libct/cg: improve ConvertMemorySwapToCgroupV2Value

Improve readability of ConvertMemorySwapToCgroupV2Value by switching
from a bunch of if statements to a switch, and adding a comment
describing each case.

No functional change.

Signed-off-by: Kir Kolyshkin <[email protected]>
(cherry picked from commit cb9f3d6)
Signed-off-by: Akihiro Suda <[email protected]>

libcontainer/cgroups/utils.go

@@ -415,26 +415,26 @@ func ConvertCPUSharesToCgroupV2Value(cpuShares uint64) uint64 {
 
 // ConvertMemorySwapToCgroupV2Value converts MemorySwap value from OCI spec
 // for use by cgroup v2 drivers. A conversion is needed since Resources.MemorySwap
-// is defined as memory+swap combined, while in cgroup v2 swap is a separate value.
+// is defined as memory+swap combined, while in cgroup v2 swap is a separate value,
+// so we need to subtract memory from it where it makes sense.
 func ConvertMemorySwapToCgroupV2Value(memorySwap, memory int64) (int64, error) {
-	// for compatibility with cgroup1 controller, set swap to unlimited in
-	// case the memory is set to unlimited, and swap is not explicitly set,
-	// treating the request as "set both memory and swap to unlimited".
-	if memory == -1 && memorySwap == 0 {
+	switch {
+	case memory == -1 && memorySwap == 0:
+		// For compatibility with cgroup1 controller, set swap to unlimited in
+		// case the memory is set to unlimited and the swap is not explicitly set,
+		// treating the request as "set both memory and swap to unlimited".
 		return -1, nil
-	}
-	if memorySwap == -1 || memorySwap == 0 {
-		// -1 is "max", 0 is "unset", so treat as is
+	case memorySwap == -1, memorySwap == 0:
+		// Treat -1 ("max") and 0 ("unset") swap as is.
 		return memorySwap, nil
-	}
-	// sanity checks
-	if memory == 0 || memory == -1 {
+	case memory == 0, memory == -1:
+		// Unset or unlimited memory, can't calculate swap.
 		return 0, errors.New("unable to set swap limit without memory limit")
-	}
-	if memory < 0 {
+	case memory < 0:
+		// Does not make sense to subtract a negative value.
 		return 0, fmt.Errorf("invalid memory value: %d", memory)
-	}
-	if memorySwap < memory {
+	case memorySwap < memory:
+		// Sanity check.
 		return 0, errors.New("memory+swap limit should be >= memory limit")
 	}