[BUG] mm/cgroupv2: memory.min may lead to an OOM error

[Lance Yang <ioworker0@gmail.com>]

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