I got new trace, attached as console-1566240256.588497319.log

Maybe this is new or just something not seen before.

After I have applied the patch, it seems issues with Call Traces
starting with "zswap_writeback_entry+0x50" are not happening any more.
Previously they were about 40% of issues.

[  590.980777] BUG: kernel NULL pointer dereference, address: 0000000000000008
[  590.985408] #PF: supervisor read access in kernel mode
[  590.989346] #PF: error_code(0x0000) - not-present page
[  590.993317] PGD 0 P4D 0
[  590.995272] Oops: 0000 [#1] SMP PTI
[  590.997963] CPU: 1 PID: 41 Comm: kworker/u8:1 Not tainted 5.3.0-rc5+ #71
[  591.003103] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009),
BIOS 1.12.0-2.fc30 04/01/2014
[  591.009620] Workqueue: zswap1 compact_page_work
[  591.013084] RIP: 0010:__list_add_valid+0x3/0x40
[  591.016570] Code: f4 ff ff ff e9 3a ff ff ff 49 c7 07 00 00 00 00
41 c7 47 08 00 00 00 00 e9 66 ff ff ff e8 d5 f4 b6 ff 90 90 90 90 90
49 89 d0 <48> 8b 52 08 48 39 f2 0f 85 7c 00 00 00 4c 8b 0a 4d 39 c1 0f
85 98
[  591.030935] RSP: 0000:ffffa6f1c0177df8 EFLAGS: 00010206
[  591.034940] RAX: 00000000000003c0 RBX: ffff8d046b5f7000 RCX: 8888888888888889
[  591.040425] RDX: 0000000000000000 RSI: ffffc6f1bf8012e8 RDI: ffff8d046b5f7000
[  591.045868] RBP: ffffc6f1bf8012e8 R08: 0000000000000000 R09: 0000000000000001
[  591.051305] R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
[  591.056762] R13: ffff8d0475f5ba00 R14: ffff8d0475f5ba08 R15: ffff8d046b5f7010
[  591.062213] FS:  0000000000000000(0000) GS:ffff8d047e600000(0000)
knlGS:0000000000000000
[  591.068390] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  591.072756] CR2: 0000000000000008 CR3: 0000000031906004 CR4: 0000000000160ee0
[  591.078217] Call Trace:
[  591.080093]  do_compact_page+0x31e/0x430
[  591.083124]  process_one_work+0x272/0x5a0
[  591.086183]  worker_thread+0x50/0x3b0
[  591.089003]  kthread+0x108/0x140
[  591.091488]  ? process_one_work+0x5a0/0x5a0
[  591.094695]  ? kthread_park+0x80/0x80
[  591.097510]  ret_from_fork+0x3a/0x50
[  591.100275] Modules linked in: ip6t_rpfilter ip6t_REJECT
nf_reject_ipv6 ipt_REJECT nf_reject_ipv4 xt_conntrack ip6table_nat
ip6table_mangle ip6table_raw ip6table_security iptable_nat nf_nat
iptable_mangle iptable_raw iptable_security nf_conntrack
nf_defrag_ipv6 nf_defrag_ipv4 libcrc32c ip_set nfnetlink
ip6table_filter ip6_tables iptable_filter ip_tables crct10dif_pclmul
crc32_pclmul ghash_clmulni_intel virtio_balloon virtio_net
net_failover intel_agp failover intel_gtt qxl drm_kms_helper
syscopyarea sysfillrect sysimgblt fb_sys_fops ttm drm crc32c_intel
serio_raw virtio_console virtio_blk agpgart qemu_fw_cfg
[  591.141313] CR2: 0000000000000008
[  591.143848] ---[ end trace adaa9348e173db62 ]---