[B][C][SRU][PATCH 0/1] z3fold: fix possible reclaim races

[Po-Hsu Lin]

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

== Justification ==
When using z3fold and zswap on a VM under overcommitted memory stress,
z3fold will complains about an "unknown buddy id 0" and fail to get a
pointer to the mapped allocation in z3fold_map().

 z3fold: unknown buddy id 0
 WARNING: CPU: 2 PID: 1584 at mm/z3fold.c:971 z3fold_zpool_map+0xce/0x100 [z3fold]

And it will leads to a null pointer dereference in zswap

 BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
 PGD 0 P4D 0
 Oops: 0000 [#1] SMP PTI
 CPU: 2 PID: 1584 Comm: stress Tainted: G        W         4.18.0-17-generic #18-Ubuntu
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.1-1ubuntu1 04/01/2014
 RIP: 0010:zswap_writeback_entry+0x4d/0x360

== Fix ==
ca0246bb (z3fold: fix possible reclaim races)

This patch has already in Disco, and can be cherry-picked into B/C.
Not needed for Xenial and older kernels as z3fold is not supported.

== Test ==
Test kernels for Bionic / Cosmic could be found here:
http://people.canonical.com/~phlin/kernel/lp-1814874-z3fold-zswap/Bionic/
http://people.canonical.com/~phlin/kernel/lp-1814874-z3fold-zswap/Cosmic/

This issue can be reproduced easily in a KVM with the following setup:
 * 8G disk, 4G RAM, 4 CPUs
 * 1G swap
 * "zswap.enabled=1 zswap.zpool=z3fold zswap.max_pool_percent=7" added to grub
 * "z3fold" module added into /etc/initramfs-tools/modules

Stress it with two childs running:
 * stress --vm-bytes 512M --vm 4 --vm-hang 3
 * stress --vm-bytes 512M --vm 4 --vm-hang 7

The VM is expected to crash within 5 minutes.

With the patched kernel, the VM can withstand this stress for over an
hour with crashing with this issue

== Regression potential ==
Small.

Fix limited to z3fold. User needs to enable it explicitly for this
feature.


Vitaly Wool (1):
  z3fold: fix possible reclaim races

 mm/z3fold.c | 101 +++++++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 62 insertions(+), 39 deletions(-)

-- 
2.7.4