Unable to read memory stats file at /sys/fs/cgroup/memory.current

[alekseyastolyarov]

Title: Unable to read memory stats file at /sys/fs/cgroup/memory.current

Description:
Envoy fail to read cgroups v1 and v2 presumably because of using readlink() for a regular file.
If I remount cgoups to a temporary directory and create symlink for /sys/fs/cgroup/memory.current and /sys/fs/cgroup/memory.max envoy starts reading that files and works without errors.

Repro steps:
take envoy of any version from v1.35.6 to v1.36.2 (latest at the moment) and apply this config in the static (aka bootstrap) configuration

overload_manager:
  refresh_interval: 0.25s
  resource_monitors:
    - name: "envoy.resource_monitors.cgroup_memory"
      typed_config:
        "@type": type.googleapis.com/envoy.extensions.resource_monitors.cgroup_memory.v3.CgroupMemoryConfig
        # max_memory_bytes: 1073741824  # 1GB
  actions:
    - name: "envoy.overload_actions.stop_accepting_requests"
      triggers:
        - name: "envoy.resource_monitors.cgroup_memory"
          threshold:
            value: 0.95  # Trigger at 95% memory utilization#
    - name: "envoy.overload_actions.disable_http_keepalive"
      triggers:
        - name: "envoy.resource_monitors.cgroup_memory"
          threshold:
            value: 0.92

Envloy logs:
"Failed to update resource envoy.resource_monitors.cgroup_memory: Unable to read memory stats file at /sys/fs/cgroup/memory.max"

What I see at the moment errors happen:

# strace -f -e trace=open,openat,read,write,stat,fstat,lstat,readlink -p 14

readlink("/sys", 0x7ffdeb836810, 1023) = -1 EINVAL (Invalid argument) [pid 14]
readlink("/sys/fs", 0x7ffdeb836810, 1023) = -1 EINVAL (Invalid argument) [pid 14]
readlink("/sys/fs/cgroup", 0x7ffdeb836810, 1023) = -1 EINVAL (Invalid argument) [pid 14]
readlink("/sys/fs/cgroup/memory.current", 0x7ffdeb836810, 1023) = -1 EINVAL (Invalid argument) [pid 14]
write(2, "{"@timestamp": "2025-11-11T11:16"..., 351) = 351 [pid 14]

according to readlink() documentation it will fail if try to open a non symlink file "EINVAL - The named file (i.e., the final filename component of path) is not a symbolic link."

what I see after replacing regular files with symlinks:

[pid    14] readlink("/sys", 0x7ffd5bdcca30, 1023) = -1 EINVAL (Invalid argument)
[pid    14] readlink("/sys/fs", 0x7ffd5bdcca30, 1023) = -1 EINVAL (Invalid argument)
[pid    14] readlink("/sys/fs/cgroup", 0x7ffd5bdcca30, 1023) = -1 EINVAL (Invalid argument)
[pid    14] readlink("/sys/fs/cgroup/memory.current", "/mnt/cgroups/memory.current", 1023) = 27
[pid    14] readlink("/mnt", 0x7ffd5bdcca30, 1023) = -1 EINVAL (Invalid argument)
[pid    14] readlink("/mnt/cgroups", 0x7ffd5bdcca30, 1023) = -1 EINVAL (Invalid argument)
[pid    14] readlink("/mnt/cgroups/memory.current", 0x7ffd5bdcca30, 1023) = -1 EINVAL (Invalid argument)

[pid    14] openat(AT_FDCWD, "/sys/fs/cgroup/memory.current", O_RDONLY) = 29
[pid    14] read(29, "97636352\n", 4096) = 9
[pid    14] read(29, "", 4096)          = 0


[pid    14] readlink("/sys", 0x7ffd5bdcca30, 1023) = -1 EINVAL (Invalid argument)
[pid    14] readlink("/sys/fs", 0x7ffd5bdcca30, 1023) = -1 EINVAL (Invalid argument)
[pid    14] readlink("/sys/fs/cgroup", 0x7ffd5bdcca30, 1023) = -1 EINVAL (Invalid argument)
[pid    14] readlink("/sys/fs/cgroup/memory.max", "/mnt/cgroups/memory.max", 1023) = 23
[pid    14] readlink("/mnt", 0x7ffd5bdcca30, 1023) = -1 EINVAL (Invalid argument)
[pid    14] readlink("/mnt/cgroups", 0x7ffd5bdcca30, 1023) = -1 EINVAL (Invalid argument)
[pid    14] readlink("/mnt/cgroups/memory.max", 0x7ffd5bdcca30, 1023) = -1 EINVAL (Invalid argument)

[pid    14] openat(AT_FDCWD, "/sys/fs/cgroup/memory.max", O_RDONLY) = 29
[pid    14] read(29, "max\n", 4096)     = 4
[pid    14] read(29, "", 4096)          = 0

I.e. finely Envoy reads cgroup files. No errors apear in the logs

[adisuissa]

cc code-owners @botengyao @KBaichoo @alesabater