RFR: 8287107: CgroupSubsystemFactory.setCgroupV2Path asserts with freezer controller [v2]

[Severin Gehwolf]

On Tue, 24 May 2022 16:15:03 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> This PR fixes a bug found on an Ubuntu host that's mostly running with cgroupv2, but there's a controller (freezer) that is mounted in cgroupv1 mode.
>> 
>> The container support code in the VM and JDK checks if we have simultaneously mounted v1 and v2 containers. If so, we revert to "hybrid" mode (which is essentially v1).
>> 
>> However, the "hybrid checks" only considers the following controllers that Java cares about:
>> 
>> - cpu
>> - cpuacct
>> - cpuset
>> - blkio
>> - memory
>> - pids
>> 
>> If only "freezer" is mounted in v1 mode, Java will start in V2 mode. Later, when `setCgroupV2Path()` sees the first line in /proc/self/cgroup, it runs into the assert.
>> 
>> 
>> $ cat /proc/self/cgroup
>> 1:freezer:/
>> 0::/user.slice/user-1001.slice/session-85.scope
>> 
>> 
>> The fix is simple -- when we get to `setCgroupV2Path()`, we have already decided that the system has not mounted any v1 controllers that we care about, so we should just ignore all the v1 controllers (which has a non-empty name).
>
> Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:
> 
>   @jerboaa comments

Looks good. Thanks for the thorough analysis.

-------------

Marked as reviewed by sgehwolf (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/8858