I've just realised I made a mistake when backporting the code. 2.6.32 and above (which the 'official' BFQ v1 was written for, when the site moved some time in 2009) calls __blk_run_queue() instead of blk_start_queueing(). As __blk_run_queue exists in 2.6.28, I assumed this was an optimization - but it turns out that in fact, blk_start_queueing was depreciated some time between .28 and .32, with its sanity checks added to __blk_run_queue; unfortunately, the .28 version (of __blk_run_queue) has almost no such checks at all. I'm just recompiling again, having changed the relevant lines back to blk_start_queueing, and will be testing in due course..