resize live /tmp tmpfs on unprivileged container [SOLVED]

[mathx]

I have a job running on a node that i've modelled elsewhere too. To speed it up, I put it in /tmp, sync back to disk when done. However I realize through modelling that when the job finishes it will flush a bunch of data to disk into /tmp and run out of space and lose all the work.

In a privileged container I can just mount -o remount,size=xxG /tmp no problem. But in an unprivleged container I get

mount: /tmp: fsconfig() failed: tmpfs: Invalid uid '100000'.

How can I resize the /tmpfs without this error? I tried a few things but got errors:

#pct exec 131 -- mount -o remount,size=34G /tmp
mount: /tmp: fsconfig() failed: tmpfs: Invalid uid '100000'.

and

#PID=$(lxc-info -n 131 -p -H)
#nsenter --mount=/proc/$PID/ns/mnt mount -o remount,size=34G /tmp
mount: /tmp: must be superuser to use mount.
       dmesg(1) may have more information after failed mount system call.

The following actually worked, gets around all the UID remapping for root (uid=0) errors, so keeping for posterity here and for others:

Create this python script:

# Config
CT_ID = "131"  # Replace with your Container ID
NEW_SIZE = "size=34G"
TARGET = "/tmp"
 
# 1. Get PID
try:
    with os.popen(f"lxc-info -n {CT_ID} -p -H") as f:
        pid = f.read().strip()
except Exception as e:
    print(f"Error finding PID: {e}")
    exit(1)
 
# 2. Open the mount namespace file
fd = os.open(f"/proc/{pid}/ns/mnt", os.O_RDONLY)
 
# 3. Join the namespace (CLONE_NEWNS = 0x00020000)
libc = ctypes.CDLL(ctypes.util.find_library('c'), use_errno=True)
if libc.setns(fd, 0x00020000) != 0:
    e = ctypes.get_errno()
    print(f"setns failed: {os.strerror(e)}")
    exit(1)
 
# 4. Perform the remount syscall directly
# MS_REMOUNT = 32
ret = libc.mount(None, TARGET.encode(), None, 32, NEW_SIZE.encode())
 
if ret != 0:
    e = ctypes.get_errno()
    print(f"Mount syscall failed: {os.strerror(e)}")
else:
    print(f"Successfully resized /tmp to {NEW_SIZE}")

put that in a file resize.py and run it with python3 resize.py

Is there a more official way without this code?