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Diffstat (limited to 'Documentation/filesystems/fiemap.txt')
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diff --git a/Documentation/filesystems/fiemap.txt b/Documentation/filesystems/fiemap.txt deleted file mode 100644 index ac87e6fda842..000000000000 --- a/Documentation/filesystems/fiemap.txt +++ /dev/null @@ -1,231 +0,0 @@ -============ -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 generally the last extent in the file. A mapping attempt past -this extent may return nothing. Some implementations set this flag to -indicate this extent is the last one in the range queried by the user -(via fiemap->fm_length). - -* 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. |