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When creating containers the setup usually involves using CLONE_NEWNS via clone3() or unshare(). This copies the caller's complete mount namespace. The runtime will also assemble a new rootfs and then use pivot_root() to switch the old mount tree with the new rootfs. Afterward it will recursively umount the old mount tree thereby getting rid of all mounts. On a basic system here where the mount table isn't particularly large this still copies about 30 mounts. Copying all of these mounts only to get rid of them later is pretty wasteful. This is exacerbated if intermediary mount namespaces are used that only exist for a very short amount of time and are immediately destroyed again causing a ton of mounts to be copied and destroyed needlessly. With a large mount table and a system where thousands or ten-thousands of containers are spawned in parallel this quickly becomes a bottleneck increasing contention on the semaphore. Extend open_tree() with a new OPEN_TREE_NAMESPACE flag. Similar to OPEN_TREE_CLONE only the indicated mount tree is copied. Instead of returning a file descriptor referring to that mount tree OPEN_TREE_NAMESPACE will cause open_tree() to return a file descriptor to a new mount namespace. In that new mount namespace the copied mount tree has been mounted on top of a copy of the real rootfs. The caller can setns() into that mount namespace and perform any additionally required setup such as move_mount() detached mounts in there. This allows OPEN_TREE_NAMESPACE to function as a combined unshare(CLONE_NEWNS) and pivot_root(). A caller may for example choose to create an extremely minimal rootfs: fd_mntns = open_tree(-EBADF, "/var/lib/containers/wootwoot", OPEN_TREE_NAMESPACE); This will create a mount namespace where "wootwoot" has become the rootfs mounted on top of the real rootfs. The caller can now setns() into this new mount namespace and assemble additional mounts. This also works with user namespaces: unshare(CLONE_NEWUSER); fd_mntns = open_tree(-EBADF, "/var/lib/containers/wootwoot", OPEN_TREE_NAMESPACE); which creates a new mount namespace owned by the earlier created user namespace with "wootwoot" as the rootfs mounted on top of the real rootfs. Link: https://patch.msgid.link/20251229-work-empty-namespace-v1-1-bfb24c7b061f@kernel.org Tested-by: Jeff Layton <jlayton@kernel.org> Reviewed-by: Aleksa Sarai <cyphar@cyphar.com> Reviewed-by: Jeff Layton <jlayton@kernel.org> Suggested-by: Christian Brauner <brauner@kernel.org> Suggested-by: Aleksa Sarai <cyphar@cyphar.com> Signed-off-by: Christian Brauner <brauner@kernel.org> |
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| clocksource | ||
| crypto | ||
| cxl | ||
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| dt-bindings | ||
| hyperv | ||
| keys | ||
| kunit | ||
| kvm | ||
| linux | ||
| math-emu | ||
| media | ||
| memory | ||
| misc | ||
| net | ||
| pcmcia | ||
| ras | ||
| rdma | ||
| rv | ||
| scsi | ||
| soc | ||
| sound | ||
| target | ||
| trace | ||
| uapi | ||
| ufs | ||
| vdso | ||
| video | ||
| xen | ||
| Kbuild | ||