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Add proposal for hiding container mountpoints from systemd
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--- | ||
title: hide-container-mountpoints | ||
authors: | ||
- "@lack" | ||
reviewers: | ||
- "@haircommander" | ||
- "@mrunalp" | ||
- "@umohnani8" | ||
approvers: | ||
- TBD | ||
creation-date: 2021-01-18 | ||
last-updated: 2021-01-18 | ||
status: implementable | ||
--- | ||
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# Hide Container Mountponts | ||
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## Release Signoff Checklist | ||
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- [ ] Enhancement is `implementable` | ||
- [ ] Design details are appropriately documented from clear requirements | ||
- [ ] Test plan is defined | ||
- [ ] Operational readiness criteria is defined | ||
- [ ] Graduation criteria for dev preview, tech preview, GA | ||
- [ ] User-facing documentation is created in [openshift-docs](https://github.com/openshift/openshift-docs/) | ||
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## Summary | ||
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The current implementation of Kubelet and CRI-O both use the top-level | ||
namespace for all container and Kubelet mountpoints. However, moving these | ||
container-specific mountpoints into a private namespace reduced systemd | ||
overhead with no difference in functionality. | ||
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## Motivation | ||
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Systemd scans and re-scans mountpoints many times, adding a lot to the CPU | ||
utilization of systemd and overall overhead of the host OS running OpenShift. | ||
Changing systemd to reduce its scanning overhead is tracked in [BZ | ||
1819868](https://bugzilla.redhat.com/show_bug.cgi?id=1819868), but we can work | ||
around this exclusively within Openshift. Using a separate mountpoint to for | ||
both CRI-O and Kubelet mounts can completely segregate all container-specific | ||
mounts away from any systemd interaction whatsoever. | ||
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### Goals | ||
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- CRI-O places all container mountpoints in a separate mount namespace from systemd | ||
- Kubelet places all container mountpoints in this same separate mount namespace from systemd | ||
- systemd does not see any CRI-O or Kubelet mountpoints | ||
- CRI-O containers still see all appropriate Kubelet mountpoints | ||
- Restarting either crio.service or kubelet.service does not result in the namespaces getting out-of-sync | ||
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### Non-Goals | ||
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- Fix systemd mountpoint scanning | ||
- Change CRI-O pinns to support pinning mount namespaces | ||
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## Proposal | ||
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We can create a separate mount namespace and cause both CRI-O and Kubelet to | ||
launch within it to hide their many many mounts from systemd by creating: | ||
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- A service called container-mount-namespace.service which spawns a separate | ||
namespace (via systemd's 'PrivateMounts' mechanism), and then sleeps forever. | ||
We don't want to create the namespace in crio.service or kubelet.service, | ||
since if either one restarts they would lose eachother's namespaces. | ||
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- An override file for each of crio.service and kubelet.service which wrap the | ||
original command under 'nsenter' so they both use the mount namespace created | ||
by 'container-mount-namespace.service' | ||
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With these in place, both Kubelet and CRI-O create their mounts in the new | ||
shared (with eachother) but private (from systemd) namespace. | ||
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### User Stories | ||
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The end-user experience should not be affected in any way by this proposal, as | ||
there is no outward API changes. There is some supportability change though, | ||
since anyone attempting to inspect the CRI-O or Kubelet mountpoints externally | ||
would need to be aware that these are now available in a different namespace | ||
than the default top-level systemd mount namespace. | ||
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### Implementation Details/Notes/Constraints | ||
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Here is an example of the container-mount-namespace.service: | ||
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[Unit] | ||
Description=Manages a mount namespace that both Kubelet and CRI-O can use to share their container-specific mounts | ||
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[Service] | ||
PrivateMounts=on | ||
ExecStart=bash -c 'while :; do sleep infinity; done' | ||
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This needs to be managed separately from either CRI-O or Kubelet to avoid the | ||
namespaces getting out of sync if either of those services restart. The systemd | ||
'PrivateMounts=on' facility does a nice job of creating a separate mountpoint, | ||
and both CRI-O and Kubelet can find the associated namespace via 'nsenter' as | ||
follows: | ||
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nsenter -m -t $(systemctl show --property MainPID --value container-mount-namespace.service) $ORIGINAL_EXECSTART | ||
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This will necessitate adding `Requires=container-mount-namespace.service` and | ||
`After=container-mount-namespace.service` to both crio.service and | ||
kubelet.service as well to ensure the PID is available. | ||
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### Risks and Mitigations | ||
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Any other external or 3rd-party tools would need to change to match. | ||
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## Design Details | ||
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### Open Questions | ||
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The fact that there is a Kubernetes test that explicitly tests that the | ||
container mountpoints are available to the parent operating system implies that | ||
this may have been desirable to someone at some level at one time in the past. | ||
> 1. What is the reason for the Kubernetes test? Is it okay to just skip or | ||
> disable the test in OpenShift? | ||
> 2. Are there any external utilities or 3rd-party tools that assume they can | ||
> have access to the CRI-O or Kubelet mountpoints in the top-level mount | ||
> namespace? | ||
### Test Plan | ||
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- Disable the existing Kubernetes e2e test that checks that all mountpoints are | ||
in the parent mount namespace | ||
- Ensure all CRI-O and Kubelet mountpoints are visible only in the child | ||
namespace and not the parent namespace | ||
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## Implementation History | ||
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- Initial proof-of-concept example (here)[https://github.com/lack/redhat-notes/tree/main/crio_unshare_mounts] | ||
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## Drawbacks | ||
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- Requires re-wrapping CRI-O and Kubelet services in bash and nsenter. | ||
- Requires get-and-set of the ExecStart stanza from both of those services. | ||
- If the namespace service restarts and then either CRI-O or Kubelet restarts, | ||
there will be a mismatch between the mount namespaces and containers will | ||
start to fail. Could be mitigated by enhancing CRI-O pinns utility to pin and | ||
reuse mount namespaces. | ||
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## Alternatives | ||
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- Enhance systemd to support unsharing namespaces at the slice level, then put | ||
crio.service and kubelet.service in the same slice | ||
- Alter both CRI-O and Kubelet executables to take a mount namespace via | ||
commandline instead of requiring `nsenter` | ||
- Do this work upstream in Kubernetes as opposed to Openshift |