[NYE PATCHRIVER 4/4] xfs: freespace defrag for online shrink

["Darrick J. Wong" <djwong@kernel.org>]

Hi everyone,

This fourth patchriver has two faces -- one way of looking at it is that
it is random odds and ends at the tail of my development tree.  A second
interpretation is that it is necessary pieces for defragmenting free
space, which is a precursor for online shrink of XFS filesystems.

The kernel side isn't that exciting -- we export refcounting information
for space extents, and add a new fallocate mode for mapping exact
portions of free filesystem space into a file.

Userspace is where things get interesting!  The free space defragmenter
is an iterative algorithm that assigns free space to a dummy file, and
then uses the GETFSMAP and GETFSREFCOUNTS information to target file
space extents in order of decreasing share counts.  Once an extent has
been targeted, it uses reflinking to freeze the space, copies it
elsewhere, and uses FIDEDUPERANGE to remap existing file data until the
dummy file is the sole owner of the targetted space.  If metadata are
involved, the defrag utility invokes online repair to rebuild the
metadata somewhere else.

Since last year, I've also merged in Dave's xfs_spaceman code to move
inodes.  I've not had much time to play with it otherwise.

When the defragmenter finishes, all the free space has been isolated to
the dummy file, which can be unlinked and closed if defragmentation was
the goal; or it could be passed to a shrinkfs operation.

--D