[dm-devel] DM brokeness with NOWAIT

[Jens Axboe]

On 9/15/23 12:54 PM, Mike Snitzer wrote:
> On Fri, Sep 15 2023 at 12:14P -0400,
> Jens Axboe <axboe at kernel.dk> wrote:
> 
>> On 9/15/23 10:04 AM, Jens Axboe wrote:
>>> Hi,
>>>
>>> Threw some db traffic into my testing mix, and that ended in tears
>>> very quickly:
>>>
>>> CPU: 7 PID: 49609 Comm: ringbuf-read.t Tainted: G        W          6.6.0-rc1-g39956d2dcd81 #129
>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
>>> Call Trace:
>>>  <TASK>
>>>  dump_stack_lvl+0x11d/0x1b0
>>>  __might_resched+0x3c3/0x5e0
>>>  ? preempt_count_sub+0x150/0x150
>>>  mempool_alloc+0x1e2/0x390
>>>  ? sanity_check_pinned_pages+0x23/0x1010
>>>  ? mempool_resize+0x7d0/0x7d0
>>>  bio_alloc_bioset+0x417/0x8c0
>>>  ? bvec_alloc+0x200/0x200
>>>  ? __gup_device_huge+0x900/0x900
>>>  bio_alloc_clone+0x53/0x100
>>>  dm_submit_bio+0x27f/0x1a20
>>>  ? lock_release+0x4b7/0x670
>>>  ? pin_user_pages_fast+0xb6/0xf0
>>>  ? blk_try_enter_queue+0x1a0/0x4d0
>>>  ? dm_dax_direct_access+0x260/0x260
>>>  ? rcu_is_watching+0x12/0xb0
>>>  ? blk_try_enter_queue+0x1cc/0x4d0
>>>  __submit_bio+0x239/0x310
>>>  ? __bio_queue_enter+0x700/0x700
>>>  ? kvm_clock_get_cycles+0x40/0x60
>>>  ? ktime_get+0x285/0x470
>>>  submit_bio_noacct_nocheck+0x4d9/0xb80
>>>  ? should_fail_request+0x80/0x80
>>>  ? preempt_count_sub+0x150/0x150
>>>  ? folio_flags+0x6c/0x1e0
>>>  submit_bio_noacct+0x53e/0x1b30
>>>  blkdev_direct_IO.part.0+0x833/0x1810
>>>  ? rcu_is_watching+0x12/0xb0
>>>  ? lock_release+0x4b7/0x670
>>>  ? blkdev_read_iter+0x40d/0x530
>>>  ? reacquire_held_locks+0x4e0/0x4e0
>>>  ? __blkdev_direct_IO_simple+0x780/0x780
>>>  ? rcu_is_watching+0x12/0xb0
>>>  ? __mark_inode_dirty+0x297/0xd50
>>>  ? preempt_count_add+0x72/0x140
>>>  blkdev_read_iter+0x2a4/0x530
>>>  ? blkdev_write_iter+0xc40/0xc40
>>>  io_read+0x369/0x1490
>>>  ? rcu_is_watching+0x12/0xb0
>>>  ? io_writev_prep_async+0x260/0x260
>>>  ? __fget_files+0x279/0x410
>>>  ? rcu_is_watching+0x12/0xb0
>>>  io_issue_sqe+0x18a/0xd90
>>>  io_submit_sqes+0x970/0x1ed0
>>>  __do_sys_io_uring_enter+0x14d4/0x2650
>>>  ? io_submit_sqes+0x1ed0/0x1ed0
>>>  ? rcu_is_watching+0x12/0xb0
>>>  ? __do_sys_io_uring_register+0x3f6/0x2190
>>>  ? io_req_caches_free+0x500/0x500
>>>  ? ksys_mmap_pgoff+0x85/0x5b0
>>>  ? rcu_is_watching+0x12/0xb0
>>>  ? trace_irq_enable.constprop.0+0xd0/0x100
>>>  do_syscall_64+0x39/0xb0
>>>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>>>
>>> which seems to demonstrate a misunderstanding on what REQ_NOWAIT is
>>> about. In particulary, it seems to assume you can then submit with
>>> atomic context? DM does an rcu_read_lock() and happily proceeds to
>>> attempt to submit IO under RCU being disabled.
>>
>> Did a quick check to see where this came from, and it got added with:
>>
>> commit 563a225c9fd207326c2a2af9d59b4097cb31ce70
>> Author: Mike Snitzer <snitzer at kernel.org>
>> Date:   Sat Mar 26 21:08:36 2022 -0400
>>
>>     dm: introduce dm_{get,put}_live_table_bio called from dm_submit_bio
>>
>> which conspiciously doesn't include any numbers on why this is necessary
>> or a good thing, and notably probably wasn't tested? This landed in 5.19
>> fwiw.
> 
> Don't recall what I was thinking, and I clearly didn't properly test
> either... should've consulted Mikulas.  Sorry for the trouble.
> 
> Would you like to send a formal patch with your Signed-off-by and I'll
> mark it for stable@ and get it to Linus?

Sure, I can do that.

-- 
Jens Axboe