Description of problem:

Current implementation of map function in dm-crypt waits for
earlies items to complete to solve insufficient memory situations.
/* out of memory -> run queues */
        if (remaining)
                blk_congestion_wait(bio_data_dir(clone), HZ/100);

This will not work after reduce stack changes in generic_make_request()
which will not process IOs immediately.
(md-dm-reduce-stack-usage-with-stacked-block-devices.patch)

Processing of map request and most of its workload should be moved
to dm-crypt workqueue.