1Direct 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 21on 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. 25 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 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) 67- calling dax_truncate_page() instead of block_truncate_page() for DAX files 68- calling dax_zero_page_range() instead of zero_user() for DAX files 69- 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 79- ext4: the fourth extended filesystem, see Documentation/filesystems/ext4.txt 80 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 88The DAX code does not work correctly on architectures which have virtually 89mapped caches such as ARM, MIPS and SPARC. 90 91Calling 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(). 98