ll_rw_block/submit_bh and request limits

[Marcelo Tosatti]

Hi, 

The following piece of code in ll_rw_block() aims to limit the number of
locked buffers by making processes throttle on IO if the number of on
flight requests is bigger than a high watermaker. IO will only start
again if we're under a low watermark.

                if (atomic_read(&queued_sectors) >= high_queued_sectors) {
                        run_task_queue(&tq_disk);
                        wait_event(blk_buffers_wait,
                        	atomic_read(&queued_sectors) < low_queued_sectors);
                }

However, if submit_bh() is used to queue IO (which is used by ->readpage()
for ext2, for example), no throttling happens.

It looks like ll_rw_block() users (writes, metadata reads) can be starved
by submit_bh() (data reads). 

If I'm not missing something, the watermark check should be moved to
submit_bh().