I will fix and resend tomorrow.
Huge thanks, Sergey!

On Tue, Apr 10, 2018 at 09:41:09AM +0900, Sergey Senozhatsky wrote:
> 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

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