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:
- brd: RAM backed block device driver
- dcssblk: s390 dcss block device driver
- pmem: NVDIMM persistent memory 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 ->read_iter and ->write_iter operations which use dax_iomap_rw()
  when inode has S_DAX flag 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, page_mkwrite, pfn_mkwrite. These
  handlers should probably call dax_iomap_fault() passing the appropriate
  fault size and iomap operations.
- calling iomap_zero_range() passing appropriate iomap operations instead of
  block_truncate_page() for DAX files
- ensuring that there is sufficient locking between reads, writes,
  truncates and page faults

The iomap handlers for allocating blocks must make sure that allocated blocks
are zeroed out and converted to written extents before being returned to avoid
exposure of uninitialized data through mmap.

These filesystems may be used for inspiration:
- ext2: see Documentation/filesystems/ext2.txt
- ext4: see Documentation/filesystems/ext4/ext4.rst
- xfs:  see Documentation/filesystems/xfs.txt


Handling Media Errors
---------------------

The libnvdimm subsystem stores a record of known media error locations for
each pmem block device (in gendisk->badblocks). If we fault at such location,
or one with a latent error not yet discovered, the application can expect
to receive a SIGBUS. Libnvdimm also allows clearing of these errors by simply
writing the affected sectors (through the pmem driver, and if the underlying
NVDIMM supports the clear_poison DSM defined by ACPI).

Since DAX IO normally doesn't go through the driver/bio path, applications or
sysadmins have an option to restore the lost data from a prior backup/inbuilt
redundancy in the following ways:

1. Delete the affected file, and restore from a backup (sysadmin route):
   This will free the file system blocks that were being used by the file,
   and the next time they're allocated, they will be zeroed first, which
   happens through the driver, and will clear bad sectors.

2. Truncate or hole-punch the part of the file that has a bad-block (at least
   an entire aligned sector has to be hole-punched, but not necessarily an
   entire filesystem block).

These are the two basic paths that allow DAX filesystems to continue operating
in the presence of media errors. More robust error recovery mechanisms can be
built on top of this in the future, for example, involving redundancy/mirroring
provided at the block layer through DM, or additionally, at the filesystem
level. These would have to rely on the above two tenets, that error clearing
can happen either by sending an IO through the driver, or zeroing (also through
the driver).


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 when there are no 'struct page' to describe
those pages.  This problem has been addressed in some device drivers
by adding optional struct page support for pages under the control of
the driver (see CONFIG_NVDIMM_PFN in drivers/nvdimm for an example of
how to do this). In the non struct page cases 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 in the
non struct page case include RDMA, sendfile() and splice().