Allow updating kmem.limit_in_bytes if initialized at cgroup creation

Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>

libcontainer/cgroups/fs/memory.go

@@ -5,6 +5,7 @@ package fs
 import (
 	"bufio"
 	"fmt"
+	"math"
 	"os"
 	"path/filepath"
 	"strconv"
@@ -12,6 +13,7 @@ import (
 
 	"github.com/opencontainers/runc/libcontainer/cgroups"
 	"github.com/opencontainers/runc/libcontainer/configs"
+	"github.com/opencontainers/runc/libcontainer/system"
 )
 
 type MemoryGroup struct {
@@ -33,7 +35,7 @@ func (s *MemoryGroup) Apply(d *cgroupData) (err error) {
 			}
 		}
 		// We have to set kernel memory here, as we can't change it once
-		// processes have been attached.
+		// processes have been attached to the cgroup.
 		if err := s.SetKernelMemory(path, d.config); err != nil {
 			return err
 		}
@@ -55,9 +57,44 @@ func (s *MemoryGroup) Apply(d *cgroupData) (err error) {
 }
 
 func (s *MemoryGroup) SetKernelMemory(path string, cgroup *configs.Cgroup) error {
-	// This has to be done separately because it has special constraints (it
-	// can't be done after there are processes attached to the cgroup).
-	if cgroup.Resources.KernelMemory > 0 {
+	// This has to be done separately because it has special
+	// constraints (it can only be initialized before setting up a
+	// hierarchy or adding a task to the cgroups. However, if
+	// sucessfully initialized, it can be updated anytime afterwards)
+	if cgroup.Resources.KernelMemory != 0 {
+		kmemInitialized := false
+		// Is kmem.limit_in_bytes already set?
+		kmemValue, err := getCgroupParamUint(path, "memory.kmem.limit_in_bytes")
+		if err != nil {
+			return err
+		}
+		switch system.GetLongBit() {
+		case 32:
+			kmemInitialized = uint32(kmemValue) != uint32(math.MaxUint32)
+		case 64:
+			kmemInitialized = kmemValue != uint64(math.MaxUint64)
+		}
+
+		if !kmemInitialized {
+			// If hierarchy is set, we can't change the limit
+			usesHierarchy, err := getCgroupParamUint(path, "memory.use_hierarchy")
+			if err != nil {
+				return err
+			}
+			if usesHierarchy != 0 {
+				return fmt.Errorf("cannot initialize kmem.limit_in_bytes if use_hierarchy is already set")
+			}
+
+			// If there's already tasks in the cgroup, we can't change the limit either
+			tasks, err := getCgroupParamString(path, "tasks")
+			if err != nil {
+				return err
+			}
+			if tasks != "" {
+				return fmt.Errorf("cannot initialize kmem.limit_in_bytes after task have joined this cgroup")
+			}
+		}
+
 		if err := writeFile(path, "memory.kmem.limit_in_bytes", strconv.FormatInt(cgroup.Resources.KernelMemory, 10)); err != nil {
 			return err
 		}
@@ -113,6 +150,10 @@ func (s *MemoryGroup) Set(path string, cgroup *configs.Cgroup) error {
 		return err
 	}
 
+	if err := s.SetKernelMemory(path, cgroup); err != nil {
+		return err
+	}
+
 	if cgroup.Resources.MemoryReservation != 0 {
 		if err := writeFile(path, "memory.soft_limit_in_bytes", strconv.FormatInt(cgroup.Resources.MemoryReservation, 10)); err != nil {
 			return err

libcontainer/cgroups/systemd/apply_systemd.go

@@ -214,11 +214,9 @@ func (m *Manager) Apply(pid int) error {
 			newProp("BlockIOWeight", uint64(c.Resources.BlkioWeight)))
 	}
 
-	// We need to set kernel memory before processes join cgroup because
-	// kmem.limit_in_bytes can only be set when the cgroup is empty.
-	// And swap memory limit needs to be set after memory limit, only
-	// memory limit is handled by systemd, so it's kind of ugly here.
-	if c.Resources.KernelMemory > 0 {
+	// We have to set kernel memory here, as we can't change it once
+	// processes have been attached to the cgroup.
+	if c.Resources.KernelMemory != 0 {
 		if err := setKernelMemory(c); err != nil {
 			return err
 		}
@@ -469,11 +467,5 @@ func setKernelMemory(c *configs.Cgroup) error {
 		return err
 	}
 
-	if err := os.MkdirAll(path, 0755); err != nil {
-		return err
-	}
-
-	// This doesn't get called by manager.Set, so we need to do it here.
-	s := &fs.MemoryGroup{}
-	return s.SetKernelMemory(path, c)
+	return os.MkdirAll(path, 0755)
 }

libcontainer/system/sysconfig.go

@@ -10,3 +10,7 @@ import "C"
 func GetClockTicks() int {
 	return int(C.sysconf(C._SC_CLK_TCK))
 }
+
+func GetLongBit() int {
+	return int(C.sysconf(C._SC_LONG_BIT))
+}