This bug reporeted downstream for openSUSE: https://bugzilla.opensuse.org/show_bug.cgi?id=906684

I was running python script to check all my files fragmentation with FIEMAP ioctl. During that plasma-desktop hang. I killed it and restarted in other console session, but the new process hang to the "disk sleep" state few seconds after it started. I rebooted pc and noticed this in journalctl logs (the time matches, so this is likely the culprit of the plasma freeze):

Dec 02 08:49:46 linux-cc13.site kernel: ------------[ cut here ]------------
Dec 02 08:49:46 linux-cc13.site kernel: kernel BUG at ../fs/btrfs/extent_io.c:2875!
Dec 02 08:49:46 linux-cc13.site kernel: invalid opcode: 0000 [#1] PREEMPT SMP 
Dec 02 08:49:46 linux-cc13.site kernel: Modules linked in: fuse ctr ccm bnep bluetooth 6lowpan_iphc af_packet xt_pkttype xt_LOG xt_limit ip6t_REJECT xt_tcpudp nf
Dec 02 08:49:46 linux-cc13.site kernel:  processor dm_mod sr_mod cdrom ata_generic btrfs xor raid6_pq ata_piix i915 i2c_algo_bit drm_kms_helper drm video button 
Dec 02 08:49:46 linux-cc13.site kernel: CPU: 0 PID: 1416 Comm: plasma-desktop Not tainted 3.16.6-2-desktop #1
Dec 02 08:49:46 linux-cc13.site kernel: Hardware name: TOSHIBA Satellite L300/Base Board Product Name, BIOS 1.30 03/19/2008
Dec 02 08:49:46 linux-cc13.site kernel: task: ffff880057a0a050 ti: ffff880057a0c000 task.ti: ffff880057a0c000
Dec 02 08:49:46 linux-cc13.site kernel: RIP: 0010:[<ffffffffa0255068>]  [<ffffffffa0255068>] __do_readpage+0x7c8/0x7d0 [btrfs]
Dec 02 08:49:46 linux-cc13.site kernel: RSP: 0000:ffff880057a0f9b8  EFLAGS: 00010206
Dec 02 08:49:46 linux-cc13.site kernel: RAX: 0000000000660000 RBX: 0000000000001000 RCX: 000000000000001a
Dec 02 08:49:46 linux-cc13.site kernel: RDX: 0000000000660000 RSI: ffff8800206a9f50 RDI: ffff8800328a0890
Dec 02 08:49:46 linux-cc13.site kernel: RBP: ffff880057a0fb78 R08: ffff8800328a0890 R09: 000000000097f000
Dec 02 08:49:46 linux-cc13.site kernel: R10: ffff88001762d000 R11: 0000000000000002 R12: ffff880057a0fb78
Dec 02 08:49:46 linux-cc13.site kernel: R13: ffff880057a0fb78 R14: 0000000000000000 R15: 000000000097f000
Dec 02 08:49:46 linux-cc13.site kernel: FS:  00007faaf16ca780(0000) GS:ffff88007f200000(0000) knlGS:0000000000000000
Dec 02 08:49:46 linux-cc13.site kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
Dec 02 08:49:46 linux-cc13.site kernel: CR2: 00007faad028f008 CR3: 000000005797f000 CR4: 00000000000007f0
Dec 02 08:49:46 linux-cc13.site kernel: Stack:
Dec 02 08:49:46 linux-cc13.site kernel:  ffff88007e82f080 ffff880057a0fb68 ffffffffa0255760 ffff880000000000
Dec 02 08:49:46 linux-cc13.site kernel:  0000000000000000 0000000000000000 000000000097ffff ffff88005da4fc10
Dec 02 08:49:46 linux-cc13.site kernel:  0000000000000000 ffffea000156de88 0000000000001000 ffff88007e82f080
Dec 02 08:49:46 linux-cc13.site kernel: Call Trace:
Dec 02 08:49:46 linux-cc13.site kernel:  [<ffffffffa0255412>] __extent_readpages.constprop.42+0x2d2/0x2f0 [btrfs]
Dec 02 08:49:46 linux-cc13.site kernel:  [<ffffffffa02566f5>] extent_readpages+0x155/0x1a0 [btrfs]
Dec 02 08:49:46 linux-cc13.site kernel:  [<ffffffff8115613b>] __do_page_cache_readahead+0x1ab/0x230
Dec 02 08:49:46 linux-cc13.site kernel:  [<ffffffff8114cab0>] filemap_fault+0x360/0x400
Dec 02 08:49:46 linux-cc13.site kernel:  [<ffffffff81171afa>] __do_fault+0x3a/0xa0
Dec 02 08:49:46 linux-cc13.site kernel:  [<ffffffff81174c9f>] do_shared_fault.isra.54+0x2f/0x1d0
Dec 02 08:49:46 linux-cc13.site kernel:  [<ffffffff81175e34>] handle_mm_fault+0x484/0x1170
Dec 02 08:49:46 linux-cc13.site kernel:  [<ffffffff81048ce8>] __do_page_fault+0x158/0x530
Dec 02 08:49:46 linux-cc13.site kernel:  [<ffffffff816237d8>] page_fault+0x28/0x30
Dec 02 08:49:46 linux-cc13.site kernel:  [<00007faaeec9c6c9>] 0x7faaeec9c6c8
Dec 02 08:49:46 linux-cc13.site kernel: Code: e8 ae db ff ff e9 e2 fd ff ff e8 14 7f db e0 e9 08 f9 ff ff bb 00 10 00 00 31 f6 c7 84 24 b4 00 00 00 00 00 00 00 e
Dec 02 08:49:46 linux-cc13.site kernel: RIP  [<ffffffffa0255068>] __do_readpage+0x7c8/0x7d0 [btrfs]
Dec 02 08:49:46 linux-cc13.site kernel:  RSP <ffff880057a0f9b8>
Dec 02 08:49:46 linux-cc13.site kernel: ---[ end trace 23cbb3588713c314 ]---
Dec 02 08:49:46 linux-cc13.site kernel: ------------[ cut here ]------------