[Bug 1800849] Re: [Ubuntu] kvm: fix deadlock when killed by oom

** Changed in: linux (Ubuntu Bionic)
Status: In Progress => Fix Committed

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Title:
[Ubuntu] kvm: fix deadlock when killed by oom

Status in Ubuntu on IBM z Systems:
Fix Committed
Status in linux package in Ubuntu:
Fix Released
Status in linux source package in Xenial:
Fix Released
Status in linux source package in Bionic:
Fix Committed
Status in linux source package in Cosmic:
Fix Released

Bug description:
== SRU Justification ==

Description: kvm: fix deadlock when killed by oom
Symptom: oom killer leaves processes in a deadlock state.
Problem: The problem arises in the rare cases in which
handle_mm_fault does not release the mm_sem.
Solution: Correct the issue by manually release the mm_sem when needed.

== Fix ==

306d6c49ac9ded11114cb53b0925da52f2c2ada1 ("s390/kvm: fix deadlock when
killed by oom")

== Patch ==

commit 306d6c49ac9ded11114cb53b0925da52f2c2ada1
Author: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Date: Mon Jul 16 10:38:57 2018 +0200

s390/kvm: fix deadlock when killed by oom

When the oom killer kills a userspace process in the page fault handler
while in guest context, the fault handler fails to release the mm_sem
if the FAULT_FLAG_RETRY_NOWAIT option is set. This leads to a deadlock
when tearing down the mm when the process terminates. This bug can only
happen when pfault is enabled, so only KVM clients are affected.

The problem arises in the rare cases in which handle_mm_fault does not
release the mm_sem. This patch fixes the issue by manually releasing
the mm_sem when needed.

Fixes: 24eb3a824c4f3 ("KVM: s390: Add FAULT_FLAG_RETRY_NOWAIT for guest fault")
Cc: <stable@vger.kernel.org> # 3.15+
Signed-off-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

== Regression Potential ==

Low and minimal, because:

- code change is s390x only
- limited to one single file: /arch/s390/mm/fault.c
- just two additional lines added (if stmt)
- Xenial and Cosmic already have this commit via upstream stable updates.
- Hence patch is just missing in Bionic.
- Test kernel was build for testting.

== Test Case ==

Create numerous KVM guests so that the host starts swapping
and memory becomes overcomitted and the oom killer is triggered.
__________

Description: kvm: fix deadlock when killed by oom

Symptom: oom killer leaves processes in a deadlock state.

Problem: The problem arises in the rare cases in which
                  handle_mm_fault does not release the mm_sem.

Solution: Correct the issue by manually relaese the mm_sem
                  when needed.

Reproduction: Create numerous KVM guests so that the host starts
                  swapping and memory becomes overcomitted and the oom
                  killer is triggered.

kernel 4.19
Upstream-ID: 306d6c49ac9ded11114cb53b0925da52f2c2ada1

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