cgroupv2: use "max" for negative values

Cgroup v1 kernel doc [1] says:

> We can write "-1" to reset the ``*.limit_in_bytes(unlimited)``.

and cgroup v2 kernel documentation [2] says:

> - If a controller implements an absolute resource guarantee and/or
>  limit, the interface files should be named "min" and "max"
>  respectively.  If a controller implements best effort resource
>  guarantee and/or limit, the interface files should be named "low"
>  and "high" respectively.
>
>  In the above four control files, the special token "max" should be
>  used to represent upward infinity for both reading and writing.

Allow -1 value to still be used for v2, converting it to "max"
where it makes sense to do so.

This fixes the following issue:

> runc update test_update --memory-swap -1:
> error while setting cgroup v2: [write /sys/fs/cgroup/machine.slice/runc-cgroups-integration-test.scope/memory.swap.max: invalid argument
> failed to write "-1" to "/sys/fs/cgroup/machine.slice/runc-cgroups-integration-test.scope/memory.swap.max"
> github.com/opencontainers/runc/libcontainer/cgroups/fscommon.WriteFile
> 	/home/kir/go/src/github.com/opencontainers/runc/libcontainer/cgroups/fscommon/fscommon.go:21
> github.com/opencontainers/runc/libcontainer/cgroups/fs2.setMemory
> 	/home/kir/go/src/github.com/opencontainers/runc/libcontainer/cgroups/fs2/memory.go:20
> github.com/opencontainers/runc/libcontainer/cgroups/fs2.(*manager).Set
> 	/home/kir/go/src/github.com/opencontainers/runc/libcontainer/cgroups/fs2/fs2.go:175
> github.com/opencontainers/runc/libcontainer/cgroups/systemd.(*UnifiedManager).Set
> 	/home/kir/go/src/github.com/opencontainers/runc/libcontainer/cgroups/systemd/unified_hierarchy.go:290
> github.com/opencontainers/runc/libcontainer.(*linuxContainer).Set
> 	/home/kir/go/src/github.com/opencontainers/runc/libcontainer/container_linux.go:211

[1] linux/Documentation/admin-guide/cgroup-v1/memory.rst
[2] linux/Documentation/admin-guide/cgroup-v2.rst

Signed-off-by: Kir Kolyshkin <[email protected]>

libcontainer/cgroups/fs2/memory.go

@@ -15,22 +15,38 @@ import (
 	"github.com/pkg/errors"
 )
 
+// numToStr converts an int64 value to a string for writing to a
+// cgroupv2 files with .min, .max, .low, or .high suffix.
+// Negative values are converted to "max" for cgroupv1 compatibility
+// (which used to write -1 to remove the limit).
+func numToStr(value int64) (ret string) {
+	if value > 0 {
+		ret = strconv.FormatInt(value, 10)
+	} else if value < 0 {
+		ret = "max"
+	} else {
+		ret = ""
+	}
+
+	return ret
+}
+
 func setMemory(dirPath string, cgroup *configs.Cgroup) error {
-	if cgroup.Resources.MemorySwap != 0 {
-		if err := fscommon.WriteFile(dirPath, "memory.swap.max", strconv.FormatInt(cgroup.Resources.MemorySwap, 10)); err != nil {
+	if val := numToStr(cgroup.Resources.MemorySwap); val != "" {
+		if err := fscommon.WriteFile(dirPath, "memory.swap.max", val); err != nil {
 			return err
 		}
 	}
-	if cgroup.Resources.Memory != 0 {
-		if err := fscommon.WriteFile(dirPath, "memory.max", strconv.FormatInt(cgroup.Resources.Memory, 10)); err != nil {
+	if val := numToStr(cgroup.Resources.Memory); val != "" {
+		if err := fscommon.WriteFile(dirPath, "memory.max", val); err != nil {
 			return err
 		}
 	}
 
 	// cgroup.Resources.KernelMemory is ignored
 
-	if cgroup.Resources.MemoryReservation != 0 {
-		if err := fscommon.WriteFile(dirPath, "memory.low", strconv.FormatInt(cgroup.Resources.MemoryReservation, 10)); err != nil {
+	if val := numToStr(cgroup.Resources.MemoryReservation); val != "" {
+		if err := fscommon.WriteFile(dirPath, "memory.low", val); err != nil {
 			return err
 		}
 	}

libcontainer/cgroups/fs2/pids.go

@@ -6,7 +6,6 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
-	"strconv"
 	"strings"
 
 	"github.com/opencontainers/runc/libcontainer/cgroups"
@@ -17,15 +16,8 @@ import (
 )
 
 func setPids(dirPath string, cgroup *configs.Cgroup) error {
-	if cgroup.Resources.PidsLimit != 0 {
-		// "max" is the fallback value.
-		limit := "max"
-
-		if cgroup.Resources.PidsLimit > 0 {
-			limit = strconv.FormatInt(cgroup.Resources.PidsLimit, 10)
-		}
-
-		if err := fscommon.WriteFile(dirPath, "pids.max", limit); err != nil {
+	if val := numToStr(cgroup.Resources.PidsLimit); val != "" {
+		if err := fscommon.WriteFile(dirPath, "pids.max", val); err != nil {
 			return err
 		}
 	}