libuv: uv_get_constrained_memory does not seem to work? (checked under cgroup2)

• Version: 1.29.1
• Platform: Linux yoga 5.1.5-arch1-2-ARCH #1 SMP PREEMPT Mon May 27 03:37:39 UTC 2019 x86_64 GNU/Linux

Running on Node.js master (libuv 1.29.1).

uv_get_constrained_memory() does not seem to work, it returns 0 when there was a limit set via cgroups.

To clarify — this does not work under unified cgroup hierarchy, i.e. when /sys/fs/cgroup has been mounted with type=cgroup2.

I do not know if this works under legacy or hybrid hierarchies (I have not tested those), but those two are deprecated and upstream is migrating off them. And they don’t allow controlling resource limits from user, that requires root access.

To test:

• Boot the system with systemd.unified_cgroup_hierarchy=1 option
• Run node (from user) with systemd-run --user --scope --property=MemoryMax=100M ./node
• Observe the return value of uv_get_constrained_memory(), which is 0. It shouldn’t be zero. E.g., Buffer.alloc(100e6).fill(0),0 fails.

Refs:

• https://github.com/libuv/libuv/pull/2289
• https://github.com/nodejs/node/pull/27508
• https://github.com/nodejs/node/pull/27805
• https://github.com/systemd/systemd/blob/master/docs/CGROUP_DELEGATION.md#three-different-tree-setups-
• https://fedoraproject.org/wiki/Changes/CGroupsV2

/cc @kjin @evanlucas