Execute-in-place for file mappings
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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.