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Based on kernel version 3.13. Page generated on 2014-01-20 22:02 EST.

1	============
2	Fiemap Ioctl
3	============
4	
5	The fiemap ioctl is an efficient method for userspace to get file
6	extent mappings. Instead of block-by-block mapping (such as bmap), fiemap
7	returns a list of extents.
8	
9	
10	Request Basics
11	--------------
12	
13	A fiemap request is encoded within struct fiemap:
14	
15	struct fiemap {
16		__u64	fm_start;	 /* logical offset (inclusive) at
17					  * which to start mapping (in) */
18		__u64	fm_length;	 /* logical length of mapping which
19					  * userspace cares about (in) */
20		__u32	fm_flags;	 /* FIEMAP_FLAG_* flags for request (in/out) */
21		__u32	fm_mapped_extents; /* number of extents that were
22					    * mapped (out) */
23		__u32	fm_extent_count; /* size of fm_extents array (in) */
24		__u32	fm_reserved;
25		struct fiemap_extent fm_extents[0]; /* array of mapped extents (out) */
26	};
27	
28	
29	fm_start, and fm_length specify the logical range within the file
30	which the process would like mappings for. Extents returned mirror
31	those on disk - that is, the logical offset of the 1st returned extent
32	may start before fm_start, and the range covered by the last returned
33	extent may end after fm_length. All offsets and lengths are in bytes.
34	
35	Certain flags to modify the way in which mappings are looked up can be
36	set in fm_flags. If the kernel doesn't understand some particular
37	flags, it will return EBADR and the contents of fm_flags will contain
38	the set of flags which caused the error. If the kernel is compatible
39	with all flags passed, the contents of fm_flags will be unmodified.
40	It is up to userspace to determine whether rejection of a particular
41	flag is fatal to its operation. This scheme is intended to allow the
42	fiemap interface to grow in the future but without losing
43	compatibility with old software.
44	
45	fm_extent_count specifies the number of elements in the fm_extents[] array
46	that can be used to return extents.  If fm_extent_count is zero, then the
47	fm_extents[] array is ignored (no extents will be returned), and the
48	fm_mapped_extents count will hold the number of extents needed in
49	fm_extents[] to hold the file's current mapping.  Note that there is
50	nothing to prevent the file from changing between calls to FIEMAP.
51	
52	The following flags can be set in fm_flags:
53	
54	* FIEMAP_FLAG_SYNC
55	If this flag is set, the kernel will sync the file before mapping extents.
56	
57	* FIEMAP_FLAG_XATTR
58	If this flag is set, the extents returned will describe the inodes
59	extended attribute lookup tree, instead of its data tree.
60	
61	
62	Extent Mapping
63	--------------
64	
65	Extent information is returned within the embedded fm_extents array
66	which userspace must allocate along with the fiemap structure. The
67	number of elements in the fiemap_extents[] array should be passed via
68	fm_extent_count. The number of extents mapped by kernel will be
69	returned via fm_mapped_extents. If the number of fiemap_extents
70	allocated is less than would be required to map the requested range,
71	the maximum number of extents that can be mapped in the fm_extent[]
72	array will be returned and fm_mapped_extents will be equal to
73	fm_extent_count. In that case, the last extent in the array will not
74	complete the requested range and will not have the FIEMAP_EXTENT_LAST
75	flag set (see the next section on extent flags).
76	
77	Each extent is described by a single fiemap_extent structure as
78	returned in fm_extents.
79	
80	struct fiemap_extent {
81		__u64	fe_logical;  /* logical offset in bytes for the start of
82				      * the extent */
83		__u64	fe_physical; /* physical offset in bytes for the start
84				      * of the extent */
85		__u64	fe_length;   /* length in bytes for the extent */
86		__u64	fe_reserved64[2];
87		__u32	fe_flags;    /* FIEMAP_EXTENT_* flags for this extent */
88		__u32	fe_reserved[3];
89	};
90	
91	All offsets and lengths are in bytes and mirror those on disk.  It is valid
92	for an extents logical offset to start before the request or its logical
93	length to extend past the request.  Unless FIEMAP_EXTENT_NOT_ALIGNED is
94	returned, fe_logical, fe_physical, and fe_length will be aligned to the
95	block size of the file system.  With the exception of extents flagged as
96	FIEMAP_EXTENT_MERGED, adjacent extents will not be merged.
97	
98	The fe_flags field contains flags which describe the extent returned.
99	A special flag, FIEMAP_EXTENT_LAST is always set on the last extent in
100	the file so that the process making fiemap calls can determine when no
101	more extents are available, without having to call the ioctl again.
102	
103	Some flags are intentionally vague and will always be set in the
104	presence of other more specific flags. This way a program looking for
105	a general property does not have to know all existing and future flags
106	which imply that property.
107	
108	For example, if FIEMAP_EXTENT_DATA_INLINE or FIEMAP_EXTENT_DATA_TAIL
109	are set, FIEMAP_EXTENT_NOT_ALIGNED will also be set. A program looking
110	for inline or tail-packed data can key on the specific flag. Software
111	which simply cares not to try operating on non-aligned extents
112	however, can just key on FIEMAP_EXTENT_NOT_ALIGNED, and not have to
113	worry about all present and future flags which might imply unaligned
114	data. Note that the opposite is not true - it would be valid for
115	FIEMAP_EXTENT_NOT_ALIGNED to appear alone.
116	
117	* FIEMAP_EXTENT_LAST
118	This is the last extent in the file. A mapping attempt past this
119	extent will return nothing.
120	
121	* FIEMAP_EXTENT_UNKNOWN
122	The location of this extent is currently unknown. This may indicate
123	the data is stored on an inaccessible volume or that no storage has
124	been allocated for the file yet.
125	
126	* FIEMAP_EXTENT_DELALLOC
127	  - This will also set FIEMAP_EXTENT_UNKNOWN.
128	Delayed allocation - while there is data for this extent, its
129	physical location has not been allocated yet.
130	
131	* FIEMAP_EXTENT_ENCODED
132	This extent does not consist of plain filesystem blocks but is
133	encoded (e.g. encrypted or compressed).  Reading the data in this
134	extent via I/O to the block device will have undefined results.
135	
136	Note that it is *always* undefined to try to update the data
137	in-place by writing to the indicated location without the
138	assistance of the filesystem, or to access the data using the
139	information returned by the FIEMAP interface while the filesystem
140	is mounted.  In other words, user applications may only read the
141	extent data via I/O to the block device while the filesystem is
142	unmounted, and then only if the FIEMAP_EXTENT_ENCODED flag is
143	clear; user applications must not try reading or writing to the
144	filesystem via the block device under any other circumstances.
145	
146	* FIEMAP_EXTENT_DATA_ENCRYPTED
147	  - This will also set FIEMAP_EXTENT_ENCODED
148	The data in this extent has been encrypted by the file system.
149	
150	* FIEMAP_EXTENT_NOT_ALIGNED
151	Extent offsets and length are not guaranteed to be block aligned.
152	
153	* FIEMAP_EXTENT_DATA_INLINE
154	  This will also set FIEMAP_EXTENT_NOT_ALIGNED
155	Data is located within a meta data block.
156	
157	* FIEMAP_EXTENT_DATA_TAIL
158	  This will also set FIEMAP_EXTENT_NOT_ALIGNED
159	Data is packed into a block with data from other files.
160	
161	* FIEMAP_EXTENT_UNWRITTEN
162	Unwritten extent - the extent is allocated but its data has not been
163	initialized.  This indicates the extent's data will be all zero if read
164	through the filesystem but the contents are undefined if read directly from
165	the device.
166	
167	* FIEMAP_EXTENT_MERGED
168	This will be set when a file does not support extents, i.e., it uses a block
169	based addressing scheme.  Since returning an extent for each block back to
170	userspace would be highly inefficient, the kernel will try to merge most
171	adjacent blocks into 'extents'.
172	
173	
174	VFS -> File System Implementation
175	---------------------------------
176	
177	File systems wishing to support fiemap must implement a ->fiemap callback on
178	their inode_operations structure. The fs ->fiemap call is responsible for
179	defining its set of supported fiemap flags, and calling a helper function on
180	each discovered extent:
181	
182	struct inode_operations {
183	       ...
184	
185	       int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start,
186	                     u64 len);
187	
188	->fiemap is passed struct fiemap_extent_info which describes the
189	fiemap request:
190	
191	struct fiemap_extent_info {
192		unsigned int fi_flags;		/* Flags as passed from user */
193		unsigned int fi_extents_mapped;	/* Number of mapped extents */
194		unsigned int fi_extents_max;	/* Size of fiemap_extent array */
195		struct fiemap_extent *fi_extents_start;	/* Start of fiemap_extent array */
196	};
197	
198	It is intended that the file system should not need to access any of this
199	structure directly.
200	
201	
202	Flag checking should be done at the beginning of the ->fiemap callback via the
203	fiemap_check_flags() helper:
204	
205	int fiemap_check_flags(struct fiemap_extent_info *fieinfo, u32 fs_flags);
206	
207	The struct fieinfo should be passed in as received from ioctl_fiemap(). The
208	set of fiemap flags which the fs understands should be passed via fs_flags. If
209	fiemap_check_flags finds invalid user flags, it will place the bad values in
210	fieinfo->fi_flags and return -EBADR. If the file system gets -EBADR, from
211	fiemap_check_flags(), it should immediately exit, returning that error back to
212	ioctl_fiemap().
213	
214	
215	For each extent in the request range, the file system should call
216	the helper function, fiemap_fill_next_extent():
217	
218	int fiemap_fill_next_extent(struct fiemap_extent_info *info, u64 logical,
219				    u64 phys, u64 len, u32 flags, u32 dev);
220	
221	fiemap_fill_next_extent() will use the passed values to populate the
222	next free extent in the fm_extents array. 'General' extent flags will
223	automatically be set from specific flags on behalf of the calling file
224	system so that the userspace API is not broken.
225	
226	fiemap_fill_next_extent() returns 0 on success, and 1 when the
227	user-supplied fm_extents array is full. If an error is encountered
228	while copying the extent to user memory, -EFAULT will be returned.
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