Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.
1 Direct Access for files 2 ----------------------- 3 4 Motivation 5 ---------- 6 7 The page cache is usually used to buffer reads and writes to files. 8 It is also used to provide the pages which are mapped into userspace 9 by a call to mmap. 10 11 For block devices that are memory-like, the page cache pages would be 12 unnecessary copies of the original storage. The DAX code removes the 13 extra copy by performing reads and writes directly to the storage device. 14 For file mappings, the storage device is mapped directly into userspace. 15 16 17 Usage 18 ----- 19 20 If you have a block device which supports DAX, you can make a filesystem 21 on it as usual. The DAX code currently only supports files with a block 22 size equal to your kernel's PAGE_SIZE, so you may need to specify a block 23 size when creating the filesystem. When mounting it, use the "-o dax" 24 option on the command line or add 'dax' to the options in /etc/fstab. 25 26 27 Implementation Tips for Block Driver Writers 28 -------------------------------------------- 29 30 To support DAX in your block driver, implement the 'direct_access' 31 block device operation. It is used to translate the sector number 32 (expressed in units of 512-byte sectors) to a page frame number (pfn) 33 that identifies the physical page for the memory. It also returns a 34 kernel virtual address that can be used to access the memory. 35 36 The direct_access method takes a 'size' parameter that indicates the 37 number of bytes being requested. The function should return the number 38 of bytes that can be contiguously accessed at that offset. It may also 39 return a negative errno if an error occurs. 40 41 In order to support this method, the storage must be byte-accessible by 42 the CPU at all times. If your device uses paging techniques to expose 43 a large amount of memory through a smaller window, then you cannot 44 implement direct_access. Equally, if your device can occasionally 45 stall the CPU for an extended period, you should also not attempt to 46 implement direct_access. 47 48 These 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 - pmem: NVDIMM persistent memory driver 53 54 55 Implementation Tips for Filesystem Writers 56 ------------------------------------------ 57 58 Filesystem support consists of 59 - adding support to mark inodes as being DAX by setting the S_DAX flag in 60 i_flags 61 - implementing ->read_iter and ->write_iter operations which use dax_iomap_rw() 62 when inode has S_DAX flag set 63 - implementing an mmap file operation for DAX files which sets the 64 VM_MIXEDMAP and VM_HUGEPAGE flags on the VMA, and setting the vm_ops to 65 include handlers for fault, pmd_fault, page_mkwrite, pfn_mkwrite. These 66 handlers should probably call dax_iomap_fault() (for fault and page_mkwrite 67 handlers), dax_iomap_pmd_fault(), dax_pfn_mkwrite() passing the appropriate 68 iomap operations. 69 - calling iomap_zero_range() passing appropriate iomap operations instead of 70 block_truncate_page() for DAX files 71 - ensuring that there is sufficient locking between reads, writes, 72 truncates and page faults 73 74 The iomap handlers for allocating blocks must make sure that allocated blocks 75 are zeroed out and converted to written extents before being returned to avoid 76 exposure of uninitialized data through mmap. 77 78 These filesystems may be used for inspiration: 79 - ext2: see Documentation/filesystems/ext2.txt 80 - ext4: see Documentation/filesystems/ext4.txt 81 - xfs: see Documentation/filesystems/xfs.txt 82 83 84 Handling Media Errors 85 --------------------- 86 87 The libnvdimm subsystem stores a record of known media error locations for 88 each pmem block device (in gendisk->badblocks). If we fault at such location, 89 or one with a latent error not yet discovered, the application can expect 90 to receive a SIGBUS. Libnvdimm also allows clearing of these errors by simply 91 writing the affected sectors (through the pmem driver, and if the underlying 92 NVDIMM supports the clear_poison DSM defined by ACPI). 93 94 Since DAX IO normally doesn't go through the driver/bio path, applications or 95 sysadmins have an option to restore the lost data from a prior backup/inbuilt 96 redundancy in the following ways: 97 98 1. Delete the affected file, and restore from a backup (sysadmin route): 99 This will free the file system blocks that were being used by the file, 100 and the next time they're allocated, they will be zeroed first, which 101 happens through the driver, and will clear bad sectors. 102 103 2. Truncate or hole-punch the part of the file that has a bad-block (at least 104 an entire aligned sector has to be hole-punched, but not necessarily an 105 entire filesystem block). 106 107 These are the two basic paths that allow DAX filesystems to continue operating 108 in the presence of media errors. More robust error recovery mechanisms can be 109 built on top of this in the future, for example, involving redundancy/mirroring 110 provided at the block layer through DM, or additionally, at the filesystem 111 level. These would have to rely on the above two tenets, that error clearing 112 can happen either by sending an IO through the driver, or zeroing (also through 113 the driver). 114 115 116 Shortcomings 117 ------------ 118 119 Even if the kernel or its modules are stored on a filesystem that supports 120 DAX on a block device that supports DAX, they will still be copied into RAM. 121 122 The DAX code does not work correctly on architectures which have virtually 123 mapped caches such as ARM, MIPS and SPARC. 124 125 Calling get_user_pages() on a range of user memory that has been mmaped 126 from a DAX file will fail when there are no 'struct page' to describe 127 those pages. This problem has been addressed in some device drivers 128 by adding optional struct page support for pages under the control of 129 the driver (see CONFIG_NVDIMM_PFN in drivers/nvdimm for an example of 130 how to do this). In the non struct page cases O_DIRECT reads/writes to 131 those memory ranges from a non-DAX file will fail (note that O_DIRECT 132 reads/writes _of a DAX file_ do work, it is the memory that is being 133 accessed that is key here). Other things that will not work in the 134 non struct page case include RDMA, sendfile() and splice().