Panic/lockup in z3fold_zpool_free

[Oleksandr Natalenko]

Hello.

Since 5.19 series, zswap went unstable for me under memory pressure, and 
occasionally I get the following:

```
watchdog: BUG: soft lockup - CPU#0 stuck for 10195s! [mariadbd:478]
Modules linked in: netconsole joydev mousedev intel_agp psmouse pcspkr 
intel_gtt cfg80211 cirrus i2c_piix4 tun rfkill mac_hid nft_ct tcp_bbr2 
nft_chain_nat nf_tables nfnetlink nf_nat nf_conntrack nf_defrag_ipv6 
nf_defrag_ipv4 fuse qemu_fw_cfg ip_tables x_tables xfs libcrc32c 
crc32c_generic dm_crypt cbc encrypted_keys trusted asn1_encoder tee tpm 
rng_core dm_mod crct10dif_pclmul crc32_pclmul crc32c_intel 
ghash_clmulni_intel virtio_net aesni_intel serio_raw net_failover 
ata_generic virtio_balloon failover pata_acpi crypto_simd virtio_blk 
atkbd libps2 vivaldi_fmap virtio_pci cryptd virtio_pci_legacy_dev 
ata_piix virtio_pci_modern_dev i8042 floppy serio usbhid
Unloaded tainted modules: intel_cstate():1 intel_uncore():1 
pcc_cpufreq():1 acpi_cpufreq():1
CPU: 0 PID: 478 Comm: mariadbd Tainted: G             L    5.19.0-pf5 #1 
12baccda8e49539e158b9dd97cbda6c7317d73af
Hardware name: Red Hat KVM, BIOS 1.11.0-2.el7 04/01/2014
RIP: 0010:z3fold_zpool_free+0x4c/0x5e0
Code: 7c 24 08 48 89 04 24 0f 85 e0 00 00 00 48 89 f5 41 bd 00 00 00 80 
48 83 e5 c0 48 83 c5 28 eb 0a 48 89 df e8 b6 8d 9f 00 f3 90 <48> 89 ef 
e8 bc 8b 9f 00 4d 8b 34 24 49 81 e6 00 f0 ff ff 49 8d 5e
RSP: 0000:ffffbeadc0e87b68 EFLAGS: 00000202
RAX: 0000000000000030 RBX: ffff99ac73d2c010 RCX: ffff99ac4e4ba380
RDX: 0000665340000000 RSI: ffffe3b540000000 RDI: ffff99ac73d2c010
RBP: ffff99ac55ef3a68 R08: ffff99ac422f0bf0 R09: 000000000000c60b
R10: ffffffffffffffc0 R11: 0000000000000000 R12: ffff99ac55ef3a50
R13: 0000000080000000 R14: ffff99ac73d2c000 R15: ffff99acf3d2c000
FS:  00007f587fcd66c0(0000) GS:ffff99ac7ec00000(0000) 
knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f587ce8bec8 CR3: 0000000005b48006 CR4: 00000000000206f0
Call Trace:
 <TASK>
 zswap_free_entry+0xb5/0x110
 zswap_frontswap_invalidate_page+0x72/0xa0
 __frontswap_invalidate_page+0x3a/0x60
 swap_range_free+0xb5/0xd0
 swapcache_free_entries+0x16e/0x2e0
 free_swap_slot+0xb4/0xc0
 put_swap_page+0x259/0x420
 delete_from_swap_cache+0x63/0xb0
 try_to_free_swap+0x1b5/0x2a0
 do_swap_page+0x24c/0xb80
 __handle_mm_fault+0xa59/0xf70
 handle_mm_fault+0x100/0x2f0
 do_user_addr_fault+0x1c7/0x6a0
 exc_page_fault+0x74/0x170
 asm_exc_page_fault+0x26/0x30
RIP: 0033:0x556e96280428
Code: a0 03 00 00 67 e8 28 64 ff ff 48 8b 83 b0 00 00 00 48 8b 0d da 18 
72 00 48 8b 10 66 48 0f 6e c1 48 85 d2 74 27 0f 1f 44 00 00 <48> c7 82 
98 00 00 00 00 00 00 00 48 8b 10 48 83 c0 08 f2 0f 11 82
RSP: 002b:00007f587fcd3980 EFLAGS: 00010206
RAX: 00007f587d028468 RBX: 00007f587cb1a818 RCX: 3ff0000000000000
RDX: 00007f587ce8be30 RSI: 0000000000000000 RDI: 00007f587cedd030
RBP: 00007f587fcd39c0 R08: 0000000000000016 R09: 0000000000000000
R10: 0000000000000008 R11: 0000556e970961a0 R12: 00007f587d1f17b8
R13: 00007f5883595598 R14: 00007f587d1f17a8 R15: 00007f587cb1a928
 </TASK>
```

This happens on the latest v5.19.10 kernel as well.

Sometimes it's not a soft lockup but GPF, although the stack trace is 
the same. So, to me it looks like a memory corruption, UAF, double free 
or something like that.

Have you got any idea regarding what's going on?

Thanks.

--
  Oleksandr Natalenko (post-factum)