1.. SPDX-License-Identifier: GPL-2.0 2 3============ 4Fiemap Ioctl 5============ 6 7The fiemap ioctl is an efficient method for userspace to get file 8extent mappings. Instead of block-by-block mapping (such as bmap), fiemap 9returns a list of extents. 10 11 12Request Basics 13-------------- 14 15A fiemap request is encoded within struct fiemap: 16 17.. kernel-doc:: include/uapi/linux/fiemap.h 18 :identifiers: fiemap 19 20fm_start, and fm_length specify the logical range within the file 21which the process would like mappings for. Extents returned mirror 22those on disk - that is, the logical offset of the 1st returned extent 23may start before fm_start, and the range covered by the last returned 24extent may end after fm_length. All offsets and lengths are in bytes. 25 26Certain flags to modify the way in which mappings are looked up can be 27set in fm_flags. If the kernel doesn't understand some particular 28flags, it will return EBADR and the contents of fm_flags will contain 29the set of flags which caused the error. If the kernel is compatible 30with all flags passed, the contents of fm_flags will be unmodified. 31It is up to userspace to determine whether rejection of a particular 32flag is fatal to its operation. This scheme is intended to allow the 33fiemap interface to grow in the future but without losing 34compatibility with old software. 35 36fm_extent_count specifies the number of elements in the fm_extents[] array 37that can be used to return extents. If fm_extent_count is zero, then the 38fm_extents[] array is ignored (no extents will be returned), and the 39fm_mapped_extents count will hold the number of extents needed in 40fm_extents[] to hold the file's current mapping. Note that there is 41nothing to prevent the file from changing between calls to FIEMAP. 42 43The following flags can be set in fm_flags: 44 45FIEMAP_FLAG_SYNC 46 If this flag is set, the kernel will sync the file before mapping extents. 47 48FIEMAP_FLAG_XATTR 49 If this flag is set, the extents returned will describe the inodes 50 extended attribute lookup tree, instead of its data tree. 51 52FIEMAP_FLAG_CACHE 53 This flag requests caching of the extents. 54 55Extent Mapping 56-------------- 57 58Extent information is returned within the embedded fm_extents array 59which userspace must allocate along with the fiemap structure. The 60number of elements in the fiemap_extents[] array should be passed via 61fm_extent_count. The number of extents mapped by kernel will be 62returned via fm_mapped_extents. If the number of fiemap_extents 63allocated is less than would be required to map the requested range, 64the maximum number of extents that can be mapped in the fm_extent[] 65array will be returned and fm_mapped_extents will be equal to 66fm_extent_count. In that case, the last extent in the array will not 67complete the requested range and will not have the FIEMAP_EXTENT_LAST 68flag set (see the next section on extent flags). 69 70Each extent is described by a single fiemap_extent structure as 71returned in fm_extents: 72 73.. kernel-doc:: include/uapi/linux/fiemap.h 74 :identifiers: fiemap_extent 75 76All offsets and lengths are in bytes and mirror those on disk. It is valid 77for an extents logical offset to start before the request or its logical 78length to extend past the request. Unless FIEMAP_EXTENT_NOT_ALIGNED is 79returned, fe_logical, fe_physical, and fe_length will be aligned to the 80block size of the file system. With the exception of extents flagged as 81FIEMAP_EXTENT_MERGED, adjacent extents will not be merged. 82 83The fe_flags field contains flags which describe the extent returned. 84A special flag, FIEMAP_EXTENT_LAST is always set on the last extent in 85the file so that the process making fiemap calls can determine when no 86more extents are available, without having to call the ioctl again. 87 88Some flags are intentionally vague and will always be set in the 89presence of other more specific flags. This way a program looking for 90a general property does not have to know all existing and future flags 91which imply that property. 92 93For example, if FIEMAP_EXTENT_DATA_INLINE or FIEMAP_EXTENT_DATA_TAIL 94are set, FIEMAP_EXTENT_NOT_ALIGNED will also be set. A program looking 95for inline or tail-packed data can key on the specific flag. Software 96which simply cares not to try operating on non-aligned extents 97however, can just key on FIEMAP_EXTENT_NOT_ALIGNED, and not have to 98worry about all present and future flags which might imply unaligned 99data. Note that the opposite is not true - it would be valid for 100FIEMAP_EXTENT_NOT_ALIGNED to appear alone. 101 102FIEMAP_EXTENT_LAST 103 This is generally the last extent in the file. A mapping attempt past 104 this extent may return nothing. Some implementations set this flag to 105 indicate this extent is the last one in the range queried by the user 106 (via fiemap->fm_length). 107 108FIEMAP_EXTENT_UNKNOWN 109 The location of this extent is currently unknown. This may indicate 110 the data is stored on an inaccessible volume or that no storage has 111 been allocated for the file yet. 112 113FIEMAP_EXTENT_DELALLOC 114 This will also set FIEMAP_EXTENT_UNKNOWN. 115 116 Delayed allocation - while there is data for this extent, its 117 physical location has not been allocated yet. 118 119FIEMAP_EXTENT_ENCODED 120 This extent does not consist of plain filesystem blocks but is 121 encoded (e.g. encrypted or compressed). Reading the data in this 122 extent via I/O to the block device will have undefined results. 123 124Note that it is *always* undefined to try to update the data 125in-place by writing to the indicated location without the 126assistance of the filesystem, or to access the data using the 127information returned by the FIEMAP interface while the filesystem 128is mounted. In other words, user applications may only read the 129extent data via I/O to the block device while the filesystem is 130unmounted, and then only if the FIEMAP_EXTENT_ENCODED flag is 131clear; user applications must not try reading or writing to the 132filesystem via the block device under any other circumstances. 133 134FIEMAP_EXTENT_DATA_ENCRYPTED 135 This will also set FIEMAP_EXTENT_ENCODED 136 The data in this extent has been encrypted by the file system. 137 138FIEMAP_EXTENT_NOT_ALIGNED 139 Extent offsets and length are not guaranteed to be block aligned. 140 141FIEMAP_EXTENT_DATA_INLINE 142 This will also set FIEMAP_EXTENT_NOT_ALIGNED 143 Data is located within a meta data block. 144 145FIEMAP_EXTENT_DATA_TAIL 146 This will also set FIEMAP_EXTENT_NOT_ALIGNED 147 Data is packed into a block with data from other files. 148 149FIEMAP_EXTENT_UNWRITTEN 150 Unwritten extent - the extent is allocated but its data has not been 151 initialized. This indicates the extent's data will be all zero if read 152 through the filesystem but the contents are undefined if read directly from 153 the device. 154 155FIEMAP_EXTENT_MERGED 156 This will be set when a file does not support extents, i.e., it uses a block 157 based addressing scheme. Since returning an extent for each block back to 158 userspace would be highly inefficient, the kernel will try to merge most 159 adjacent blocks into 'extents'. 160 161FIEMAP_EXTENT_SHARED 162 This flag is set to request that space be shared with other files. 163 164VFS -> File System Implementation 165--------------------------------- 166 167File systems wishing to support fiemap must implement a ->fiemap callback on 168their inode_operations structure. The fs ->fiemap call is responsible for 169defining its set of supported fiemap flags, and calling a helper function on 170each discovered extent:: 171 172 struct inode_operations { 173 ... 174 175 int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start, 176 u64 len); 177 178->fiemap is passed struct fiemap_extent_info which describes the 179fiemap request: 180 181.. kernel-doc:: include/linux/fiemap.h 182 :identifiers: fiemap_extent_info 183 184It is intended that the file system should not need to access any of this 185structure directly. Filesystem handlers should be tolerant to signals and return 186EINTR once fatal signal received. 187 188 189Flag checking should be done at the beginning of the ->fiemap callback via the 190fiemap_prep() helper:: 191 192 int fiemap_prep(struct inode *inode, struct fiemap_extent_info *fieinfo, 193 u64 start, u64 *len, u32 supported_flags); 194 195The struct fieinfo should be passed in as received from ioctl_fiemap(). The 196set of fiemap flags which the fs understands should be passed via fs_flags. If 197fiemap_prep finds invalid user flags, it will place the bad values in 198fieinfo->fi_flags and return -EBADR. If the file system gets -EBADR, from 199fiemap_prep(), it should immediately exit, returning that error back to 200ioctl_fiemap(). Additionally the range is validate against the supported 201maximum file size. 202 203 204For each extent in the request range, the file system should call 205the helper function, fiemap_fill_next_extent():: 206 207 int fiemap_fill_next_extent(struct fiemap_extent_info *info, u64 logical, 208 u64 phys, u64 len, u32 flags, u32 dev); 209 210fiemap_fill_next_extent() will use the passed values to populate the 211next free extent in the fm_extents array. 'General' extent flags will 212automatically be set from specific flags on behalf of the calling file 213system so that the userspace API is not broken. 214 215fiemap_fill_next_extent() returns 0 on success, and 1 when the 216user-supplied fm_extents array is full. If an error is encountered 217while copying the extent to user memory, -EFAULT will be returned. 218