Add the kernel module: transparent huge pages set per machine (hq issue 746)

With THP 'always' journald's mmapped journal files sit in 2 MiB folios, so each small
journal write dirties and writes back 2 MiB; on the control node that saturated the
root mirror holding the bus store and the store, and the controller lost its lease.
The module sets the mode from a per-machine setting (default always, the kernel's own,
so assigning it changes nothing until a machine sets madvise), verifies it took, rotates
the journal once so new files are mapped under the new mode, gives back what it found
when stopped, and serves kernel_huge_pages to measure file_thp per unit.

modules/kernel/module.json

@@ -0,0 +1,69 @@
+{
+  "module": "kernel",
+  "version": "1",
+  "capabilities": [
+    "service-manager"
+  ],
+  "settings": {
+    "transparent-huge-pages": {
+      "kind": "preference",
+      "default": "always",
+      "why": "always is what the distribution's kernel boots with, so assigning the module changes nothing until a machine sets another value; madvise gives huge pages only to programs that ask for them, which stops a small write to a mapped file (the journal) dirtying and writing back a whole 2 MiB page on a machine whose disks are slow (novox/hq issue 746)"
+    }
+  },
+  "tools": [
+    "kernel_huge_pages"
+  ],
+  "resources": [
+    {
+      "id": "state",
+      "type": "directory",
+      "mode": "0700",
+      "place": "."
+    },
+    {
+      "id": "huge-pages",
+      "type": "file",
+      "path": "${dir:state}/huge-pages",
+      "mode": "0755",
+      "trusted": true,
+      "content": "#!/bin/sh\n# Transparent huge pages as the mesh sets them on this machine (module kernel, novox/hq issue 746).\n# Written by the mesh from this machine's setting transparent-huge-pages and run by\n# mesh-kernel-huge-pages.service: `apply` at boot and whenever the setting changes, `restore` when the\n# unit stops (the module unassigned, or the machine shutting down). Replaced on every push; change the\n# setting, never this file.\n#\n#   huge-pages apply|restore [<the kernel's transparent_hugepage directory> [<this module's state>]]\n#\n# The two directories are arguments only so the module's tests can run this against a stand-in tree.\nset -eu\n\nwant='${setting:transparent-huge-pages}'\nsys=/sys/kernel/mm/transparent_hugepage\nstate='${dir:state}'\n[ $# -ge 2 ] && sys=$2\n[ $# -ge 3 ] && state=$3\n\ncase \"$want\" in\nalways | madvise | never) ;;\n*)\n\techo \"the setting transparent-huge-pages is '$want'; the kernel takes always, madvise or never\" >&2\n\texit 2\n\t;;\nesac\n[ -f \"$sys/enabled\" ] || {\n\techo \"this kernel has no $sys/enabled: it was built without transparent huge pages, so there is nothing to set\" >&2\n\texit 1\n}\n\n# The mode in force is the word the kernel brackets: \"always [madvise] never\". A file holding one bare\n# word (the stand-in tree of the tests) is read as that word.\nmode() { sed -n -e 's/.*\\[\\([a-z]*\\)\\].*/\\1/p' -e t -e 's/^ *\\([a-z]*\\) *$/\\1/p' \"$sys/enabled\"; }\n\n[ $# -ge 1 ] || set -- usage\ncase \"$1\" in\napply)\n\tnow=$(mode)\n\t# What the machine had before the mesh first set it, kept once, so `restore` gives it back.\n\t[ -s \"$state/found\" ] || printf '%s\\n' \"$now\" >\"$state/found\"\n\tif [ \"$now\" = \"$want\" ]; then\n\t\techo \"transparent huge pages: $want, as set\"\n\t\texit 0\n\tfi\n\tprintf '%s\\n' \"$want\" >\"$sys/enabled\"\n\tgot=$(mode)\n\t[ \"$got\" = \"$want\" ] || {\n\t\techo \"wrote $want to $sys/enabled and the kernel says $got\" >&2\n\t\texit 1\n\t}\n\techo \"transparent huge pages: $now -> $want\"\n\t# A file folio already in the page cache keeps its size: the active journal files, mapped by journald\n\t# and by every reader, stay in 2 MiB folios until they are archived (issue 746). Rotating starts new\n\t# files that are mapped under the new mode. A failed rotation is said and does not fail the setting.\n\tif [ \"$want\" != always ] && command -v journalctl >/dev/null 2>&1; then\n\t\tjournalctl --rotate || echo \"the journal could not be rotated: its active files keep their huge pages until they are archived\" >&2\n\tfi\n\t;;\nrestore)\n\tfound=$(cat \"$state/found\" 2>/dev/null || true)\n\tcase \"$found\" in\n\talways | madvise | never)\n\t\tprintf '%s\\n' \"$found\" >\"$sys/enabled\"\n\t\techo \"transparent huge pages: back to $found, as the machine had them\"\n\t\t;;\n\t*) echo \"transparent huge pages: nothing recorded as found, left at $(mode)\" ;;\n\tesac\n\t;;\n*)\n\techo \"usage: huge-pages apply|restore [<transparent_hugepage directory> [<state directory>]]\" >&2\n\texit 2\n\t;;\nesac\n"
+    },
+    {
+      "id": "unit",
+      "type": "file",
+      "path": "/etc/systemd/system/mesh-kernel-huge-pages.service",
+      "mode": "0644",
+      "content": "[Unit]\nDescription=Transparent huge pages as the mesh sets them on this machine\n# Module kernel, novox/hq issue 746. Starting this unit sets the mode from the setting\n# transparent-huge-pages; stopping it gives back the mode the machine had before. It runs early in boot,\n# before the services that fill the page cache, and again whenever the setting changes.\nDefaultDependencies=no\nAfter=local-fs.target\nBefore=sysinit.target shutdown.target\nConflicts=shutdown.target\n\n[Service]\nType=oneshot\nRemainAfterExit=yes\nExecStart=${dir:state}/huge-pages apply\nExecStop=${dir:state}/huge-pages restore\n\n[Install]\nWantedBy=sysinit.target\n"
+    },
+    {
+      "id": "apply",
+      "type": "service",
+      "unit": "mesh-kernel-huge-pages.service",
+      "state": "running",
+      "boot": "enabled",
+      "restart-on": [
+        "huge-pages",
+        "unit"
+      ],
+      "health": {
+        "kind": "unit"
+      }
+    }
+  ],
+  "build": {
+    "artifacts": [
+      {
+        "name": "tools",
+        "kind": "bundle",
+        "language": "go",
+        "system": "arch",
+        "from": "cmd/kernel-tools",
+        "binary": "kernel-tools",
+        "loads": [
+          "kernel-tools"
+        ]
+      }
+    ]
+  }
+}