--- title: Inotify watcher leaks in Kubelet date: 2024-04-18 16:35 -0400 description: cover: '/static/2024-04/kubelet_inotify_leak_logo.png' category: tags: ["Kubernetes", "Kubelet", "Debug", "Inotify"] published: true sitemap: true permalink: author: Pengzhan Hao --- ## Symptom Recently, I faced an issue where Kubelet on a node reported error message failed to create file descriptors. ```bash error creating file watcher: too many open files error creating file watcher: no space left on device ``` After short checking, I found the node has `max_user_watches` of 10000, but the `TotalinotifyWatches` is beyond this value. (P. S still not sure why watcher can initiate more than cap). In order to find which process occupied the most watchers. I used following command[^flbit_ino] to find it out. ```bash echo -e "COUNT\tPID\tUSER\tCOMMAND"; sudo find /proc/[0-9]*/fdinfo -type f 2>/dev/null | sudo xargs grep ^inotify 2>/dev/null | cut -d/ -f 3 | uniq -c | sort -nr | { while read -rs COUNT PID; do echo -en "$COUNT\t$PID\t"; ps -p $PID -o user=,command=; done} COUNT PID USER COMMAND 7491 8412 root /home/kubernetes/bin/kubelet --v=2 --cloud-provide=gce --experi 2620 1 root /sbin/init .... ``` Surprisingly, Kubelet initiated more than 7000 inotify watchers. I assumed there was an inotify leakage in Kubelet. ## Leakage check ### Clean Kubelet To better understand the situation, I created a clean cluster with only 1 clean node on GKE. Roughly 70 inotify watchers were there. I created a single nginx pod and the number increased by 3. Theoretically, these 3 watchers are used by Kubelet to monitor any changes on `rootfs `, `kube-api-access` and `PodSandbox`. But to verify it, we need to check more details on which inodes are monitored by Kubelet. ### Check inotify file descriptors To do so, let's take a look how to track a single inotify file descriptor. Opened processes' `fdinfo` folder, we can examine each or them to find an inotify fd. ```bash # Find kubelet pid ps -aux | grep kubelet KPID=2430 # File the an example fd sudo ls /proc/2430/fdinfo 0 1 10 11 12 13 14 2 3 4 5 6 7 8 9 ... sudo cat /proc/2430/fdinfo/8 pos: 0 flags: 02004000 mnt_id: 15 ino: 1057 inotify wd:1 ino:3f327 sdev:800001 mask:fc6 ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:27f30300e5059ea2 ``` This is very confusing, so I rely on `man proc`[^man_proc] to understand every piece of them. In given fd, the needed information to continue sit in the last line. It's an inotify entry represents the 1 file or folder to be monitored. And the most useful data is `ino:3f327` which represents the inode number of target file (in hexadecimal). And `sdev:800001`, which represents the ID of device where the inode sit on, and it's also in hex. Using `lsblk`, I can see there's only 1 disk I'm using on the node, so finding the target file would be easy. ```bash # Cast to decimal ino=3f327 dec="$((16#${ino}))" # Find the target file loc="debugfs -R 'ncheck ${dec}' /dev/sda1" sudo eval $loc 2>/dev/null debugfd 1.46.5 (30-Dec-2021) Inode Pathname 258855 /etc/srv/kubernetes/pki/ca-certificates.crt ``` Put all processes above into one single script, I can retrieve all target files, that would help to understand if there's a real leakage. Also, I count the unique inode amount, this could also help to know which inode are monitored multiple times. ```bash cat << EOF | sudo tee -a test.sh echo "kubelet pid="${PID} in_fds=$(find /proc/${PID}/fdinfo -type f 2>/dev/null | xargs grep ^inotify | cut -d " " -f 3 | cut -d ":" -f 2) echo ${in_fds} echo "Count: $(find /proc/${PID}/fdinfo -type f 2>/dev/null | xargs grep ^inotify | wc -l)" uniq_fds=$(echo "${in_fds[@]}" | sort | uniq) echo ${uniq_fds} while read -r element; do count=$(echo "${in_fds[@]}" | grep -o "${element}" | wc -l) dec="$((16#${element}))" loc="debugfs -R 'ncheck ${dec}' /dev/sda1" loc=$(eval $loc 2>/dev/null | tail -1 | cut -d " " -f 4) printf "%-6s %-10s %-6s %s\n" "${element}" "${dec}" "${count}" "${loc}" done <<< "${uniq_fds}" EOF sudo bash test.sh kubelet pid=2430 3f327 3f321 ... Count: 120 1 10b 1259 128a ... 1 1 72 Inode Pathname 10b 267 1 267 /etc/systemd/system/multi-user.target.wants/snapd.service ... ``` The given results are consists by following parts: - One line for get Kubelet pid - One line for all target inode numbers - One line for tell how many unique inode (120) - One line of sorted target - Following 120 lines, each of them represents a unique inode number, its decimal number, count, another time of decimal number and the target file path. I used the same script to the problematic node, and it showed the following result. In summary, most Kubelet watchers were targeting `ino:1 `. And there are 6649 targets files, which likely to be leakage, because there were only 150 pods on this pod. Unfortunately, `debugfs` can't find any target files, so the output showed as meaningless string `"Inode Pathname"`. ``` kubelet pid=8412 ... Count: 7491 ... 1 1 6649 Inode Pathname ``` ### Bad apple Why `debugfs` can't help anymore? The reason is simple, each cgroup for a pod is using its own rootfs. This means the watcher are somehow residing on different rootfs and using independent inode index. There are some other ways to do it, I choose the most common tool `grep` to find out. ``` sudo grep / -inum 1 /home/kubernetes/containerized_mounter/rootfs/dev /home/kubernetes/containerized_mounter/rootfs/proc ... /home/kubernetes/containerized_mounter/rootfs/var/lib/kubelet/pods/5325873d-f2a0-48df-83e2-0b911df2f77f/volumes/kubernetes.io~projected/kube-api-access-227jg ... /dev /boot/efi ... ``` This turns things easy, because I can just use pod ID to compare between running pods to find out if there are any terminated pods are there. And it did show there's some non-existence pods still being watched somehow. ## What to expect next - [How Kubelet leaked inotify watchers?]() - [debugfs]() ## References [^flbit_ino]: [Fluentbit error "cannot adjust chunk size" on GKE](https://stackoverflow.com/a/76712244) [^man_proc]: [proc(5)](https://manpages.courier-mta.org/htmlman5/proc.5.html) [^list_ino]: [Listing the files that are being watched by `inotify` instances](https://unix.stackexchange.com/a/646113)