Merge pull request #6 from maximumadmin/detect-vm

Optionally skip initialization if running on a VM

README.md

@@ -53,7 +53,7 @@ See also https://fedoraproject.org/wiki/Changes/SwapOnZRAM#Benefit_to_Fedora
 
 * zramd start --help
   ```
-  Usage: zramd start [--algorithm ALGORITHM] [--fraction FRACTION] [--max-size MAX_SIZE] [--num-devices NUM_DEVICES] [--priority PRIORITY]
+  Usage: zramd start [--algorithm ALGORITHM] [--fraction FRACTION] [--max-size MAX_SIZE] [--num-devices NUM_DEVICES] [--priority PRIORITY] [--skip-vm]
 
   Options:
     --algorithm ALGORITHM, -a ALGORITHM
@@ -66,6 +66,7 @@ See also https://fedoraproject.org/wiki/Changes/SwapOnZRAM#Benefit_to_Fedora
                            maximum number of zram devices to create [default: 1]
     --priority PRIORITY, -p PRIORITY
                            swap priority [default: 100]
+    --skip-vm, -s          skip initialization if running on a VM [default: false]
     --help, -h             display this help and exit
   ```
 
@@ -106,6 +107,7 @@ See also https://fedoraproject.org/wiki/Changes/SwapOnZRAM#Benefit_to_Fedora
 * **Avoid** using other zram-related packages along this one, `zramd` loads and unloads the zram kernel module assuming that the system is not using zram for other stuff e.g. tmpfs.
 * Do **not** use zswap with zram, it would unnecessarily cause data to be [compressed and decompressed back and forth](https://www.phoronix.com/forums/forum/software/distributions/1231542-fedora-34-looking-to-tweak-default-zram-configuration/page5#post1232327).
 * When dealing with virtual machines, zram should be used on the **host** OS so guest memory can be compressed transparently, see also comments on original zram [implementation](https://code.google.com/archive/p/compcache/).
+  * If you boot the same system on a real computer as well as on a virtual machine, you can use the `--skip-vm` parameter to avoid initialization when running inside a virtual machine.
 * For best results install `systemd-oomd` or `earlyoom` (they may not be available on all distributions).
 * You can use `swapon -show` or `zramctl` to see all swap devices currently in use, this is useful if you want to confirm that all of the zram devices were setup correctly.
 * To quickly fill the memory, you can use `tail /dev/zero` but keep in mind that your system may become unresponsive if you do not have an application like `earlyoom` to kill `tail` just before it reaches the memory limit.

cspell.json

@@ -5,6 +5,7 @@
         "itertools",
         "lsmod",
         "mkswap",
+        "virt",
         "zram",
         "zramd",
         "zstd"

extra/zramd.default

@@ -12,3 +12,6 @@
 
 # Swap priority
 # PRIORITY=100
+
+# Skip initialization if running inside a virtual machine
+# SKIP_VM=false

src/system/system.go

@@ -0,0 +1,45 @@
+package system
+
+import (
+	"errors"
+	"os"
+	"os/exec"
+	"regexp"
+	"strings"
+)
+
+// IsRoot will check if current process was started by the init system (e.g.
+// systemd) from which we expect to handle this process' capabilities, otherwise
+// check if the current process is running as root.
+func IsRoot() bool {
+	return os.Getppid() == 1 || os.Geteuid() == 0
+}
+
+func cpuInfo() *[]byte {
+	data, err := os.ReadFile("/proc/cpuinfo")
+	if err != nil {
+		panic(err)
+	}
+	return &data
+}
+
+// IsVM detects if we are currently running inside a VM, see also
+// https://man.archlinux.org/man/systemd-detect-virt.1.en.
+func IsVM() bool {
+	// Try to run systemd-detect-virt which is more accurate but is not present on
+	// all systems.
+	cmd := exec.Command("systemd-detect-virt", "--vm")
+	out, err := cmd.Output()
+	if err == nil {
+		return strings.TrimSpace(string(out)) != "none"
+	}
+	// If error happened because systemd-detect-virt is not available on the
+	// system, try to use cpuinfo (less accurate but available everywhere).
+	if errors.Is(err, exec.ErrNotFound) {
+		info := cpuInfo()
+		pattern := "(?m)^flags\\s*\\:.*\\s+hypervisor(?:\\s+.*)?$"
+		match, _ := regexp.Match(pattern, *info)
+		return match
+	}
+	panic(err)
+}

src/zramd.go

@@ -7,6 +7,7 @@ import (
 	"sync"
 	"zramd/src/kernelversion"
 	"zramd/src/memory"
+	"zramd/src/system"
 	"zramd/src/zram"
 
 	"github.com/alexflint/go-arg"
@@ -26,6 +27,7 @@ type startCmd struct {
 	MaxSizeMB    int     `arg:"-m,--max-size,env:MAX_SIZE" default:"8192" placeholder:"MAX_SIZE" help:"maximum total MB of swap to allocate"`
 	NumDevices   int     `arg:"-n,--num-devices,env:NUM_DEVICES" default:"1" placeholder:"NUM_DEVICES" help:"maximum number of zram devices to create"`
 	SwapPriority int     `arg:"-p,--priority,env:PRIORITY" default:"100" placeholder:"PRIORITY" help:"swap priority"`
+	SkipVM       bool    `arg:"-s,--skip-vm,env:SKIP_VM" default:"false" help:"skip initialization if running on a VM"`
 }
 
 type stopCmd struct {
@@ -139,13 +141,6 @@ func deinitializeZram() int {
 	return ret
 }
 
-// canRun will check if current process was started by the init system (e.g.
-// systemd) from which we expect to handle this process' capabilities, otherwise
-// check if the current process is running as root.
-func canRun() bool {
-	return os.Getppid() == 1 || os.Geteuid() == 0
-}
-
 func run() int {
 	if len(os.Args) > 1 && os.Args[1] == "--version" {
 		fmt.Printf("zramd %s %s %s\n", Version, CommitDate, runtime.GOARCH)
@@ -199,11 +194,19 @@ func run() int {
 		if args.Start.SwapPriority < -1 || args.Start.SwapPriority > 32767 {
 			parser.Fail("--priority must have a value between -1 and 32767")
 		}
+		// Avoid initializing zram if running on a virtual machine, we are not
+		// relying on systemd's "ConditionVirtualization=!vm" because it requires
+		// systemd, also we want this to be an opt-in setting unlike
+		// ConditionVirtualization which would behave the other way around.
+		if args.Start.SkipVM && system.IsVM() {
+			fmt.Println("virtual machine detected, initialization skipped")
+			return 0
+		}
 		if zram.IsLoaded() {
 			errorf("the zram module is already loaded")
 			return 1
 		}
-		if !canRun() {
+		if !system.IsRoot() {
 			errorf("root privileges are required")
 			return 1
 		}
@@ -214,7 +217,7 @@ func run() int {
 			errorf("the zram module is not loaded")
 			return 1
 		}
-		if !canRun() {
+		if !system.IsRoot() {
 			errorf("root privileges are required")
 			return 1
 		}