motivation
  ----------
  File mappings are performed by mapping page cache pages to userspace. In
  addition, read&write type file operations also transfer data from/to the page
  cache.
  
  For memory backed storage devices that use the block device interface, the page
  cache pages are in fact copies of the original storage. Various approaches
  exist to work around the need for an extra copy. The ramdisk driver for example
  does read the data into the page cache, keeps a reference, and discards the
  original data behind later on.
  
  Execute-in-place solves this issue the other way around: instead of keeping
  data in the page cache, the need to have a page cache copy is eliminated
  completely. With execute-in-place, read&write type operations are performed
  directly from/to the memory backed storage device. For file mappings, the
  storage device itself is mapped directly into userspace.

  This implementation was initially written for shared memory segments between

  different virtual machines on s390 hardware to allow multiple machines to
  share the same binaries and libraries.
  
  Implementation
  --------------
  Execute-in-place is implemented in three steps: block device operation,
  address space operation, and file operations.
  
  A block device operation named direct_access is used to retrieve a
  reference (pointer) to a block on-disk. The reference is supposed to be
  cpu-addressable, physical address and remain valid until the release operation
  is performed. A struct block_device reference is used to address the device,
  and a sector_t argument is used to identify the individual block. As an
  alternative, memory technology devices can be used for this.
  
  The block device operation is optional, these block devices support it as of
  today:
  - dcssblk: s390 dcss block device driver

  An address space operation named get_xip_mem is used to retrieve references
  to a page frame number and a kernel address. To obtain these values a reference
  to an address_space is provided. This function assigns values to the kmem and
  pfn parameters. The third argument indicates whether the function should allocate
  blocks if needed.

  
  This address space operation is mutually exclusive with readpage&writepage that
  do page cache read/write operations.
  The following filesystems support it as of today:
  - ext2: the second extended filesystem, see Documentation/filesystems/ext2.txt
  
  A set of file operations that do utilize get_xip_page can be found in
  mm/filemap_xip.c . The following file operation implementations are provided:
  - aio_read/aio_write
  - readv/writev
  - sendfile
  
  The generic file operations do_sync_read/do_sync_write can be used to implement
  classic synchronous IO calls.
  
  Shortcomings
  ------------
  This implementation is limited to storage devices that are cpu addressable at
  all times (no highmem or such). It works well on rom/ram, but enhancements are
  needed to make it work with flash in read+write mode.
  Putting the Linux kernel and/or its modules on a xip filesystem does not mean
  they are not copied.