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oci: use readonly mount to read user/group info
In linux kernel, the umount writable-mountpoint will try to do sync-fs to make sure that the dirty pages to the underlying filesystems. The many number of umount actions in the same time maybe introduce performance issue in IOPS limited disk. When CRI-plugin creates container, it will temp-mount rootfs to read that UID/GID info for entrypoint. Basically, the rootfs is writable snapshotter and then after read, umount will invoke sync-fs action. For example, using overlayfs on ext4 and use bcc-tools to monitor ext4_sync_fs call. ``` // uname -a Linux chaofan 5.13.0-27-generic #29~20.04.1-Ubuntu SMP Fri Jan 14 00:32:30 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux // open terminal 1 kubectl run --image=nginx --image-pull-policy=IfNotPresent nginx-pod // open terminal 2 /usr/share/bcc/tools/stackcount ext4_sync_fs -i 1 -v -P ext4_sync_fs sync_filesystem ovl_sync_fs __sync_filesystem sync_filesystem generic_shutdown_super kill_anon_super deactivate_locked_super deactivate_super cleanup_mnt __cleanup_mnt task_work_run exit_to_user_mode_prepare syscall_exit_to_user_mode do_syscall_64 entry_SYSCALL_64_after_hwframe syscall.Syscall.abi0 github.com/containerd/containerd/mount.unmount github.com/containerd/containerd/mount.UnmountAll github.com/containerd/containerd/mount.WithTempMount.func2 github.com/containerd/containerd/mount.WithTempMount github.com/containerd/containerd/oci.WithUserID.func1 github.com/containerd/containerd/oci.WithUser.func1 github.com/containerd/containerd/oci.ApplyOpts github.com/containerd/containerd.WithSpec.func1 github.com/containerd/containerd.(*Client).NewContainer github.com/containerd/containerd/pkg/cri/server.(*criService).CreateContainer github.com/containerd/containerd/pkg/cri/server.(*instrumentedService).CreateContainer k8s.io/cri-api/pkg/apis/runtime/v1._RuntimeService_CreateContainer_Handler.func1 github.com/containerd/containerd/services/server.unaryNamespaceInterceptor github.com/grpc-ecosystem/go-grpc-middleware.ChainUnaryServer.func1.1.1 github.com/grpc-ecosystem/go-grpc-prometheus.(*ServerMetrics).UnaryServerInterceptor.func1 github.com/grpc-ecosystem/go-grpc-middleware.ChainUnaryServer.func1.1.1 go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc.UnaryServerInterceptor.func1 github.com/grpc-ecosystem/go-grpc-middleware.ChainUnaryServer.func1.1.1 github.com/grpc-ecosystem/go-grpc-middleware.ChainUnaryServer.func1 k8s.io/cri-api/pkg/apis/runtime/v1._RuntimeService_CreateContainer_Handler google.golang.org/grpc.(*Server).processUnaryRPC google.golang.org/grpc.(*Server).handleStream google.golang.org/grpc.(*Server).serveStreams.func1.2 runtime.goexit.abi0 containerd [34771] 1 ``` If there are comming several create requestes, umount actions might bring high IO pressure on the /var/lib/containerd's underlying disk. After checkout the kernel code[1], the kernel will not call __sync_filesystem if the mount is readonly. Based on this, containerd should use readonly mount to get UID/GID information. Reference: * [1] https://elixir.bootlin.com/linux/v5.13/source/fs/sync.c#L61 Closes: containerd#4604 Signed-off-by: Wei Fu <fuweid89@gmail.com>
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