linux/Documentation/filesystems/fiemap.rst
<<
>>
Prefs
   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.
 237