libcontainer: reuse tmpfs for directory masks#5262
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dims wants to merge 1 commit intoopencontainers:mainfrom
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libcontainer: reuse tmpfs for directory masks#5262dims wants to merge 1 commit intoopencontainers:mainfrom
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AkihiroSuda
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Apr 25, 2026
Kubernetes may add one sysfs thermal_throttle entry per CPU to maskedPaths. On large Intel systems this can produce many directory masks for a single container. runc currently handles each directory mask with a separate read-only tmpfs mount, and therefore a separate tmpfs superblock. On Linux 4.18/RHEL 8 kernels, creating and tearing down many tmpfs superblocks can contend on the global shrinker_rwsem when containers start or stop concurrently. Use one read-only tmpfs for directory masks and bind-mount it over the remaining directory targets. The first non-procfs-fd directory mount is reopened through the container root fd before it is reused. File masks still bind /dev/null, and procfs fd targets keep the existing one-tmpfs-per-target behavior because they are fd aliases rather than stable rootfs paths. The bind mounts do not create additional tmpfs superblocks. They also retain the read-only mount flag inherited from the source vfsmount, so the masking semantics remain unchanged. Signed-off-by: Davanum Srinivas <davanum@gmail.com>
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( i think the fedora ci break is a flake ) |
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Kicked CI; its green now |
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A similar fix was done to crun a while back, and it makes sense (especially with more masked paths and containers). (I wish a kernel had a special mount flag for in-container /proc and /sys but until we have it...) |
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thanks @thaJeztah @kolyshkin ! |
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Wondering if a problem that showed up recently in k8s when we added more masked paths can be handled better in
runcitself? please see details below:Kubernetes may add one sysfs thermal_throttle entry per CPU to maskedPaths. On large Intel systems this can produce many directory masks for a single container. runc currently handles each directory mask with a separate read-only tmpfs mount, and therefore a separate tmpfs superblock.
On Linux 4.18/RHEL 8 kernels, creating and tearing down many tmpfs superblocks can contend on the global shrinker_rwsem when containers start or stop concurrently.
Use one read-only tmpfs for directory masks and bind-mount it over the remaining directory targets. The first non-procfs-fd directory mount is reopened through the container root fd before it is reused. File masks still bind /dev/null, and procfs fd targets keep the existing one-tmpfs-per-target behavior because they are fd aliases rather than stable rootfs paths.
The bind mounts do not create additional tmpfs superblocks. They also retain the read-only mount flag inherited from the source vfsmount, so the masking semantics remain unchanged.
xref: kubernetes/kubernetes#138512
xref: kubernetes/kubernetes#138388
xref: kubernetes/kubernetes#131018
(With some assistance from claude/codex)