[linux-2.6-block.git] / Documentation / filesystems / dax.txt

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().
Commit	Line	Data
95ec8dab MW	1	Direct Access for files
	2	-----------------------
	3
	4	Motivation
	5	----------
	6
	7	The page cache is usually used to buffer reads and writes to files.
	8	It is also used to provide the pages which are mapped into userspace
	9	by a call to mmap.
	10
	11	For block devices that are memory-like, the page cache pages would be
	12	unnecessary copies of the original storage. The DAX code removes the
	13	extra copy by performing reads and writes directly to the storage device.
	14	For file mappings, the storage device is mapped directly into userspace.
	15
	16
	17	Usage
	18	-----
	19
	20	If you have a block device which supports DAX, you can make a filesystem
44f4c054 MW	21	on it as usual. The DAX code currently only supports files with a block
	22	size equal to your kernel's PAGE_SIZE, so you may need to specify a block
	23	size when creating the filesystem. When mounting it, use the "-o dax"
	24	option on the command line or add 'dax' to the options in /etc/fstab.
95ec8dab MW	25
	26
	27	Implementation Tips for Block Driver Writers
	28	--------------------------------------------
	29
	30	To support DAX in your block driver, implement the 'direct_access'
	31	block device operation. It is used to translate the sector number
	32	(expressed in units of 512-byte sectors) to a page frame number (pfn)
	33	that identifies the physical page for the memory. It also returns a
	34	kernel virtual address that can be used to access the memory.
	35
	36	The direct_access method takes a 'size' parameter that indicates the
	37	number of bytes being requested. The function should return the number
	38	of bytes that can be contiguously accessed at that offset. It may also
	39	return a negative errno if an error occurs.
	40
	41	In order to support this method, the storage must be byte-accessible by
	42	the CPU at all times. If your device uses paging techniques to expose
	43	a large amount of memory through a smaller window, then you cannot
	44	implement direct_access. Equally, if your device can occasionally
	45	stall the CPU for an extended period, you should also not attempt to
	46	implement direct_access.
	47
	48	These block devices may be used for inspiration:
	49	- axonram: Axon DDR2 device driver
	50	- brd: RAM backed block device driver
	51	- dcssblk: s390 dcss block device driver
	52
	53
	54	Implementation Tips for Filesystem Writers
	55	------------------------------------------
	56
	57	Filesystem support consists of
	58	- adding support to mark inodes as being DAX by setting the S_DAX flag in
	59	i_flags
	60	- implementing the direct_IO address space operation, and calling
	61	dax_do_io() instead of blockdev_direct_IO() if S_DAX is set
	62	- implementing an mmap file operation for DAX files which sets the
844f35db MW	63	VM_MIXEDMAP and VM_HUGEPAGE flags on the VMA, and setting the vm_ops to
	64	include handlers for fault, pmd_fault and page_mkwrite (which should
	65	probably call dax_fault(), dax_pmd_fault() and dax_mkwrite(), passing the
	66	appropriate get_block() callback)
95ec8dab	67	- calling dax_truncate_page() instead of block_truncate_page() for DAX files
25726bc1	68	- calling dax_zero_page_range() instead of zero_user() for DAX files
95ec8dab MW	69	- ensuring that there is sufficient locking between reads, writes,
	70	truncates and page faults
	71
	72	The get_block() callback passed to the DAX functions may return
	73	uninitialised extents. If it does, it must ensure that simultaneous
	74	calls to get_block() (for example by a page-fault racing with a read()
	75	or a write()) work correctly.
	76
	77	These filesystems may be used for inspiration:
	78	- ext2: the second extended filesystem, see Documentation/filesystems/ext2.txt
923ae0ff	79	- ext4: the fourth extended filesystem, see Documentation/filesystems/ext4.txt
95ec8dab MW	80
	81
	82	Shortcomings
	83	------------
	84
	85	Even if the kernel or its modules are stored on a filesystem that supports
	86	DAX on a block device that supports DAX, they will still be copied into RAM.
	87
d92576f1 MW	88	The DAX code does not work correctly on architectures which have virtually
	89	mapped caches such as ARM, MIPS and SPARC.
	90
95ec8dab MW	91	Calling get_user_pages() on a range of user memory that has been mmaped
	92	from a DAX file will fail as there are no 'struct page' to describe
	93	those pages. This problem is being worked on. That means that O_DIRECT
	94	reads/writes to those memory ranges from a non-DAX file will fail (note
	95	that O_DIRECT reads/writes _of a DAX file_ do work, it is the memory
	96	that is being accessed that is key here). Other things that will not
	97	work include RDMA, sendfile() and splice().