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d2bdb9b61bWith nullfs as the real rootfs userspace can pivot_root() away from the
initramfs and unmount it:
chdir(rootfs);
pivot_root(".", ".");
umount2(".", MNT_DETACH);
That's what systemd does and the initramfs is supposed to go away at
that point together with all the memory it uses. But on every machine
with more than one possible cpu the initramfs superblock stays around
forever (cf. [1]). With ramfs as rootfs the unpacked initramfs shows up
as Unevictable in /proc/meminfo, with tmpfs as Shmem, and it is never
freed. In the reports that's between 70MB and 150MB.
The idle threads of the secondary cpus are created by fork_idle() from
smp_init(). That runs in kernel_init(), i.e., in PID 1, after
init_userspace_fs() moved PID 1 from nullfs into the initramfs.
fork_idle() doesn't pass CLONE_FS so each idle thread gets a private
copy of PID 1's fs_struct and with it a reference to the initramfs as
root and as pwd.
pivot_root() relies on chroot_fs_refs() to move everyone whose root or
pwd is the old root over to the new root. Idle threads aren't on the
task list though so chroot_fs_refs() never finds them. They never exit
either. So they keep the initramfs mount and with it the superblock
alive for good. I overlooked this when I added nullfs. Before that the
rootfs could never be unmounted anyway so it didn't matter what pinned
it.
Idle threads are kernel threads and init_task, the idle thread of the
boot cpu, already uses init_fs. So let the idle threads of the other
cpus share init_fs as well instead of copying PID 1's filesystem state.
Note that kernels before 7.3 need a different backport. There PID 1
still shares init_fs with all kthreads so passing CLONE_FS in
fork_idle() is enough. They also have a second pin: devtmpfsd unshares
a copy of the initial mount namespace, initramfs mount included. That
was fixed in commit
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Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. 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Start your kernel development journey here: * Getting Started: Documentation/process/development-process.rst * Your First Patch: Documentation/process/submitting-patches.rst * Coding Style: Documentation/process/coding-style.rst * Build System: Documentation/kbuild/index.rst * Development Tools: Documentation/dev-tools/index.rst * Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst * Core APIs: Documentation/core-api/index.rst Academic Researcher ------------------- Explore the kernel's architecture and internals: * Researcher Guidelines: Documentation/process/researcher-guidelines.rst * Memory Management: Documentation/mm/index.rst * Scheduler: Documentation/scheduler/index.rst * Networking Stack: Documentation/networking/index.rst * Filesystems: Documentation/filesystems/index.rst * RCU (Read-Copy Update): Documentation/RCU/index.rst * Locking Primitives: Documentation/locking/index.rst * Power Management: Documentation/power/index.rst Security Expert --------------- Security documentation and hardening guides: * Security Documentation: Documentation/security/index.rst * LSM Development: Documentation/security/lsm-development.rst * Self Protection: Documentation/security/self-protection.rst * Reporting Vulnerabilities: Documentation/process/security-bugs.rst * CVE Procedures: Documentation/process/cve.rst * Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst * Security Features: Documentation/userspace-api/seccomp_filter.rst Backport/Maintenance Engineer ----------------------------- Maintain and stabilize kernel versions: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * Backporting Guide: Documentation/process/backporting.rst * Applying Patches: Documentation/process/applying-patches.rst * Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git for Maintainers: Documentation/maintainer/configure-git.rst System Administrator -------------------- Configure, tune, and troubleshoot Linux systems: * Admin Guide: Documentation/admin-guide/index.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst * Tracing/Debugging: Documentation/trace/index.rst * Performance Security: Documentation/admin-guide/perf-security.rst * Hardware Monitoring: Documentation/hwmon/index.rst Maintainer ---------- Lead kernel subsystems and manage contributions: * Maintainer Handbook: Documentation/maintainer/index.rst * Pull Requests: Documentation/maintainer/pull-requests.rst * Managing Patches: Documentation/maintainer/modifying-patches.rst * Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst * Development Process: Documentation/process/maintainer-handbooks.rst * Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git Configuration: Documentation/maintainer/configure-git.rst Hardware Vendor --------------- Write drivers and support new hardware: * Driver API Guide: Documentation/driver-api/index.rst * Driver Model: Documentation/driver-api/driver-model/driver.rst * Device Drivers: Documentation/driver-api/infrastructure.rst * Bus Types: Documentation/driver-api/driver-model/bus.rst * Device Tree Bindings: Documentation/devicetree/bindings/ * Power Management: Documentation/driver-api/pm/index.rst * DMA API: Documentation/core-api/dma-api.rst Distribution Maintainer ----------------------- Package and distribute the kernel: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * ABI Documentation: Documentation/ABI/README * Kernel Configuration: Documentation/kbuild/kconfig.rst * Module Signing: Documentation/admin-guide/module-signing.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst AI Coding Assistant ------------------- CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and follow the AI coding assistants documentation before contributing to the Linux kernel: * Documentation/process/coding-assistants.rst This documentation contains essential requirements about licensing, attribution, and the Developer Certificate of Origin that all AI tools must comply with. 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