cgroups: nokmem: error out on explicitly-set kmemcg limits

When built with nokmem we explicitly are disabling support for kmemcg,
but it is a strict specification requirement that if we cannot fulfil an
aspect of the container configuration that we error out.

Completely ignoring explicitly-requested kmemcg limits with nokmem would
undoubtably lead to problems.

Fixes: 6a2c155968 ("libcontainer: ability to compile without kmem")
Signed-off-by: Aleksa Sarai <asarai@suse.de>

libcontainer/cgroups/fs/kmem.go

@@ -3,6 +3,7 @@
 package fs
 
 import (
+	"errors"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -17,7 +18,12 @@ import (
 const cgroupKernelMemoryLimit = "memory.kmem.limit_in_bytes"
 
 func EnableKernelMemoryAccounting(path string) error {
-	// Check if kernel memory is enabled
+	// Ensure that kernel memory is available in this kernel build. If it
+	// isn't, we just ignore it because EnableKernelMemoryAccounting is
+	// automatically called for all memory limits.
+	if !cgroups.PathExists(filepath.Join(path, cgroupKernelMemoryLimit)) {
+		return nil
+	}
 	// We have to limit the kernel memory here as it won't be accounted at all
 	// until a limit is set on the cgroup and limit cannot be set once the
 	// cgroup has children, or if there are already tasks in the cgroup.
@@ -34,8 +40,9 @@ func setKernelMemory(path string, kernelMemoryLimit int64) error {
 		return fmt.Errorf("no such directory for %s", cgroupKernelMemoryLimit)
 	}
 	if !cgroups.PathExists(filepath.Join(path, cgroupKernelMemoryLimit)) {
-		// kernel memory is not enabled on the system so we should do nothing
-		return nil
+		// We have specifically been asked to set a kmem limit. If the kernel
+		// doesn't support it we *must* error out.
+		return errors.New("kernel memory accounting not supported by this kernel")
 	}
 	if err := ioutil.WriteFile(filepath.Join(path, cgroupKernelMemoryLimit), []byte(strconv.FormatInt(kernelMemoryLimit, 10)), 0700); err != nil {
 		// Check if the error number returned by the syscall is "EBUSY"

libcontainer/cgroups/fs/kmem_disabled.go

@@ -2,10 +2,14 @@
 
 package fs
 
+import (
+	"errors"
+)
+
 func EnableKernelMemoryAccounting(path string) error {
 	return nil
 }
 
 func setKernelMemory(path string, kernelMemoryLimit int64) error {
-	return nil
+	return errors.New("kernel memory accounting disabled in this runc build")
 }