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Matthew Wilcox95ec8da2015-02-16 15:59:09 -08001Direct Access for files
2-----------------------
3
4Motivation
5----------
6
7The page cache is usually used to buffer reads and writes to files.
8It is also used to provide the pages which are mapped into userspace
9by a call to mmap.
10
11For block devices that are memory-like, the page cache pages would be
12unnecessary copies of the original storage. The DAX code removes the
13extra copy by performing reads and writes directly to the storage device.
14For file mappings, the storage device is mapped directly into userspace.
15
16
17Usage
18-----
19
20If you have a block device which supports DAX, you can make a filesystem
Matthew Wilcox44f4c052015-07-03 10:40:38 -040021on it as usual. The DAX code currently only supports files with a block
22size equal to your kernel's PAGE_SIZE, so you may need to specify a block
23size when creating the filesystem. When mounting it, use the "-o dax"
24option on the command line or add 'dax' to the options in /etc/fstab.
Matthew Wilcox95ec8da2015-02-16 15:59:09 -080025
26
27Implementation Tips for Block Driver Writers
28--------------------------------------------
29
30To support DAX in your block driver, implement the 'direct_access'
31block device operation. It is used to translate the sector number
32(expressed in units of 512-byte sectors) to a page frame number (pfn)
33that identifies the physical page for the memory. It also returns a
34kernel virtual address that can be used to access the memory.
35
36The direct_access method takes a 'size' parameter that indicates the
37number of bytes being requested. The function should return the number
38of bytes that can be contiguously accessed at that offset. It may also
39return a negative errno if an error occurs.
40
41In order to support this method, the storage must be byte-accessible by
42the CPU at all times. If your device uses paging techniques to expose
43a large amount of memory through a smaller window, then you cannot
44implement direct_access. Equally, if your device can occasionally
45stall the CPU for an extended period, you should also not attempt to
46implement direct_access.
47
48These 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
54Implementation Tips for Filesystem Writers
55------------------------------------------
56
57Filesystem 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
Matthew Wilcox844f35d2015-09-08 14:58:57 -070063 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)
Matthew Wilcox95ec8da2015-02-16 15:59:09 -080067- calling dax_truncate_page() instead of block_truncate_page() for DAX files
Matthew Wilcox25726bc2015-02-16 15:59:35 -080068- calling dax_zero_page_range() instead of zero_user() for DAX files
Matthew Wilcox95ec8da2015-02-16 15:59:09 -080069- ensuring that there is sufficient locking between reads, writes,
70 truncates and page faults
71
72The get_block() callback passed to the DAX functions may return
73uninitialised extents. If it does, it must ensure that simultaneous
74calls to get_block() (for example by a page-fault racing with a read()
75or a write()) work correctly.
76
77These filesystems may be used for inspiration:
78- ext2: the second extended filesystem, see Documentation/filesystems/ext2.txt
Ross Zwisler923ae0f2015-02-16 15:59:38 -080079- ext4: the fourth extended filesystem, see Documentation/filesystems/ext4.txt
Matthew Wilcox95ec8da2015-02-16 15:59:09 -080080
81
82Shortcomings
83------------
84
85Even if the kernel or its modules are stored on a filesystem that supports
86DAX on a block device that supports DAX, they will still be copied into RAM.
87
Matthew Wilcoxd92576f2015-02-16 15:59:44 -080088The DAX code does not work correctly on architectures which have virtually
89mapped caches such as ARM, MIPS and SPARC.
90
Matthew Wilcox95ec8da2015-02-16 15:59:09 -080091Calling get_user_pages() on a range of user memory that has been mmaped
92from a DAX file will fail as there are no 'struct page' to describe
93those pages. This problem is being worked on. That means that O_DIRECT
94reads/writes to those memory ranges from a non-DAX file will fail (note
95that O_DIRECT reads/writes _of a DAX file_ do work, it is the memory
96that is being accessed that is key here). Other things that will not
97work include RDMA, sendfile() and splice().