From: William Lee Irwin III <wli@holomorphy.com>

If one's goal is to free highmem pages, shrink_slab() is an ineffective
method of recovering them, as slab pages are all ZONE_NORMAL or ZONE_DMA.
Hence, this "FIXME: do not do for zone highmem".  Presumably this is a
question of policy, as highmem allocations may be satisfied by reaping slab
pages and handing them back; but the FIXME says what we should do.