• Zygo Blaxell's avatar
    btrfs: add a flag to iterate_inodes_from_logical to find all extent refs for uncompressed extents · c995ab3c
    Zygo Blaxell authored
    The LOGICAL_INO ioctl provides a backward mapping from extent bytenr and
    offset (encoded as a single logical address) to a list of extent refs.
    LOGICAL_INO complements TREE_SEARCH, which provides the forward mapping
    (extent ref -> extent bytenr and offset, or logical address).  These are
    useful capabilities for programs that manipulate extents and extent
    references from userspace (e.g. dedup and defrag utilities).
    
    When the extents are uncompressed (and not encrypted and not other),
    check_extent_in_eb performs filtering of the extent refs to remove any
    extent refs which do not contain the same extent offset as the 'logical'
    parameter's extent offset.  This prevents LOGICAL_INO from returning
    references to more than a single block.
    
    To find the set of extent references to an uncompressed extent from [a, b),
    userspace has to run a loop like this pseudocode:
    
    	for (i = a; i < b; ++i)
    		extent_ref_set += LOGICAL_INO(i);
    
    At each iteration of the loop (up to 32768 iterations for a 128M extent),
    data we are interested in is collected in the kernel, then deleted by
    the filter in check_extent_in_eb.
    
    When the extents are compressed (or encrypted or other), the 'logical'
    parameter must be an extent bytenr (the 'a' parameter in the loop).
    No filtering by extent offset is done (or possible?) so the result is
    the complete set of extent refs for the entire extent.  This removes
    the need for the loop, since we get all the extent refs in one call.
    
    Add an 'ignore_offset' argument to iterate_inodes_from_logical,
    [...several levels of function call graph...], and check_extent_in_eb, so
    that we can disable the extent offset filtering for uncompressed extents.
    This flag can be set by an improved version of the LOGICAL_INO ioctl to
    get either behavior as desired.
    
    There is no functional change in this patch.  The new flag is always
    false.
    Signed-off-by: default avatarZygo Blaxell <ce3g8jdj@umail.furryterror.org>
    Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
    [ minor coding style fixes ]
    Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
    c995ab3c
backref.c 59.2 KB