1 .. SPDX-License-Identifier: GPL-2.0
2
3 ============
4 Fiemap Ioctl
5 ============
6
7 The fiemap ioctl is an efficient method for userspace to get file
8 extent mappings. Instead of block-by-block mapping (such as bmap), fiemap
9 returns a list of extents.
10
11
12 Request Basics
13 --------------
14
15 A 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
31 fm_start, and fm_length specify the logical range within the file
32 which the process would like mappings for. Extents returned mirror
33 those on disk - that is, the logical offset of the 1st returned extent
34 may start before fm_start, and the range covered by the last returned
35 extent may end after fm_length. All offsets and lengths are in bytes.
36
37 Certain flags to modify the way in which mappings are looked up can be
38 set in fm_flags. If the kernel doesn't understand some particular
39 flags, it will return EBADR and the contents of fm_flags will contain
40 the set of flags which caused the error. If the kernel is compatible
41 with all flags passed, the contents of fm_flags will be unmodified.
42 It is up to userspace to determine whether rejection of a particular
43 flag is fatal to its operation. This scheme is intended to allow the
44 fiemap interface to grow in the future but without losing
45 compatibility with old software.
46
47 fm_extent_count specifies the number of elements in the fm_extents[] array
48 that can be used to return extents. If fm_extent_count is zero, then the
49 fm_extents[] array is ignored (no extents will be returned), and the
50 fm_mapped_extents count will hold the number of extents needed in
51 fm_extents[] to hold the file's current mapping. Note that there is
52 nothing to prevent the file from changing between calls to FIEMAP.
53
54 The following flags can be set in fm_flags:
55
56 FIEMAP_FLAG_SYNC
57 If this flag is set, the kernel will sync the file before mapping extents.
58
59 FIEMAP_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
64 Extent Mapping
65 --------------
66
67 Extent information is returned within the embedded fm_extents array
68 which userspace must allocate along with the fiemap structure. The
69 number of elements in the fiemap_extents[] array should be passed via
70 fm_extent_count. The number of extents mapped by kernel will be
71 returned via fm_mapped_extents. If the number of fiemap_extents
72 allocated is less than would be required to map the requested range,
73 the maximum number of extents that can be mapped in the fm_extent[]
74 array will be returned and fm_mapped_extents will be equal to
75 fm_extent_count. In that case, the last extent in the array will not
76 complete the requested range and will not have the FIEMAP_EXTENT_LAST
77 flag set (see the next section on extent flags).
78
79 Each extent is described by a single fiemap_extent structure as
80 returned 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
93 All offsets and lengths are in bytes and mirror those on disk. It is valid
94 for an extents logical offset to start before the request or its logical
95 length to extend past the request. Unless FIEMAP_EXTENT_NOT_ALIGNED is
96 returned, fe_logical, fe_physical, and fe_length will be aligned to the
97 block size of the file system. With the exception of extents flagged as
98 FIEMAP_EXTENT_MERGED, adjacent extents will not be merged.
99
100 The fe_flags field contains flags which describe the extent returned.
101 A special flag, FIEMAP_EXTENT_LAST is always set on the last extent in
102 the file so that the process making fiemap calls can determine when no
103 more extents are available, without having to call the ioctl again.
104
105 Some flags are intentionally vague and will always be set in the
106 presence of other more specific flags. This way a program looking for
107 a general property does not have to know all existing and future flags
108 which imply that property.
109
110 For example, if FIEMAP_EXTENT_DATA_INLINE or FIEMAP_EXTENT_DATA_TAIL
111 are set, FIEMAP_EXTENT_NOT_ALIGNED will also be set. A program looking
112 for inline or tail-packed data can key on the specific flag. Software
113 which simply cares not to try operating on non-aligned extents
114 however, can just key on FIEMAP_EXTENT_NOT_ALIGNED, and not have to
115 worry about all present and future flags which might imply unaligned
116 data. Note that the opposite is not true - it would be valid for
117 FIEMAP_EXTENT_NOT_ALIGNED to appear alone.
118
119 FIEMAP_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
125 FIEMAP_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
130 FIEMAP_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
136 FIEMAP_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
141 Note that it is *always* undefined to try to update the data
142 in-place by writing to the indicated location without the
143 assistance of the filesystem, or to access the data using the
144 information returned by the FIEMAP interface while the filesystem
145 is mounted. In other words, user applications may only read the
146 extent data via I/O to the block device while the filesystem is
147 unmounted, and then only if the FIEMAP_EXTENT_ENCODED flag is
148 clear; user applications must not try reading or writing to the
149 filesystem via the block device under any other circumstances.
150
151 FIEMAP_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
155 FIEMAP_EXTENT_NOT_ALIGNED
156 Extent offsets and length are not guaranteed to be block aligned.
157
158 FIEMAP_EXTENT_DATA_INLINE
159 This will also set FIEMAP_EXTENT_NOT_ALIGNED
160 Data is located within a meta data block.
161
162 FIEMAP_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
166 FIEMAP_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
172 FIEMAP_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
179 VFS -> File System Implementation
180 ---------------------------------
181
182 File systems wishing to support fiemap must implement a ->fiemap callback on
183 their inode_operations structure. The fs ->fiemap call is responsible for
184 defining its set of supported fiemap flags, and calling a helper function on
185 each 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
194 fiemap 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
203 It is intended that the file system should not need to access any of this
204 structure directly. Filesystem handlers should be tolerant to signals and return
205 EINTR once fatal signal received.
206
207
208 Flag checking should be done at the beginning of the ->fiemap callback via the
209 fiemap_prep() helper::
210
211 int fiemap_prep(struct inode *inode, struct fiemap_extent_info *fieinfo,
212 u64 start, u64 *len, u32 supported_flags);
213
214 The struct fieinfo should be passed in as received from ioctl_fiemap(). The
215 set of fiemap flags which the fs understands should be passed via fs_flags. If
216 fiemap_prep finds invalid user flags, it will place the bad values in
217 fieinfo->fi_flags and return -EBADR. If the file system gets -EBADR, from
218 fiemap_prep(), it should immediately exit, returning that error back to
219 ioctl_fiemap(). Additionally the range is validate against the supported
220 maximum file size.
221
222
223 For each extent in the request range, the file system should call
224 the 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
229 fiemap_fill_next_extent() will use the passed values to populate the
230 next free extent in the fm_extents array. 'General' extent flags will
231 automatically be set from specific flags on behalf of the calling file
232 system so that the userspace API is not broken.
233
234 fiemap_fill_next_extent() returns 0 on success, and 1 when the
235 user-supplied fm_extents array is full. If an error is encountered
236 while copying the extent to user memory, -EFAULT will be returned.
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