I think zram_debugfs_unregister() is called both from zram_remove() and
from destroy_devices(), which explodes:
[ 1906.467981] BUG: unable to handle kernel NULL pointer dereference at
00000000000000b8
[ 1906.468007] PGD 80000003d5050067 P4D 80000003d5050067 PUD 3fa97d067 PMD 0
[ 1906.468029] Oops: 0002 [#1] PREEMPT SMP PTI
[ 1906.468035] Modules linked in: zram(-) zsmalloc mousedev hid_generic
rndis_host usbhid cdc_ether usbnet hid snd_hda_codec_realtek
snd_hda_codec_generic snd_hda_intel snd_hda_codec snd_hda_core coretemp snd_pcm
hwmon snd_timer snd r8169 i2c_i801 soundcore mii xhci_pci xhci_hcd usbcore
sd_mod usb_common
[ 1906.468069] RIP: 0010:down_write+0x29/0x46
[ 1906.468073] RSP: 0018:ffffb9dd0480fdd0 EFLAGS: 00010246
[ 1906.468077] RAX: 00000000000000b8 RBX: 00000000000000b8 RCX: 0000000000000001
[ 1906.468082] RDX: ffffffff00000001 RSI: 0000000000000045 RDI: ffffffffb1baa40a
[ 1906.468086] RBP: ffffa189fce9fbd8 R08: ffffb9dd0480fddc R09: 0000000000000007
[ 1906.468091] R10: ffffb9dd0480fe40 R11: 0000000000000000 R12: ffffa189fce9fbd8
[ 1906.468095] R13: ffffffffb23cab54 R14: 0000000000000000 R15: 0000000000000000
[ 1906.468100] FS: 00007fbc72cd2b80(0000) GS:ffffa18a1ed00000(0000)
knlGS:0000000000000000
[ 1906.468105] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1906.468110] CR2: 00000000000000b8 CR3: 00000003b4570003 CR4: 00000000001606e0
[ 1906.468114] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 1906.468119] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 1906.468123] Call Trace:
[ 1906.468130] debugfs_remove_recursive+0x40/0x15e
[ 1906.468136] zram_remove+0x7f/0xf6 [zram]
[ 1906.468141] ? hot_remove_store+0x93/0x93 [zram]
[ 1906.468146] zram_remove_cb+0xd/0x10 [zram]
[ 1906.468150] idr_for_each+0x50/0xc7
[ 1906.468155] destroy_devices+0x32/0x5c [zram]
[ 1906.468160] SyS_delete_module+0xfd/0x19a
[ 1906.468165] do_syscall_64+0x17e/0x195
[ 1906.468169] entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[ 1906.468174] RIP: 0033:0x7fbc723cb5d7
[ 1906.468177] RSP: 002b:00007ffc40316d48 EFLAGS: 00000206 ORIG_RAX:
00000000000000b0
[ 1906.468182] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fbc723cb5d7
[ 1906.468187] RDX: 000000000000000a RSI: 0000000000000800 RDI: 00000000013e7778
[ 1906.468191] RBP: 00000000013e7710 R08: 00007ffc40315cc1 R09: 0000000000000000
[ 1906.468196] R10: 00000000000008b2 R11: 0000000000000206 R12: 00007ffc403188b4
[ 1906.468201] R13: 0000000000000000 R14: 00000000013e7710 R15: 00000000013e7260
[ 1906.468761] Code: ff ff 0f 1f 44 00 00 53 31 d2 48 89 fb be 45 00 00 00 48
c7 c7 0a a4 ba b1 e8 ef bf b9 ff 48 89 d8 48 ba 01 00 00 00 ff ff ff ff <f0> 48
0f c1 10 85 d2 74 05 e8 9e bd ff ff 65 48 8b 04 25 80 4d
[ 1906.469296] RIP: down_write+0x29/0x46 RSP: ffffb9dd0480fdd0
[ 1906.469995] CR2: 00000000000000b8
[ 1906.475689] ---[ end trace 10d42324537dfb4c ]---
-ss
