============
  Fiemap Ioctl
  ============
  
  The fiemap ioctl is an efficient method for userspace to get file
  extent mappings. Instead of block-by-block mapping (such as bmap), fiemap
  returns a list of extents.
  
  
  Request Basics
  --------------
  
  A fiemap request is encoded within struct fiemap:
  
  struct fiemap {
  	__u64	fm_start;	 /* logical offset (inclusive) at
  				  * which to start mapping (in) */
  	__u64	fm_length;	 /* logical length of mapping which
  				  * userspace cares about (in) */
  	__u32	fm_flags;	 /* FIEMAP_FLAG_* flags for request (in/out) */
  	__u32	fm_mapped_extents; /* number of extents that were
  				    * mapped (out) */
  	__u32	fm_extent_count; /* size of fm_extents array (in) */
  	__u32	fm_reserved;
  	struct fiemap_extent fm_extents[0]; /* array of mapped extents (out) */
  };
  
  
  fm_start, and fm_length specify the logical range within the file
  which the process would like mappings for. Extents returned mirror
  those on disk - that is, the logical offset of the 1st returned extent
  may start before fm_start, and the range covered by the last returned
  extent may end after fm_length. All offsets and lengths are in bytes.
  
  Certain flags to modify the way in which mappings are looked up can be
  set in fm_flags. If the kernel doesn't understand some particular
  flags, it will return EBADR and the contents of fm_flags will contain
  the set of flags which caused the error. If the kernel is compatible
  with all flags passed, the contents of fm_flags will be unmodified.
  It is up to userspace to determine whether rejection of a particular
  flag is fatal to its operation. This scheme is intended to allow the
  fiemap interface to grow in the future but without losing
  compatibility with old software.
  
  fm_extent_count specifies the number of elements in the fm_extents[] array
  that can be used to return extents.  If fm_extent_count is zero, then the
  fm_extents[] array is ignored (no extents will be returned), and the
  fm_mapped_extents count will hold the number of extents needed in
  fm_extents[] to hold the file's current mapping.  Note that there is
  nothing to prevent the file from changing between calls to FIEMAP.
  
  The following flags can be set in fm_flags:
  
  * FIEMAP_FLAG_SYNC
  If this flag is set, the kernel will sync the file before mapping extents.
  
  * FIEMAP_FLAG_XATTR
  If this flag is set, the extents returned will describe the inodes
  extended attribute lookup tree, instead of its data tree.
  
  
  Extent Mapping
  --------------
  
  Extent information is returned within the embedded fm_extents array
  which userspace must allocate along with the fiemap structure. The
  number of elements in the fiemap_extents[] array should be passed via
  fm_extent_count. The number of extents mapped by kernel will be
  returned via fm_mapped_extents. If the number of fiemap_extents
  allocated is less than would be required to map the requested range,
  the maximum number of extents that can be mapped in the fm_extent[]
  array will be returned and fm_mapped_extents will be equal to
  fm_extent_count. In that case, the last extent in the array will not
  complete the requested range and will not have the FIEMAP_EXTENT_LAST
  flag set (see the next section on extent flags).
  
  Each extent is described by a single fiemap_extent structure as
  returned in fm_extents.
  
  struct fiemap_extent {
  	__u64	fe_logical;  /* logical offset in bytes for the start of
  			      * the extent */
  	__u64	fe_physical; /* physical offset in bytes for the start
  			      * of the extent */
  	__u64	fe_length;   /* length in bytes for the extent */
  	__u64	fe_reserved64[2];
  	__u32	fe_flags;    /* FIEMAP_EXTENT_* flags for this extent */
  	__u32	fe_reserved[3];
  };
  
  All offsets and lengths are in bytes and mirror those on disk.  It is valid
  for an extents logical offset to start before the request or its logical
  length to extend past the request.  Unless FIEMAP_EXTENT_NOT_ALIGNED is
  returned, fe_logical, fe_physical, and fe_length will be aligned to the
  block size of the file system.  With the exception of extents flagged as
  FIEMAP_EXTENT_MERGED, adjacent extents will not be merged.
  
  The fe_flags field contains flags which describe the extent returned.
  A special flag, FIEMAP_EXTENT_LAST is always set on the last extent in
  the file so that the process making fiemap calls can determine when no
  more extents are available, without having to call the ioctl again.
  
  Some flags are intentionally vague and will always be set in the
  presence of other more specific flags. This way a program looking for
  a general property does not have to know all existing and future flags
  which imply that property.
  
  For example, if FIEMAP_EXTENT_DATA_INLINE or FIEMAP_EXTENT_DATA_TAIL
  are set, FIEMAP_EXTENT_NOT_ALIGNED will also be set. A program looking
  for inline or tail-packed data can key on the specific flag. Software
  which simply cares not to try operating on non-aligned extents
  however, can just key on FIEMAP_EXTENT_NOT_ALIGNED, and not have to
  worry about all present and future flags which might imply unaligned
  data. Note that the opposite is not true - it would be valid for
  FIEMAP_EXTENT_NOT_ALIGNED to appear alone.
  
  * FIEMAP_EXTENT_LAST
  This is the last extent in the file. A mapping attempt past this
  extent will return nothing.
  
  * FIEMAP_EXTENT_UNKNOWN
  The location of this extent is currently unknown. This may indicate
  the data is stored on an inaccessible volume or that no storage has
  been allocated for the file yet.
  
  * FIEMAP_EXTENT_DELALLOC
    - This will also set FIEMAP_EXTENT_UNKNOWN.
  Delayed allocation - while there is data for this extent, its
  physical location has not been allocated yet.
  
  * FIEMAP_EXTENT_ENCODED
  This extent does not consist of plain filesystem blocks but is
  encoded (e.g. encrypted or compressed).  Reading the data in this
  extent via I/O to the block device will have undefined results.
  
  Note that it is *always* undefined to try to update the data
  in-place by writing to the indicated location without the
  assistance of the filesystem, or to access the data using the
  information returned by the FIEMAP interface while the filesystem
  is mounted.  In other words, user applications may only read the
  extent data via I/O to the block device while the filesystem is
  unmounted, and then only if the FIEMAP_EXTENT_ENCODED flag is
  clear; user applications must not try reading or writing to the
  filesystem via the block device under any other circumstances.
  
  * FIEMAP_EXTENT_DATA_ENCRYPTED
    - This will also set FIEMAP_EXTENT_ENCODED
  The data in this extent has been encrypted by the file system.
  
  * FIEMAP_EXTENT_NOT_ALIGNED
  Extent offsets and length are not guaranteed to be block aligned.
  
  * FIEMAP_EXTENT_DATA_INLINE
    This will also set FIEMAP_EXTENT_NOT_ALIGNED
  Data is located within a meta data block.
  
  * FIEMAP_EXTENT_DATA_TAIL
    This will also set FIEMAP_EXTENT_NOT_ALIGNED
  Data is packed into a block with data from other files.
  
  * FIEMAP_EXTENT_UNWRITTEN
  Unwritten extent - the extent is allocated but its data has not been
  initialized.  This indicates the extent's data will be all zero if read
  through the filesystem but the contents are undefined if read directly from
  the device.
  
  * FIEMAP_EXTENT_MERGED
  This will be set when a file does not support extents, i.e., it uses a block
  based addressing scheme.  Since returning an extent for each block back to
  userspace would be highly inefficient, the kernel will try to merge most
  adjacent blocks into 'extents'.
  
  
  VFS -> File System Implementation
  ---------------------------------
  
  File systems wishing to support fiemap must implement a ->fiemap callback on
  their inode_operations structure. The fs ->fiemap call is responsible for
  defining its set of supported fiemap flags, and calling a helper function on
  each discovered extent:
  
  struct inode_operations {
         ...
  
         int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start,
                       u64 len);
  
  ->fiemap is passed struct fiemap_extent_info which describes the
  fiemap request:
  
  struct fiemap_extent_info {
  	unsigned int fi_flags;		/* Flags as passed from user */
  	unsigned int fi_extents_mapped;	/* Number of mapped extents */
  	unsigned int fi_extents_max;	/* Size of fiemap_extent array */
  	struct fiemap_extent *fi_extents_start;	/* Start of fiemap_extent array */
  };
  
  It is intended that the file system should not need to access any of this
  structure directly. Filesystem handlers should be tolerant to signals and return
  EINTR once fatal signal received.
  
  
  Flag checking should be done at the beginning of the ->fiemap callback via the
  fiemap_check_flags() helper:
  
  int fiemap_check_flags(struct fiemap_extent_info *fieinfo, u32 fs_flags);
  
  The struct fieinfo should be passed in as received from ioctl_fiemap(). The
  set of fiemap flags which the fs understands should be passed via fs_flags. If
  fiemap_check_flags finds invalid user flags, it will place the bad values in
  fieinfo->fi_flags and return -EBADR. If the file system gets -EBADR, from
  fiemap_check_flags(), it should immediately exit, returning that error back to
  ioctl_fiemap().
  
  
  For each extent in the request range, the file system should call
  the helper function, fiemap_fill_next_extent():
  
  int fiemap_fill_next_extent(struct fiemap_extent_info *info, u64 logical,
  			    u64 phys, u64 len, u32 flags, u32 dev);
  
  fiemap_fill_next_extent() will use the passed values to populate the
  next free extent in the fm_extents array. 'General' extent flags will
  automatically be set from specific flags on behalf of the calling file
  system so that the userspace API is not broken.
  
  fiemap_fill_next_extent() returns 0 on success, and 1 when the
  user-supplied fm_extents array is full. If an error is encountered
  while copying the extent to user memory, -EFAULT will be returned.