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-============
-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.
-
-
-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.