From: Hugh Dickins <hugh@veritas.com>

What purpose does loop_make_request's blk_queue_bounce serve?  None, it's
just a relic from before the kmaps were added to loop's transfers, and ties
up mempooled resources - in the file-backed case, with no guarantee they'll
soon be freed.  And what purpose does loop_set_fd's blk_queue_bounce_limit
serve?  None, blk_queue_make_request did that.