Re: [bug] mkfs.ext[23] hangs on loop device (aarch64, 5.8+)

[Jan Stancek <jstancek@redhat.com>]

----- Original Message -----
> I saw this kind io hang in ltp/fs_fill test reliably and the loop is
> over image in tmpfs:
> 
> https://lkml.org/lkml/2020/7/26/77
> 
> And I have verified that the following patch can fix the issue:
> 
> https://lore.kernel.org/linux-block/bc5fa941-3b7c-f28e-dd46-1a1d6e5c40a8@kernel.dk/T/#t

Thanks, I'll test your patch with my setup.

In my case, I traced requests going up to blk_mq_sched_insert_requests(),
but they never made it to loop driver code (loop_queue_rq / lo_complete_rq),
so I assumed they are getting lost somewhere in mq scheduling.

After hang, there were always several requests stuck "inflight":
# cat /sys/kernel/debug/block/loop0/rqos/wbt/inflight
0: inflight 41
1: inflight 0
2: inflight 0

With some additional traces I could see requests being at dispatch list
and state == 0, which appears to fit description of problem you've seen:

  blk_mq_sched_insert_requests: blk_mq_sched_insert_requests hctx: ffff000168598000, ctx: fffffdffbff16dc0
  wbt_wait: wbt_wait rqos: ffff00016a5e1358, rqw: ffff00016a5e1388, bio: ffff0000da6bbd00, inflight: 41
  <wbt_wait goes to sleep>

crash> bio.bi_disk ffff0000da6bbd00
  bi_disk = 0xffff000168599800
crash> gendisk.disk_name 0xffff000168599800
  disk_name = "loop0\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000" 

crash> blk_mq_hw_ctx.queue 0xffff000168598000
  queue = 0xffff000117c06800
crash> request_queue.rq_qos 0xffff000117c06800
  rq_qos = 0xffff00016a5e1358

crash> blk_mq_hw_ctx.state 0xffff000168598000
    state = 0
crash> list blk_mq_hw_ctx.dispatch -h 0xffff000168598000 | wc -l
42

Regards,
Jan