Hi all,
It's possible to encounter an OOM error if both parent and child cgroups are
configured such that memory.min and memory.max are set to the same values, as
is practice in Kubernetes.
Hmm... I'm not sure that whether this behavior is a bug or an expected aspect of
the kernel design.
To reproduce the bug, we can follow these command-based steps:
1. Check Kernel Version and OS release:
```
$ uname -r
6.10.0-rc5+
$ cat /etc/os-release
PRETTY_NAME="Ubuntu 24.04 LTS"
NAME="Ubuntu"
VERSION_ID="24.04"
VERSION="24.04 LTS (Noble Numbat)"
VERSION_CODENAME=noble
ID=ubuntu
ID_LIKE=debian
HOME_URL="<https://www.ubuntu.com/>"
SUPPORT_URL="<https://help.ubuntu.com/>"
BUG_REPORT_URL="<https://bugs.launchpad.net/ubuntu/>"
PRIVACY_POLICY_URL="<https://www.ubuntu.com/legal/terms-and-policies/privacy-policy>"
UBUNTU_CODENAME=noble
LOGO=ubuntu-logo
```
2. Navigate to the cgroup v2 filesystem, create a test cgroup, and set memory settings:
```
$ cd /sys/fs/cgroup/
$ stat -fc %T /sys/fs/cgroup
cgroup2fs
$ mkdir test
$ echo "+memory" > cgroup.subtree_control
$ mkdir test/test-child
$ echo 1073741824 > memory.max
$ echo 1073741824 > memory.min
$ cat memory.max
1073741824
$ cat memory.min
1073741824
$ cat memory.low
0
$ cat memory.high
max
```
3. Set up and check memory settings in the child cgroup:
```
$ cd test-child
$ echo 1073741824 > memory.max
$ echo 1073741824 > memory.min
$ cat memory.max
1073741824
$ cat memory.min
1073741824
$ cat memory.low
0
$ cat memory.high
max
```
4. Add process to the child cgroup and verify:
```
$ echo $$ > cgroup.procs
$ cat cgroup.procs
1131
1320
$ ps -ef|grep 1131
root 1131 1014 0 10:45 pts/0 00:00:00 -bash
root 1321 1131 99 11:06 pts/0 00:00:00 ps -ef
root 1322 1131 0 11:06 pts/0 00:00:00 grep --color=auto 1131
```
5. Attempt to create a large file using dd and observe the process being killed:
```
$ dd if=/dev/zero of=/tmp/2gbfile bs=10M count=200
Killed
```
6. Check kernel messages related to the OOM event:
```
$ dmesg
...
[ 1341.112388] oom-kill:constraint=CONSTRAINT_MEMCG,nodemask=(null),cpuset=/,mems_allowed=0,oom_memcg=/test,task_memcg=/test/test-child,task=dd,pid=1324,uid=0
[ 1341.112418] Memory cgroup out of memory: Killed process 1324 (dd) total-vm:15548kB, anon-rss:10240kB, file-rss:1764kB, shmem-rss:0kB, UID:0 pgtables:76kB oom_score_adj:0
```
7. Reduce the `memory.min` setting in the child cgroup and attempt the same large file creation, and then this issue is resolved.
```
# echo 107374182 > memory.min
# dd if=/dev/zero of=/tmp/2gbfile bs=10M count=200
200+0 records in
200+0 records out
2097152000 bytes (2.1 GB, 2.0 GiB) copied, 1.8713 s, 1.1 GB/s
```
Thanks,
Lance
