[dm-devel] DM brokeness with NOWAIT

[Jens Axboe]

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.

-- 
Jens Axboe