issue 1841/S16: kernel_boot answers the loader, entries, kernels, booted entry and the fallback
ADR 0508 §9: the operator sees whether every boot menu lists the fallback kernel, and the room left on /boot.
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@@ -1,8 +1,9 @@
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# kernel
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The kernel's run-time settings on a machine (novox/hq to-be 42 Phase 1, row 11). Today it holds one:
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**transparent huge pages**, set per machine. The rest of the row (the kernel package, microcode, boot
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entries) is not built yet.
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The kernel's run-time settings on a machine (novox/hq to-be 42 Phase 1, row 11). It holds two things:
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**transparent huge pages**, set per machine, and **the fallback kernel** (`linux-lts` with its headers) and
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its place on the boot menu (novox/hq ADR 0508 §9 and §10). The default kernel's package, and microcode, are
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not built yet.
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## Why (novox/hq issue 746)
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@@ -35,16 +36,34 @@ under the new mode. A failed rotation is said in the unit's journal and does not
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rotation is made while the machine is still starting (`systemctl is-system-running` says `initializing`
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or `starting`): nothing is in huge pages yet, and the persistent journal opens after this unit.
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## The fallback kernel (novox/hq ADR 0508, issue 1841)
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Every machine keeps a second, older kernel to start from, so a kernel upgrade that does not boot is not the
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end of the machine. The module declares the packages `linux-lts` and `linux-lts-headers` (so every DKMS module is
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built for the fallback kernel too) after its boot-entries service, so the entry is in place before the kernel is.
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| | what | notes |
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|---|---|---|
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| file | `${dir:state}/boot-entries` | the script. It reads from the machine which loader booted it: the EFI variable `LoaderEntrySelected` present means systemd-boot, a `/boot/grub/grub.cfg` without it means GRUB, neither fails the unit loudly |
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| | systemd-boot | writes `/boot/loader/entries/mesh-linux-lts.conf` from `/proc/cmdline` without `initrd=` and `BOOT_IMAGE=`, with the microcode images present in `/boot` and the fallback initramfs as initrd lines; `loader.conf` and the default entry are never touched |
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| | GRUB | runs `grub-mkconfig -o /boot/grub/grub.cfg` only when the set of `/boot/vmlinuz-*` differs from the kernels the menu names; the default is left as it is |
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| | room | while `/boot/vmlinuz-linux-lts` is not there yet, less than 96 MiB free on `/boot` fails the unit and says so (the fallback kernel writes about 72 MB, issue 1836) |
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| | a kernel-copying hook | a hook in `/etc/pacman.d/hooks` that the mesh did not write, triggered by kernel images and copying to one fixed file name in `/boot`, would put the fallback kernel in the default kernel's place: it is moved into `${dir:state}/taken-over/` and named in the unit's journal. Each kernel is then placed under its own package's name by the distribution's hooks |
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| file | `/etc/systemd/system/mesh-kernel-boot-entries.service` | oneshot, sandboxed: the file system read-only but for `/boot`, `/etc/pacman.d/hooks` and the module's state; no home, no network |
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| file | `/etc/pacman.d/hooks/90-mesh-kernel-boot-entries.hook` | runs the script after a kernel image is installed, upgraded or removed |
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| packages | `linux-lts`, `linux-lts-headers` | declared after the service and the hook |
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## Tools
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| tool | | answers |
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| `kernel_boot` | r | the loader (`systemd-boot`, `grub` or `none`), the menu's entries, the kernels in `/boot`, the entry booted (systemd-boot) or the kernel image booted (GRUB), whether the fallback kernel is on the menu with its kernel in `/boot`, and the room free on `/boot` in KiB |
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| `kernel_huge_pages` | r | the mode, defrag and shared-memory mode; each per-size control and what `inherit` resolves to; huge pages by kind from `/proc/meminfo` (MiB); the units holding the most file pages in huge folios (`memory.stat` `file_thp`) with their dirty and writeback bytes, khugepaged's collapses and the bytes their cgroup wrote (`io.stat`, the most any one device saw: a mirror and its members each count the same write) |
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## Tests
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`go test ./...`: the tool over a stand-in tree holding the control node's own files of 2026-10-14; the
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manifest's shape (the default, the order, the unit and its sandbox, reload, restart and health); and the script as the mesh
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manifest's shape (the default, the order, the unit and its sandbox, reload, restart and health); the boot-entries script run with `sh` over stand-in roots (a systemd-boot entries directory, a GRUB menu, a hook that copies kernels to one fixed name): the entry from the booted command line, a menu generated again only when the kernels changed, no loader, no room, the taken-over hook; `kernel_boot` over the same roots; and the huge-pages script as the mesh
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writes it, run with `sh` against a stand-in directory: it sets and reads back, keeps what it found once,
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rotates the journal once and not for `always` nor while the machine starts, goes straight to a new value on a
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reload without rewriting what it found, gives back what it found and then keeps nothing, refuses a value the kernel
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@@ -0,0 +1,89 @@
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package main
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// kernel_boot: which loader booted the machine, its entries, the kernels in /boot, the entry it booted and
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// whether the fallback kernel is on the menu (novox/hq ADR 0508 §9, issue 1841). Reads only.
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import (
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"os"
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"path/filepath"
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"regexp"
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"sort"
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"strings"
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"syscall"
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"unicode/utf16"
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)
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const loaderSelected = "sys/firmware/efi/efivars/LoaderEntrySelected-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f"
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// BootState is what kernel_boot answers.
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type BootState struct {
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Loader string `json:"loader"`
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Entries []string `json:"entries"`
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Kernels []string `json:"kernels"`
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Booted string `json:"booted"`
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FallbackPresent bool `json:"fallback_present"`
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BootFreeKiB int64 `json:"boot_free_kib"`
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}
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var grubKernel = regexp.MustCompile(`(?m)^\s*linux\s+(?:/boot)?/(vmlinuz-\S+)`)
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var grubTitle = regexp.MustCompile(`(?m)^\s*menuentry\s+'([^']*)'`)
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var bootImage = regexp.MustCompile(`BOOT_IMAGE=(?:/boot)?/?(\S+)`)
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// Boot reads the machine, the loader the way the boot-entries script does: the loader's EFI variable means
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// systemd-boot, a grub.cfg without it GRUB.
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func (m *Machine) Boot() (*BootState, error) {
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b := &BootState{Loader: "none", Entries: []string{}, Kernels: []string{}}
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files, _ := filepath.Glob(m.path("boot/vmlinuz-*"))
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for _, f := range files {
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b.Kernels = append(b.Kernels, filepath.Base(f))
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}
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sort.Strings(b.Kernels)
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hasKernel := func(name string) bool {
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for _, k := range b.Kernels {
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if k == name {
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return true
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}
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}
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return false
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}
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var s syscall.Statfs_t
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if err := syscall.Statfs(m.path("boot"), &s); err == nil {
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b.BootFreeKiB = int64(s.Bavail) * int64(s.Bsize) / 1024
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}
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if raw, err := os.ReadFile(m.path(loaderSelected)); err == nil {
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b.Loader = "systemd-boot"
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if len(raw) > 4 {
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var u []uint16
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for i := 4; i+1 < len(raw); i += 2 {
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u = append(u, uint16(raw[i])|uint16(raw[i+1])<<8)
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}
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b.Booted = strings.TrimRight(string(utf16.Decode(u)), "\x00")
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}
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files, _ := filepath.Glob(m.path("boot/loader/entries/*.conf"))
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for _, f := range files {
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b.Entries = append(b.Entries, filepath.Base(f))
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}
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sort.Strings(b.Entries)
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if raw, err := os.ReadFile(m.path("boot/loader/entries/mesh-linux-lts.conf")); err == nil {
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b.FallbackPresent = strings.Contains(string(raw), "/vmlinuz-linux-lts") && hasKernel("vmlinuz-linux-lts")
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}
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return b, nil
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}
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if raw, err := os.ReadFile(m.path("boot/grub/grub.cfg")); err == nil {
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b.Loader = "grub"
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for _, t := range grubTitle.FindAllStringSubmatch(string(raw), -1) {
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b.Entries = append(b.Entries, t[1])
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}
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for _, k := range grubKernel.FindAllStringSubmatch(string(raw), -1) {
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if k[1] == "vmlinuz-linux-lts" && hasKernel("vmlinuz-linux-lts") {
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b.FallbackPresent = true
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}
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}
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if cmd, err := m.read("proc/cmdline"); err == nil {
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if i := bootImage.FindStringSubmatch(cmd); i != nil {
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b.Booted = i[1]
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}
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}
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}
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return b, nil
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}
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@@ -0,0 +1,90 @@
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package main
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// kernel_boot over stand-in roots: a systemd-boot machine with and without the fallback entry, a GRUB machine
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// with and without the fallback kernel in its menu, and a machine with neither (novox/hq ADR 0508 §9, issue 1841).
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import (
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"os"
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"path/filepath"
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"testing"
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)
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const selectedVar = "sys/firmware/efi/efivars/LoaderEntrySelected-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f"
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func put(t *testing.T, root, p, content string) {
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t.Helper()
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full := filepath.Join(root, p)
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if err := os.MkdirAll(filepath.Dir(full), 0o755); err != nil {
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t.Fatal(err)
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}
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write(t, full, content, 0o644)
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}
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func bootOf(t *testing.T, root string) *BootState {
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t.Helper()
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got, err := (&Machine{Root: root}).Boot()
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if err != nil {
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t.Fatal(err)
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}
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return got
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}
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func TestKernelBootReadsASystemdBootMachineWithItsFallbackEntry(t *testing.T) {
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root := t.TempDir()
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put(t, root, selectedVar, "\x07\x00\x00\x00a\x00r\x00c\x00h\x00.\x00c\x00o\x00n\x00f\x00\x00\x00")
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put(t, root, "boot/loader/entries/arch.conf", defaultEntry)
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put(t, root, "boot/loader/entries/mesh-linux-lts.conf", "title Arch Linux (linux-lts, fallback)\nlinux /vmlinuz-linux-lts\n")
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put(t, root, "boot/vmlinuz-linux", "k")
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put(t, root, "boot/vmlinuz-linux-lts", "k")
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got := bootOf(t, root)
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if got.Loader != "systemd-boot" || got.Booted != "arch.conf" || !got.FallbackPresent {
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t.Fatalf("%+v", got)
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}
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if len(got.Entries) != 2 || got.Entries[0] != "arch.conf" || got.Entries[1] != "mesh-linux-lts.conf" {
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t.Fatalf("entries: %v", got.Entries)
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}
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if len(got.Kernels) != 2 || got.Kernels[0] != "vmlinuz-linux" || got.Kernels[1] != "vmlinuz-linux-lts" {
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t.Fatalf("kernels: %v", got.Kernels)
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}
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if got.BootFreeKiB <= 0 {
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t.Fatalf("free: %d", got.BootFreeKiB)
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}
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}
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func TestKernelBootSaysASystemdBootMachineHasNoFallbackYet(t *testing.T) {
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root := t.TempDir()
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put(t, root, selectedVar, "\x07\x00\x00\x00a\x00r\x00c\x00h\x00.\x00c\x00o\x00n\x00f\x00\x00\x00")
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put(t, root, "boot/loader/entries/arch.conf", defaultEntry)
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put(t, root, "boot/vmlinuz-linux", "k")
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// The entry file alone is not the fallback: the kernel it names must be in /boot.
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put(t, root, "boot/loader/entries/mesh-linux-lts.conf", "linux /vmlinuz-linux-lts\n")
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if got := bootOf(t, root); got.FallbackPresent {
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t.Fatalf("a fallback entry without its kernel: %+v", got)
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}
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}
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func TestKernelBootReadsAGrubMachine(t *testing.T) {
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root := t.TempDir()
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put(t, root, "boot/grub/grub.cfg", grubMenu)
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put(t, root, "boot/vmlinuz-linux", "k")
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put(t, root, "proc/cmdline", bootedCmdline)
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got := bootOf(t, root)
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if got.Loader != "grub" || got.FallbackPresent || got.Booted != "vmlinuz-linux" {
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t.Fatalf("%+v", got)
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}
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if len(got.Entries) != 2 || got.Entries[0] != "Arch Linux" {
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t.Fatalf("entries: %v", got.Entries)
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}
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put(t, root, "boot/vmlinuz-linux-lts", "k")
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put(t, root, "boot/grub/grub.cfg", grubMenu+"menuentry 'Arch Linux LTS' {\n\tlinux\t/boot/vmlinuz-linux-lts root=x rw\n}\n")
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if got := bootOf(t, root); !got.FallbackPresent {
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t.Fatalf("%+v", got)
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}
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}
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func TestKernelBootSaysWhenThereIsNoLoader(t *testing.T) {
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got := bootOf(t, t.TempDir())
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if got.Loader != "none" || got.FallbackPresent || len(got.Entries) != 0 {
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t.Fatalf("%+v", got)
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}
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}
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@@ -46,5 +46,15 @@ func tools(m *Machine) []stdio.Tool {
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return m.HugePages(n)
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},
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},
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{
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Name: "kernel_boot",
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Description: "How this machine boots and whether the fallback kernel is on its menu (novox/hq ADR 0508 §9): the loader that booted it " +
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"(systemd-boot, read from the EFI variable it sets, grub, or none), the menu's entries, the kernels in /boot, the entry " +
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"the machine booted (systemd-boot) or the kernel image it booted (grub), whether the fallback kernel linux-lts is on " +
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"the menu with its kernel in /boot, and the room free on /boot in KiB (the fallback kernel needs about 72 MB). " +
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"Replaces ls /boot, bootctl list, reading grub.cfg and df /boot. (r)",
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Input: schema(map[string]any{}),
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Run: func(args map[string]any) (any, error) { return m.Boot() },
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},
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}
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}
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@@ -13,7 +13,8 @@
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}
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},
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"tools": [
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"kernel_huge_pages"
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"kernel_huge_pages",
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"kernel_boot"
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],
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"resources": [
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{
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