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

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

Hi all,

This fourth patch deluge 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.

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.

NOTE: There's also an experimental vectorization interface for scrub.
Given how long it's likely to take to get to this fourth deluge, it
might make more sense to integrate with io_uring when that day comes.

As a warning, the patches will likely take several days to trickle in.

--D