forked from Minki/linux
844f35db10
This is the support code for DAX-enabled filesystems to allow them to provide huge pages in response to faults. Signed-off-by: Matthew Wilcox <willy@linux.intel.com> Cc: Hillf Danton <dhillf@gmail.com> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> Cc: Theodore Ts'o <tytso@mit.edu> Cc: Jan Kara <jack@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
98 lines
4.1 KiB
Plaintext
98 lines
4.1 KiB
Plaintext
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().
|