NFSv4.2 is broken across different hosts

[MichaelEischer]

Description

With flatcar 3975.2.1 we see very weird behavior of NFS 4.2 where one pod writes a file but a pod on a different host is unable to see the just written file content.

NFS 3 / 4.1 works as expected. (Haven't tested 4.0). Flatcar 3815.2.5 is also unaffected.

Impact

NFS 4.2 mount is unusable.

Environment and steps to reproduce

1. Set-up:

• at least two nodes in a k8s cluster running flatcar 3975.2.1
• Setup nfs-ganesh:

helm repo add nfs-ganesha-server-and-external-provisioner https://kubernetes-sigs.github.io/nfs-ganesha-server-and-external-provisioner/
helm install my-release nfs-ganesha-server-and-external-provisioner/nfs-server-provisioner

Update mount options in `StorageClass` `nfs`

allowVolumeExpansion: true
apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  annotations:
    meta.helm.sh/release-name: my-release
    meta.helm.sh/release-namespace: default
  labels:
    app: nfs-server-provisioner
    app.kubernetes.io/managed-by: Helm
    chart: nfs-server-provisioner-1.8.0
    heritage: Helm
    release: my-release
  name: nfs
mountOptions:
- hard
- retrans=3
- proto=tcp
- nfsvers=4.2
- rsize=4096
- wsize=4096
- noatime
- nodiratime
provisioner: cluster.local/my-release-nfs-server-provisioner
reclaimPolicy: Delete
volumeBindingMode: Immediate

create pvc

kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: test-dynamic-volume-claim
spec:
  storageClassName: "nfs"
  accessModes:
    - ReadWriteMany
  resources:
    requests:
      storage: 100Mi

create pods (must be executed on different hosts)

apiVersion: v1
kind: Pod
metadata:
  name: test-pod-1
  labels:
    app: nginx
spec:
  containers:
    - name: test
      image: nginx
      volumeMounts:
        - name: config
          mountPath: /test
  volumes:
    - name: config
      persistentVolumeClaim:
        claimName: test-dynamic-volume-claim
  affinity:
    podAntiAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
      - labelSelector:
          matchLabels:
            app: nginx
        topologyKey: "kubernetes.io/hostname"
---
apiVersion: v1
kind: Pod
metadata:
  name: test-pod-2
  labels:
    app: nginx
spec:
  containers:
    - name: test
      image: nginx
      volumeMounts:
        - name: config
          mountPath: /test
  volumes:
    - name: config
      persistentVolumeClaim:
        claimName: test-dynamic-volume-claim
  affinity:
    podAntiAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
      - labelSelector:
          matchLabels:
            app: nginx
        topologyKey: "kubernetes.io/hostname"

2. Action(s):
   a. kubectl exec -it test-pod-1 -- bash -c 'echo "def" > /test/testfile'
   b. kubectl exec -it test-pod-2 -- bash -c 'cat /test/testfile'
3. Error: The call to cat should return "def", but returns nothing. Note that both pods see accurate metadata (using ls -la /test) for the file

Expected behavior

cat from test-pod-2 should be able to read the just written file content. Note that test-pod-1 is able to read the file contents.

[MichaelEischer]

Just repeated the test with 3975.2.2 and it is also affected.

Edit: https://cdn.kernel.org/pub/linux/kernel/v6.x/ChangeLog-6.6.55 contains a few NFS related fixes, but it's unclear to me whether that would resolve this issue.

[ader1990]

Hello, I could reproduce this behaviour on a two node ARM64 Flatcar latest alpha (4116.0.0) env.

I think the issue is related to a host problem as the files from the secondary host are empty as shown from the host perspective:

# from k8s node 2
cat /var/lib/kubelet/pods/2cf32713-9a7a-412e-b4fc-998741deb125/volumes/kubernetes.io~nfs/pvc-8f47140d-1164-42a3-816f-05b41a9633c9/test

# empty output

[ader1990]

Flatcar main with kernel 6.6.56 is also affected.

[ader1990]

Tested with Flatcar using kernel 6.10.9 and the issue is present there too, this seems to be a Linux kernel regression. Or a tooling / containerd issue - needs debugging to repro this case outside of k8s first and to better pin-point the actual cause.

[ader1990]

torvalds/linux@9cf2744#diff-a24af2ce5442597efe8051684905db2be615f41703247fbce9a446e77f2e9587R214 -> from the linux tree, this is the only thing I see it has changed that might affect NFS 6.6 or 6.10 vs previous ones.

[MichaelEischer]

I'm wondering whether the underlying issue might be a bug in the ganesha NFS server that is now exposed by the read_plus default change.

Edit: I did some additional testing and the output of cat /test/testfile is actually not empty, but rather consists only of null bytes with the expected length.

[ader1990]

I'm wondering whether the underlying issue might be a bug in the ganesha NFS server that is now exposed by the read_plus default change.

Edit: I did some additional testing and the output of cat /test/testfile is actually not empty, but rather consists only of null bytes with the expected length.

I am trying now to build a kernel with the read_plus disabled, let's see how that goes.

[tormath1]

We might be interested to update our NFS test then to catch further regressions like this. (https://github.com/flatcar/mantle/blob/02348d65a5f9bd72f3e7412da54a688b7f972790/kola/tests/kubeadm/kubeadm.go#L237)

[ader1990]

Tested with NFS_V4_2_READ_PLUS=n and the issue got solved. This is an upstream issue - kernel or nfs implementation and needs to be properly reported, any idea where is best to have it reported?

[jepio]

Normally https://lore.kernel.org/linux-nfs and or the upstream for the server implementation but... the nfs-ganesha-server-and-external-provisioner repo (https://github.com/kubernetes-sigs/nfs-ganesha-server-and-external-provisioner) is still on Ganesha V4.0.8 whereas upstream just released V6. So there is some reason to think that this might be fixed in newer versions.

[ader1990]

kubernetes-sigs/nfs-ganesha-server-and-external-provisioner#152 -> there is a PR to update the chart to use Ganesha v6.

[ader1990]

nfs-ganesha/nfs-ganesha@24da5c3#diff-d4e3191eebe00b04019cafa02691fef13becc8cb3cc098ae6c177653cea40561R776 -> this commit is the best candidate to have a fix for this issue.

[MichaelEischer]

Is there any update here yet?

[ader1990]

Hello, the just released Flatcar versions alpha/beta/stable from https://www.flatcar.org/releases have the Linux kernel fix.

[MichaelEischer]

I just gave the new flatcar version a try and NFS in works again. Thanks!