New warning in nvme_setup_discard

[Oleksandr Natalenko]

Hello.

After a v5.13.2 massive update I encountered this:

```
[19231.556665] ------------[ cut here ]------------
[19231.556674] WARNING: CPU: 20 PID: 502 at drivers/nvme/host/core.c:850 
nvme_setup_discard+0x188/0x1f0
...
[19231.556776] CPU: 20 PID: 502 Comm: kworker/20:1H Not tainted 5.13.2 #1
[19231.556780] Hardware name: ASUS System Product Name/Pro WS X570-ACE, BIOS 
3601 05/26/2021
[19231.556784] Workqueue: kblockd blk_mq_run_work_fn
[19231.556789] RIP: 0010:nvme_setup_discard+0x188/0x1f0
[19231.556794] Code: 49 8b 44 24 10 4c 8b 90 40 0b 00 00 4c 2b 15 8f 09 d8 00 
49 c1 fa 06 49 c1 e2 0c 4c 03 15 90 09 d8 00 4d 89 d0 e9 b9 fe ff ff <0f> 0b b8 
00 00 00 80 49 01 c2 72 52 48 c7 c0 00 00 00 80 48 2b 05
[19231.556798] RSP: 0018:ffffaed2416efc00 EFLAGS: 00010287
[19231.556802] RAX: ffff8e67fb580000 RBX: ffff8e640bbe5240 RCX: 0000000000000020
[19231.556805] RDX: ffff8e67fb580000 RSI: 000000000000001f RDI: 0000000000000000
[19231.556808] RBP: ffff8e640bbe5388 R08: ffff8e677b580000 R09: 0000000008abb800
[19231.556811] R10: ffff8e677b580000 R11: 0000000000000400 R12: ffff8e6405999c00
[19231.556814] R13: 000000000000001f R14: ffff8e6405693000 R15: ffff8e640bbaf800
[19231.556816] FS:  0000000000000000(0000) GS:ffff8e6b0ef00000(0000) knlGS:
0000000000000000
[19231.556819] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[19231.556822] CR2: ffff888c76ece000 CR3: 000000047a184000 CR4: 0000000000350ee0
[19231.556825] Call Trace:
[19231.556830]  nvme_setup_cmd+0x2d0/0x670
[19231.556834]  nvme_queue_rq+0x79/0xc90
[19231.556837]  ? __sbitmap_get_word+0x30/0x80
[19231.556842]  ? sbitmap_get+0x85/0x180
[19231.556846]  blk_mq_dispatch_rq_list+0x15c/0x810
[19231.556851]  ? list_sort+0x21d/0x2f0
[19231.556856]  __blk_mq_do_dispatch_sched+0x196/0x320
[19231.556860]  __blk_mq_sched_dispatch_requests+0x14d/0x190
[19231.556864]  blk_mq_sched_dispatch_requests+0x2f/0x60
[19231.556867]  blk_mq_run_work_fn+0x43/0xc0
[19231.556871]  process_one_work+0x24e/0x430
[19231.556876]  worker_thread+0x54/0x4d0
[19231.556880]  ? process_one_work+0x430/0x430
[19231.556883]  kthread+0x1b3/0x1e0
[19231.556886]  ? __kthread_init_worker+0x50/0x50
[19231.556889]  ret_from_fork+0x22/0x30
[19231.556895] ---[ end trace d9abdf019a56b4c7 ]---
[19231.556906] blk_update_request: I/O error, dev nvme1n1, sector 632935424 op 
0x3:(DISCARD) flags 0x0 phys_seg 31 prio class 0
```

or, in code:

```
 850     if (WARN_ON_ONCE(n != segments)) {
 851         if (virt_to_page(range) == ns->ctrl->discard_page)
 852             clear_bit_unlock(0, &ns->ctrl->discard_page_busy);
 853         else
 854             kfree(range);
 855         return BLK_STS_IOERR;
 856     }
```

BFQ scheduler is in use.

Something similar was already fixed by a958937ff166fc60d1c3a721036f6ff41bfa2821, 
but I do not have a multipath device here, it's just 2 NVMe SSDs in a soft 
RAID10 with LUKS and LVM.

Any idea what this might mean? v5.13.2 brought some commit into a stable tree 
that are, as I still suspect, causing unreproducible panics [1] [2]. 
Previously, I dropped that extra stuff from my kernel build and had no issues. 
This time I also do not have any extra commits in the block layer, only those 
that are in v5.13.2.

Thanks.

[1] https://lore.kernel.org/linux-block/3533087.dJKXTdksHR@spock/
[2] https://lore.kernel.org/linux-block/2957867.CS06ZTPI5V@spock/

-- 
Oleksandr Natalenko (post-factum)