fiemap.rst (10059B)
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 struct fiemap { 18 __u64 fm_start; /* logical offset (inclusive) at 19 * which to start mapping (in) */ 20 __u64 fm_length; /* logical length of mapping which 21 * userspace cares about (in) */ 22 __u32 fm_flags; /* FIEMAP_FLAG_* flags for request (in/out) */ 23 __u32 fm_mapped_extents; /* number of extents that were 24 * mapped (out) */ 25 __u32 fm_extent_count; /* size of fm_extents array (in) */ 26 __u32 fm_reserved; 27 struct fiemap_extent fm_extents[0]; /* array of mapped extents (out) */ 28 }; 29 30 31fm_start, and fm_length specify the logical range within the file 32which the process would like mappings for. Extents returned mirror 33those on disk - that is, the logical offset of the 1st returned extent 34may start before fm_start, and the range covered by the last returned 35extent may end after fm_length. All offsets and lengths are in bytes. 36 37Certain flags to modify the way in which mappings are looked up can be 38set in fm_flags. If the kernel doesn't understand some particular 39flags, it will return EBADR and the contents of fm_flags will contain 40the set of flags which caused the error. If the kernel is compatible 41with all flags passed, the contents of fm_flags will be unmodified. 42It is up to userspace to determine whether rejection of a particular 43flag is fatal to its operation. This scheme is intended to allow the 44fiemap interface to grow in the future but without losing 45compatibility with old software. 46 47fm_extent_count specifies the number of elements in the fm_extents[] array 48that can be used to return extents. If fm_extent_count is zero, then the 49fm_extents[] array is ignored (no extents will be returned), and the 50fm_mapped_extents count will hold the number of extents needed in 51fm_extents[] to hold the file's current mapping. Note that there is 52nothing to prevent the file from changing between calls to FIEMAP. 53 54The following flags can be set in fm_flags: 55 56FIEMAP_FLAG_SYNC 57 If this flag is set, the kernel will sync the file before mapping extents. 58 59FIEMAP_FLAG_XATTR 60 If this flag is set, the extents returned will describe the inodes 61 extended attribute lookup tree, instead of its data tree. 62 63 64Extent Mapping 65-------------- 66 67Extent information is returned within the embedded fm_extents array 68which userspace must allocate along with the fiemap structure. The 69number of elements in the fiemap_extents[] array should be passed via 70fm_extent_count. The number of extents mapped by kernel will be 71returned via fm_mapped_extents. If the number of fiemap_extents 72allocated is less than would be required to map the requested range, 73the maximum number of extents that can be mapped in the fm_extent[] 74array will be returned and fm_mapped_extents will be equal to 75fm_extent_count. In that case, the last extent in the array will not 76complete the requested range and will not have the FIEMAP_EXTENT_LAST 77flag set (see the next section on extent flags). 78 79Each extent is described by a single fiemap_extent structure as 80returned in fm_extents:: 81 82 struct fiemap_extent { 83 __u64 fe_logical; /* logical offset in bytes for the start of 84 * the extent */ 85 __u64 fe_physical; /* physical offset in bytes for the start 86 * of the extent */ 87 __u64 fe_length; /* length in bytes for the extent */ 88 __u64 fe_reserved64[2]; 89 __u32 fe_flags; /* FIEMAP_EXTENT_* flags for this extent */ 90 __u32 fe_reserved[3]; 91 }; 92 93All offsets and lengths are in bytes and mirror those on disk. It is valid 94for an extents logical offset to start before the request or its logical 95length to extend past the request. Unless FIEMAP_EXTENT_NOT_ALIGNED is 96returned, fe_logical, fe_physical, and fe_length will be aligned to the 97block size of the file system. With the exception of extents flagged as 98FIEMAP_EXTENT_MERGED, adjacent extents will not be merged. 99 100The fe_flags field contains flags which describe the extent returned. 101A special flag, FIEMAP_EXTENT_LAST is always set on the last extent in 102the file so that the process making fiemap calls can determine when no 103more extents are available, without having to call the ioctl again. 104 105Some flags are intentionally vague and will always be set in the 106presence of other more specific flags. This way a program looking for 107a general property does not have to know all existing and future flags 108which imply that property. 109 110For example, if FIEMAP_EXTENT_DATA_INLINE or FIEMAP_EXTENT_DATA_TAIL 111are set, FIEMAP_EXTENT_NOT_ALIGNED will also be set. A program looking 112for inline or tail-packed data can key on the specific flag. Software 113which simply cares not to try operating on non-aligned extents 114however, can just key on FIEMAP_EXTENT_NOT_ALIGNED, and not have to 115worry about all present and future flags which might imply unaligned 116data. Note that the opposite is not true - it would be valid for 117FIEMAP_EXTENT_NOT_ALIGNED to appear alone. 118 119FIEMAP_EXTENT_LAST 120 This is generally the last extent in the file. A mapping attempt past 121 this extent may return nothing. Some implementations set this flag to 122 indicate this extent is the last one in the range queried by the user 123 (via fiemap->fm_length). 124 125FIEMAP_EXTENT_UNKNOWN 126 The location of this extent is currently unknown. This may indicate 127 the data is stored on an inaccessible volume or that no storage has 128 been allocated for the file yet. 129 130FIEMAP_EXTENT_DELALLOC 131 This will also set FIEMAP_EXTENT_UNKNOWN. 132 133 Delayed allocation - while there is data for this extent, its 134 physical location has not been allocated yet. 135 136FIEMAP_EXTENT_ENCODED 137 This extent does not consist of plain filesystem blocks but is 138 encoded (e.g. encrypted or compressed). Reading the data in this 139 extent via I/O to the block device will have undefined results. 140 141Note that it is *always* undefined to try to update the data 142in-place by writing to the indicated location without the 143assistance of the filesystem, or to access the data using the 144information returned by the FIEMAP interface while the filesystem 145is mounted. In other words, user applications may only read the 146extent data via I/O to the block device while the filesystem is 147unmounted, and then only if the FIEMAP_EXTENT_ENCODED flag is 148clear; user applications must not try reading or writing to the 149filesystem via the block device under any other circumstances. 150 151FIEMAP_EXTENT_DATA_ENCRYPTED 152 This will also set FIEMAP_EXTENT_ENCODED 153 The data in this extent has been encrypted by the file system. 154 155FIEMAP_EXTENT_NOT_ALIGNED 156 Extent offsets and length are not guaranteed to be block aligned. 157 158FIEMAP_EXTENT_DATA_INLINE 159 This will also set FIEMAP_EXTENT_NOT_ALIGNED 160 Data is located within a meta data block. 161 162FIEMAP_EXTENT_DATA_TAIL 163 This will also set FIEMAP_EXTENT_NOT_ALIGNED 164 Data is packed into a block with data from other files. 165 166FIEMAP_EXTENT_UNWRITTEN 167 Unwritten extent - the extent is allocated but its data has not been 168 initialized. This indicates the extent's data will be all zero if read 169 through the filesystem but the contents are undefined if read directly from 170 the device. 171 172FIEMAP_EXTENT_MERGED 173 This will be set when a file does not support extents, i.e., it uses a block 174 based addressing scheme. Since returning an extent for each block back to 175 userspace would be highly inefficient, the kernel will try to merge most 176 adjacent blocks into 'extents'. 177 178 179VFS -> File System Implementation 180--------------------------------- 181 182File systems wishing to support fiemap must implement a ->fiemap callback on 183their inode_operations structure. The fs ->fiemap call is responsible for 184defining its set of supported fiemap flags, and calling a helper function on 185each discovered extent:: 186 187 struct inode_operations { 188 ... 189 190 int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start, 191 u64 len); 192 193->fiemap is passed struct fiemap_extent_info which describes the 194fiemap request:: 195 196 struct fiemap_extent_info { 197 unsigned int fi_flags; /* Flags as passed from user */ 198 unsigned int fi_extents_mapped; /* Number of mapped extents */ 199 unsigned int fi_extents_max; /* Size of fiemap_extent array */ 200 struct fiemap_extent *fi_extents_start; /* Start of fiemap_extent array */ 201 }; 202 203It is intended that the file system should not need to access any of this 204structure directly. Filesystem handlers should be tolerant to signals and return 205EINTR once fatal signal received. 206 207 208Flag checking should be done at the beginning of the ->fiemap callback via the 209fiemap_prep() helper:: 210 211 int fiemap_prep(struct inode *inode, struct fiemap_extent_info *fieinfo, 212 u64 start, u64 *len, u32 supported_flags); 213 214The struct fieinfo should be passed in as received from ioctl_fiemap(). The 215set of fiemap flags which the fs understands should be passed via fs_flags. If 216fiemap_prep finds invalid user flags, it will place the bad values in 217fieinfo->fi_flags and return -EBADR. If the file system gets -EBADR, from 218fiemap_prep(), it should immediately exit, returning that error back to 219ioctl_fiemap(). Additionally the range is validate against the supported 220maximum file size. 221 222 223For each extent in the request range, the file system should call 224the helper function, fiemap_fill_next_extent():: 225 226 int fiemap_fill_next_extent(struct fiemap_extent_info *info, u64 logical, 227 u64 phys, u64 len, u32 flags, u32 dev); 228 229fiemap_fill_next_extent() will use the passed values to populate the 230next free extent in the fm_extents array. 'General' extent flags will 231automatically be set from specific flags on behalf of the calling file 232system so that the userspace API is not broken. 233 234fiemap_fill_next_extent() returns 0 on success, and 1 when the 235user-supplied fm_extents array is full. If an error is encountered 236while copying the extent to user memory, -EFAULT will be returned.