kubeadm: update warning message for the swap check

[carlory]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

• NodeSwap graduates to Beta2 in Kubernetes 1.30.
• In Beta2, the NodeSwap feature gate is on by default but fail-on-swap=false still needs to be provided to kubelet.

Which issue(s) this PR fixes:

xref kubernetes/kubeadm#2563

Special notes for your reviewer:

Does this PR introduce a user-facing change?

kubeadm: improve the warning message about the NodeSwap check which kubeadm performs on preflight.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

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[carlory]

/cc @neolit123 @pacoxu

[SataQiu]

Is there a document link about the NodeSwap feature gate we can add here?

[carlory]

@SataQiu

Yes, I find two links that are related to the swap memory configuration in Kubernetes.

• https://kubernetes.io/docs/concepts/architecture/nodes/#swap-memory

• (text is out-date, need to update) https://kubernetes.io/docs/setup/production-environment/tools/kubeadm/install-kubeadm/#before-you-begin
  Swap configuration. The default behavior of a kubelet was to fail to start if swap memory was detected on a node.
  Swap has been supported since v1.22. And since v1.28, Swap is supported for cgroup v2 only;
  the NodeSwap feature gate of the kubelet is beta but disabled by default.
  ...

  update kubeadm prerequisites website#46560

Which link is preferred?

[SataQiu]

I'm +1 for https://kubernetes.io/docs/concepts/architecture/nodes/#swap-memory

[carlory]

I remove "the NodeSwap feature gate of the kubelet is beta and enabled by default; ". once the feature is GA, we don't need to change warning messages again.

cc @pacoxu @neolit123 @SataQiu

https://kubernetes.io/docs/concepts/architecture/nodes/#swap-memory contains this information.

To enable swap on a node, the NodeSwap feature gate must be enabled on the kubelet (default is true), 

[neolit123]

once the feature is GA we should remove the kubeadm check entirely.

[carlory]

Thank you @kannon92

Any suggestions on my changes? please take another look, thank you. @pacoxu @SataQiu @neolit123

[neolit123]

continuing to default a flag with a certain value so that the kubelet fails for a supposedly supported feature would be confusing for future and existing users...

does this mean we need to maintain a warning about swap forever in kubeadm?

[carlory]

According to #125157 (comment), I think so.

[neolit123]

i recall discussions in kubernetes/kubeadm#2563 about removing the kubeadm preflight check about swap completely.

[kannon92]

I agree it’s not the best UX. Unfortunately the fail swap on flag predates the node swap feature by a few years.

It’s important to state that one can provision kubelet with swap enabled but the feature is disabled. This can mean that kubelet runs on a swap node but no pods actually use swap. The swap feature is about how certain workloads (Burstable QoS) could use swap memory.

We haven’t really discussed the option for defaulting fail swap to false as part of this KEP.

If one provisions kubelet with swap they don’t need to set anything with swap feature unless they want workloads to actually use swap. Default is to have NodeSwap on with swap disabled for all workloads.

[pacoxu]

/lgtm
And does this need a cherry-pick to v1.30?

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 49b5fa83a915179358bbd925b42e5985c7ee16a7

[neolit123]

And does this need a cherry-pick to v1.30?

this does not fall under the critical, blocking bug category, so we should try not to backport similar changes to reduce backport noise.

[neolit123]

/release-note-edit

kubeadm: improve the warning message about the NodeSwap check which kubeadm performs on preflight.

[neolit123]

please don't block the changes to the warning text on my comments.

[neolit123]

/approve

[neolit123]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 76e3e8b4df9c56001058603bb9e5b2aa43c163bd

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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