Merge branch 'akpm' (patches from Andrew)

	- 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.

	int (*releasepage) (struct page *, int);

	void (*freepage)(struct page *);

	int (*direct_IO)(int, struct kiocb *, struct iov_iter *iter, loff_t offset);

	int (*get_xip_mem)(struct address_space *, pgoff_t, int, void **,

				unsigned long *);

	int (*migratepage)(struct address_space *, struct page *, struct page *);

	int (*launder_page)(struct page *);

	int (*is_partially_uptodate)(struct page *, unsigned long, unsigned long);

releasepage:		yes

freepage:		yes

direct_IO:

get_xip_mem:					maybe

migratepage:		yes (both)

launder_page:		yes

is_partially_uptodate:	yes

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

- 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().

check=none, nocheck	(*)	Don't do extra checking of bitmaps on mount

				(check=normal and check=strict options removed)

dax				Use direct access (no page cache).  See

				Documentation/filesystems/dax.txt.

debug				Extra debugging information is sent to the

				kernel syslog.  Useful for developers.

nobh				Do not attach buffer_heads to file pagecache.

xip				Use execute in place (no caching) if possible

grpquota,noquota,quota,usrquota	Quota options are silently ignored by ext2.

i_version		Enable 64-bit inode version support. This option is

			off by default.

dax			Use direct access (no page cache).  See

			Documentation/filesystems/dax.txt.  Note that

			this option is incompatible with data=journal.

Data Mode

=========

There are 3 different data modes: