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| -rw-r--r-- | Documentation/filesystems/fiemap.txt | 228 |
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diff --git a/Documentation/filesystems/fiemap.txt b/Documentation/filesystems/fiemap.txt new file mode 100644 index 000000000000..1e3defcfe50b --- /dev/null +++ b/Documentation/filesystems/fiemap.txt @@ -0,0 +1,228 @@ +============ +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 it's 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 it's 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 it's 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, it's +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 it's 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 it's 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 recieved 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. |