Re: [PATCH 1/3] btrfs: add a flag to iterate_inodes_from_logical to find all extent refs for uncompressed extents

[David Sterba <dsterba@suse.cz>]

On Fri, Sep 22, 2017 at 01:58:45PM -0400, Zygo Blaxell wrote:
> 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: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>

Reviewed-by: David Sterba <dsterba@suse.com>