237 lines · plain
1.. SPDX-License-Identifier: GPL-2.02 3============4Fiemap Ioctl5============6 7The fiemap ioctl is an efficient method for userspace to get file8extent mappings. Instead of block-by-block mapping (such as bmap), fiemap9returns a list of extents.10 11 12Request Basics13--------------14 15A fiemap request is encoded within struct fiemap::16 17 struct fiemap {18 __u64 fm_start; /* logical offset (inclusive) at19 * which to start mapping (in) */20 __u64 fm_length; /* logical length of mapping which21 * userspace cares about (in) */22 __u32 fm_flags; /* FIEMAP_FLAG_* flags for request (in/out) */23 __u32 fm_mapped_extents; /* number of extents that were24 * 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 file32which the process would like mappings for. Extents returned mirror33those on disk - that is, the logical offset of the 1st returned extent34may start before fm_start, and the range covered by the last returned35extent 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 be38set in fm_flags. If the kernel doesn't understand some particular39flags, it will return EBADR and the contents of fm_flags will contain40the set of flags which caused the error. If the kernel is compatible41with all flags passed, the contents of fm_flags will be unmodified.42It is up to userspace to determine whether rejection of a particular43flag is fatal to its operation. This scheme is intended to allow the44fiemap interface to grow in the future but without losing45compatibility with old software.46 47fm_extent_count specifies the number of elements in the fm_extents[] array48that can be used to return extents. If fm_extent_count is zero, then the49fm_extents[] array is ignored (no extents will be returned), and the50fm_mapped_extents count will hold the number of extents needed in51fm_extents[] to hold the file's current mapping. Note that there is52nothing to prevent the file from changing between calls to FIEMAP.53 54The following flags can be set in fm_flags:55 56FIEMAP_FLAG_SYNC57 If this flag is set, the kernel will sync the file before mapping extents.58 59FIEMAP_FLAG_XATTR60 If this flag is set, the extents returned will describe the inodes61 extended attribute lookup tree, instead of its data tree.62 63 64Extent Mapping65--------------66 67Extent information is returned within the embedded fm_extents array68which userspace must allocate along with the fiemap structure. The69number of elements in the fiemap_extents[] array should be passed via70fm_extent_count. The number of extents mapped by kernel will be71returned via fm_mapped_extents. If the number of fiemap_extents72allocated 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 to75fm_extent_count. In that case, the last extent in the array will not76complete the requested range and will not have the FIEMAP_EXTENT_LAST77flag set (see the next section on extent flags).78 79Each extent is described by a single fiemap_extent structure as80returned in fm_extents::81 82 struct fiemap_extent {83 __u64 fe_logical; /* logical offset in bytes for the start of84 * the extent */85 __u64 fe_physical; /* physical offset in bytes for the start86 * 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 valid94for an extents logical offset to start before the request or its logical95length to extend past the request. Unless FIEMAP_EXTENT_NOT_ALIGNED is96returned, fe_logical, fe_physical, and fe_length will be aligned to the97block size of the file system. With the exception of extents flagged as98FIEMAP_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 in102the file so that the process making fiemap calls can determine when no103more extents are available, without having to call the ioctl again.104 105Some flags are intentionally vague and will always be set in the106presence of other more specific flags. This way a program looking for107a general property does not have to know all existing and future flags108which imply that property.109 110For example, if FIEMAP_EXTENT_DATA_INLINE or FIEMAP_EXTENT_DATA_TAIL111are set, FIEMAP_EXTENT_NOT_ALIGNED will also be set. A program looking112for inline or tail-packed data can key on the specific flag. Software113which simply cares not to try operating on non-aligned extents114however, can just key on FIEMAP_EXTENT_NOT_ALIGNED, and not have to115worry about all present and future flags which might imply unaligned116data. Note that the opposite is not true - it would be valid for117FIEMAP_EXTENT_NOT_ALIGNED to appear alone.118 119FIEMAP_EXTENT_LAST120 This is generally the last extent in the file. A mapping attempt past121 this extent may return nothing. Some implementations set this flag to122 indicate this extent is the last one in the range queried by the user123 (via fiemap->fm_length).124 125FIEMAP_EXTENT_UNKNOWN126 The location of this extent is currently unknown. This may indicate127 the data is stored on an inaccessible volume or that no storage has128 been allocated for the file yet.129 130FIEMAP_EXTENT_DELALLOC131 This will also set FIEMAP_EXTENT_UNKNOWN.132 133 Delayed allocation - while there is data for this extent, its134 physical location has not been allocated yet.135 136FIEMAP_EXTENT_ENCODED137 This extent does not consist of plain filesystem blocks but is138 encoded (e.g. encrypted or compressed). Reading the data in this139 extent via I/O to the block device will have undefined results.140 141Note that it is *always* undefined to try to update the data142in-place by writing to the indicated location without the143assistance of the filesystem, or to access the data using the144information returned by the FIEMAP interface while the filesystem145is mounted. In other words, user applications may only read the146extent data via I/O to the block device while the filesystem is147unmounted, and then only if the FIEMAP_EXTENT_ENCODED flag is148clear; user applications must not try reading or writing to the149filesystem via the block device under any other circumstances.150 151FIEMAP_EXTENT_DATA_ENCRYPTED152 This will also set FIEMAP_EXTENT_ENCODED153 The data in this extent has been encrypted by the file system.154 155FIEMAP_EXTENT_NOT_ALIGNED156 Extent offsets and length are not guaranteed to be block aligned.157 158FIEMAP_EXTENT_DATA_INLINE159 This will also set FIEMAP_EXTENT_NOT_ALIGNED160 Data is located within a meta data block.161 162FIEMAP_EXTENT_DATA_TAIL163 This will also set FIEMAP_EXTENT_NOT_ALIGNED164 Data is packed into a block with data from other files.165 166FIEMAP_EXTENT_UNWRITTEN167 Unwritten extent - the extent is allocated but its data has not been168 initialized. This indicates the extent's data will be all zero if read169 through the filesystem but the contents are undefined if read directly from170 the device.171 172FIEMAP_EXTENT_MERGED173 This will be set when a file does not support extents, i.e., it uses a block174 based addressing scheme. Since returning an extent for each block back to175 userspace would be highly inefficient, the kernel will try to merge most176 adjacent blocks into 'extents'.177 178 179VFS -> File System Implementation180---------------------------------181 182File systems wishing to support fiemap must implement a ->fiemap callback on183their inode_operations structure. The fs ->fiemap call is responsible for184defining its set of supported fiemap flags, and calling a helper function on185each 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 the194fiemap 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 this204structure directly. Filesystem handlers should be tolerant to signals and return205EINTR once fatal signal received.206 207 208Flag checking should be done at the beginning of the ->fiemap callback via the209fiemap_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(). The215set of fiemap flags which the fs understands should be passed via fs_flags. If216fiemap_prep finds invalid user flags, it will place the bad values in217fieinfo->fi_flags and return -EBADR. If the file system gets -EBADR, from218fiemap_prep(), it should immediately exit, returning that error back to219ioctl_fiemap(). Additionally the range is validate against the supported220maximum file size.221 222 223For each extent in the request range, the file system should call224the 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 the230next free extent in the fm_extents array. 'General' extent flags will231automatically be set from specific flags on behalf of the calling file232system so that the userspace API is not broken.233 234fiemap_fill_next_extent() returns 0 on success, and 1 when the235user-supplied fm_extents array is full. If an error is encountered236while copying the extent to user memory, -EFAULT will be returned.237