Can be reproduced easily with Cosmic kernel, patch works as expected. A
test kernel could be found here:
http://people.canonical.com/~phlin/kernel/lp-1814874-z3fold-
zswap/Cosmic/

Will try this on Bionic next.

** Also affects: linux (Ubuntu Cosmic)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1814874

Title:
  NULL pointer dereference when using z3fold and zswap

Status in Linux:
  Fix Released
Status in linux package in Ubuntu:
  In Progress
Status in linux source package in Cosmic:
  New

Bug description:
  Under memory pressure, my VM locks up. This has been reported upstream
  though I don't know how far any solution has progressed.

  https://bugzilla.kernel.org/show_bug.cgi?id=201603

  Feb  6 07:15:42 vps632258 kernel: [151336.450064] z3fold: unknown buddy id 0
  Feb  6 07:15:42 vps632258 kernel: [151336.454450] BUG: unable to handle 
kernel NULL pointer dereference at 0000000000000008

  The little bit of log I managed to salvage is attached.

  This has happened to two identical VMs. Unusually it has not occurred
  on a third VM which is configured the same but has less RAM (fingers
  crossed it won't).

  Irrelevant information:
  I thought the lock-ups were due to me using a BTRFS filesystem, however I 
swapped over to NILFS2 and this still occurs. The only difference seems to be 
that I am now able to grab some of the kernel output.

  ProblemType: Bug
  DistroRelease: Ubuntu 18.04
  Package: linux-image-4.18.0-14-generic 4.18.0-14.15~18.04.1
  ProcVersionSignature: Ubuntu 4.18.0-14.15~18.04.1-generic 4.18.20
  Uname: Linux 4.18.0-14-generic x86_64
  ApportVersion: 2.20.9-0ubuntu7.5
  Architecture: amd64
  Date: Wed Feb  6 10:55:05 2019
  ProcEnviron:
   TERM=xterm
   PATH=(custom, no user)
   XDG_RUNTIME_DIR=<set>
   LANG=en_GB.UTF-8
   SHELL=/bin/bash
  SourcePackage: linux-signed-hwe
  UpgradeStatus: No upgrade log present (probably fresh install)

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