Using LXCFS together with Podman

JP was puzzled that using podman run --memory=2G … would not result in the 2G limit being visible inside the container. While we were able to identify this as a visualization problem — tools like free(1) only look at /proc/meminfo and that is not virtualized inside a container, you'd have to look at /sys/fs/cgroup/memory.max and friends instead — I couldn't leave it at that. And then I remembered there is actually something that can provide a virtual (cgroup-aware) /proc for containers: LXCFS!

But does it work with Podman?! I always used it with LXC, but there is technically no reason why it wouldn't work with a different container solution — cgroups are cgroups after all.

As we all know: there is only one way to find out!

Take a fresh Debian 12 VM, install podman and verify things behave as expected:

user@debian12:~$ podman run -ti --rm --memory=2G centos:stream9
bash-5.1# grep MemTotal /proc/meminfo
MemTotal:        6067396 kB
bash-5.1# cat /sys/fs/cgroup/memory.max
2147483648

And after installing (and starting) lxcfs, we can use the virtual /proc/meminfo it generates by bind-mounting it into the container (LXC does that part automatically for us):

user@debian12:~$ podman run -ti --rm --memory=2G --mount=type=bind,source=/var/lib/lxcfs/proc/meminfo,destination=/proc/meminfo centos:stream9
bash-5.1# grep MemTotal /proc/meminfo
MemTotal:        2097152 kB
bash-5.1# cat /sys/fs/cgroup/memory.max
2147483648

The same of course works with all the other proc entries lxcfs provides (cpuinfo, diskstats, loadavg, meminfo, slabinfo, stat, swaps, and uptime here), just bind-mount them.

And yes, free(1) now works too!

bash-5.1# free -m
               total        used        free      shared  buff/cache   available
Mem:            2048           3        1976           0          67        2044
Swap:              0           0           0

Just don't blindly mount the whole /var/lib/lxcfs/proc over the container's /proc. It did work (as in: "bash and free didn't crash") for me, but with /proc/$PID etc missing, I bet things will go south pretty quickly.