dax: replace XIP documentation with DAX documentation

	- directory containing configfs documentation and example code.

cramfs.txt

	- info on the cram filesystem for small storage (ROMs etc).

dax.txt

	- info on avoiding the page cache for files stored on CPU-addressable

	  storage devices.

debugfs.txt

	- info on the debugfs filesystem.

devpts.txt

	- info on XFS Self Describing Metadata.

xfs.txt

	- info and mount options for the XFS filesystem.

xip.txt

	- info on execute-in-place for file mappings.

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.  When mounting it, use the -o dax option manually

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 flag on the VMA, and setting the vm_ops to include handlers

  for fault and page_mkwrite (which should probably call dax_fault() and

  dax_mkwrite(), passing the appropriate get_block() callback)

- calling dax_truncate_page() instead of block_truncate_page() 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

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.

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().

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 translate the

block device sector number 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.

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.