Comment 4 for bug 1812845

Spotted on X / D / mainline 5.0rc2 as well.

syslog for 5.0rc2:
[ 157.662397] ------------[ cut here ]------------
[ 157.662401] kernel BUG at mm/highmem.c:349!
[ 157.666725] invalid opcode: 0000 [#1] SMP PTI
[ 157.671086] CPU: 1 PID: 162 Comm: kworker/u16:4 Not tainted 5.0.0-050000rc2-generic #201901171452
[ 157.680017] Hardware name: Dell Inc. PowerEdge R310/05XKKK, BIOS 1.11.0 09/18/2012
[ 157.687684] Workqueue: btrfs-endio-raid56 btrfs_endio_raid56_helper [btrfs]
[ 157.694774] EIP: kunmap_high+0xb2/0xc0
[ 157.698661] Code: d4 8d b4 26 00 00 00 00 6a 00 b8 a0 d7 cf d9 b9 01 00 00 00 ba 03 00 00 00 e8 4a 79 ec ff 8b 5d fc 58 c9 c3 8d 74 26 00 0f 0b <0f> 0b 8d b4 26 00 00 00 00 8d 74 26 00 90 66 66 66 66 90 55 89 e5
[ 157.717962] EAX: 000001d5 EBX: fffff000 ECX: 00000001 EDX: 00000000
[ 157.724560] ESI: 00000004 EDI: 00000004 EBP: f45dfe5c ESP: f45dfe58
[ 157.730918] DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068 EFLAGS: 00010246
[ 157.737962] CR0: 80050033 CR2: 0225610c CR3: 19eca000 CR4: 000006f0
[ 157.744384] Call Trace:
[ 157.747009] kunmap+0x3e/0x50
[ 157.750192] finish_parity_scrub+0x315/0x570 [btrfs]
[ 157.755316] validate_rbio_for_parity_scrub+0xca/0xd0 [btrfs]
[ 157.761318] raid56_parity_scrub_end_io+0x53/0x70 [btrfs]
[ 157.766833] bio_endio+0xee/0x150
[ 157.770333] ? end_workqueue_fn+0x2c/0x40 [btrfs]
[ 157.775211] end_workqueue_fn+0x33/0x40 [btrfs]
[ 157.779799] normal_work_helper+0xc5/0x2e0 [btrfs]
[ 157.784868] btrfs_endio_raid56_helper+0x10/0x20 [btrfs]
[ 157.790313] process_one_work+0x1d4/0x3a0
[ 157.794479] worker_thread+0x37/0x400
[ 157.798310] kthread+0xf0/0x110
[ 157.801620] ? pwq_unbound_release_workfn+0xb0/0xb0
[ 157.806656] ? kthread_park+0x90/0x90
[ 157.810478] ret_from_fork+0x2e/0x38
[ 157.814220] Modules linked in: intel_powerclamp coretemp kvm_intel dcdbas kvm ipmi_ssif irqbypass joydev input_leds ipmi_si intel_cstate acpi_power_meter mac_hid ipmi_devintf i7core_edac ipmi_msghandler sch_fq_codel ib_iser rdma_cm iw_cm ib_cm ib_core iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi ip_tables x_tables autofs4 btrfs zstd_compress raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c raid1 raid0 multipath linear hid_generic gpio_ich mgag200 i2c_algo_bit ttm drm_kms_helper syscopyarea usbhid sysfillrect hid sysimgblt mpt3sas fb_sys_fops drm pata_acpi raid_class lpc_ich bnx2 scsi_transport_sas wmi
[ 157.873824] ---[ end trace f9b1fc7e1f35ab27 ]---
[ 157.887583] EIP: kunmap_high+0xb2/0xc0
[ 157.891437] Code: d4 8d b4 26 00 00 00 00 6a 00 b8 a0 d7 cf d9 b9 01 00 00 00 ba 03 00 00 00 e8 4a 79 ec ff 8b 5d fc 58 c9 c3 8d 74 26 00 0f 0b <0f> 0b 8d b4 26 00 00 00 00 8d 74 26 00 90 66 66 66 66 90 55 89 e5
[ 157.910746] EAX: 000001d5 EBX: fffff000 ECX: 00000001 EDX: 00000000
[ 157.917276] ESI: 00000004 EDI: 00000004 EBP: f45dfe5c ESP: d9ed1e5c
[ 157.923699] DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068 EFLAGS: 00010246
[ 157.930744] CR0: 80050033 CR2: 0225610c CR3: 19eca000 CR4: 000006f0