linux-imx

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Fiemap Ioctl

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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.

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Request Basics

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A fiemap request is encoded within struct fiemap:

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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) */

};

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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.

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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.

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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.

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The following flags can be set in fm_flags:

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* FIEMAP_FLAG_SYNC

If this flag is set, the kernel will sync the file before mapping extents.

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* FIEMAP_FLAG_XATTR

If this flag is set, the extents returned will describe the inodes

extended attribute lookup tree, instead of its data tree.

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Extent Mapping

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