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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
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().
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