As I checked mongoDB gained support for memory constraints over cgroup in linux with
SERVER-16571. However it seems that it doesn't have the ability to check memory limit if host is using cgroups v2 resulting hostInfo.system.memLimitMB to be equal to system memory in a containerized environment.
Current code to read memory limits in src/mongo/util/processinfo_linux.cpp
But in cgroups v2 the memory limit is at /sys/fs/cgroup/memory.max.
I think engine should check if the cgroups v2 is enabled on the host and then read the limits according to that.
Tested on mongodb 4.2.6 in docker containers at host Fedore CoreOS 34.20210904.3.0. But I do believe newer versions have the same since source code seems same since first implementation.