Remove kernel.Kernel.savedMFOwners.

Replace it with a savable struct. Such checkpoint/restore metadata information
does not belong in the Kernel struct. Also refactor logic to save/load private
memory files into one place. This will be useful later when we implement
checkpoint restore for multicontainer, in which case there will multiple sets
of saved private memory files, but only 1 kernel.

PiperOrigin-RevId: 590069275

pkg/sentry/kernel/kernel.go

@@ -332,10 +332,6 @@ type Kernel struct {
 	// devGofers maps container ID to its device gofer client.
 	devGofers   map[string]*devutil.GoferClient `state:"nosave"`
 	devGofersMu sync.Mutex                      `state:"nosave"`
-
-	// savedMFOwners is the list of filesystem unique IDs (in order) for
-	// filesystems that have saved their MemoryFile in the state file.
-	savedMFOwners []string
 }
 
 // InitKernelArgs holds arguments to Init.
@@ -507,6 +503,57 @@ func (k *Kernel) Init(args InitKernelArgs) error {
 	return nil
 }
 
+// +stateify savable
+type privateMemoryFileMetadata struct {
+	owners []string
+}
+
+func savePrivateMFs(ctx context.Context, w wire.Writer, mfsToSave map[string]*pgalloc.MemoryFile) error {
+	var meta privateMemoryFileMetadata
+	// Generate the order in which private memory files are saved.
+	for fsID := range mfsToSave {
+		meta.owners = append(meta.owners, fsID)
+	}
+	// Save the metadata.
+	if _, err := state.Save(ctx, w, &meta); err != nil {
+		return err
+	}
+	// Followed by the private memory files in order.
+	for _, fsID := range meta.owners {
+		if err := mfsToSave[fsID].SaveTo(ctx, w); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func loadPrivateMFs(ctx context.Context, r wire.Reader) error {
+	// Load the metadata.
+	var meta privateMemoryFileMetadata
+	if _, err := state.Load(ctx, r, &meta); err != nil {
+		return err
+	}
+	var mfmap map[string]*pgalloc.MemoryFile
+	if mfmapv := ctx.Value(vfs.CtxFilesystemMemoryFileMap); mfmapv != nil {
+		mfmap = mfmapv.(map[string]*pgalloc.MemoryFile)
+	}
+	// Ensure that it is consistent with CtxFilesystemMemoryFileMap.
+	if len(mfmap) != len(meta.owners) {
+		return fmt.Errorf("inconsistent private memory files on restore: savedMFOwners = %v, CtxFilesystemMemoryFileMap = %v", meta.owners, mfmap)
+	}
+	// Load all private memory files.
+	for _, fsID := range meta.owners {
+		mf, ok := mfmap[fsID]
+		if !ok {
+			return fmt.Errorf("saved memory file for %q was not configured on restore", fsID)
+		}
+		if err := mf.LoadFrom(ctx, r); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 // SaveTo saves the state of k to w.
 //
 // Preconditions: The kernel must be paused throughout the call to SaveTo.
@@ -539,12 +586,6 @@ func (k *Kernel) SaveTo(ctx context.Context, w wire.Writer) error {
 	if err := k.vfs.PrepareSave(vfsCtx); err != nil {
 		return err
 	}
-	// Generate the order in which memory files are saved in the state file.
-	// Note that this must happen before saving the kernel state.
-	k.savedMFOwners = make([]string, 0, len(mfsToSave))
-	for fsID := range mfsToSave {
-		k.savedMFOwners = append(k.savedMFOwners, fsID)
-	}
 
 	// Save the CPUID FeatureSet before the rest of the kernel so we can
 	// verify its compatibility on restore before attempting to restore the
@@ -582,10 +623,8 @@ func (k *Kernel) SaveTo(ctx context.Context, w wire.Writer) error {
 	if err := k.mf.SaveTo(ctx, w); err != nil {
 		return err
 	}
-	for _, fsID := range k.savedMFOwners {
-		if err := mfsToSave[fsID].SaveTo(ctx, w); err != nil {
-			return err
-		}
+	if err := savePrivateMFs(ctx, w, mfsToSave); err != nil {
+		return err
 	}
 	log.Infof("Memory files save took [%s].", time.Since(memoryStart))
 
@@ -665,21 +704,8 @@ func (k *Kernel) LoadFrom(ctx context.Context, r wire.Reader, timeReady chan str
 	if err := k.mf.LoadFrom(ctx, r); err != nil {
 		return err
 	}
-	var mfmap map[string]*pgalloc.MemoryFile
-	if mfmapv := ctx.Value(vfs.CtxFilesystemMemoryFileMap); mfmapv != nil {
-		mfmap = mfmapv.(map[string]*pgalloc.MemoryFile)
-	}
-	if len(mfmap) != len(k.savedMFOwners) {
-		return fmt.Errorf("inconsistent private memory files on restore: savedMFOwners = %v, CtxFilesystemMemoryFileMap = %v", k.savedMFOwners, mfmap)
-	}
-	for _, fsID := range k.savedMFOwners {
-		mf, ok := mfmap[fsID]
-		if !ok {
-			return fmt.Errorf("saved memory file for %q was not configured on restore", fsID)
-		}
-		if err := mf.LoadFrom(ctx, r); err != nil {
-			return err
-		}
+	if err := loadPrivateMFs(ctx, r); err != nil {
+		return err
 	}
 	log.Infof("Memory files load took [%s].", time.Since(memoryStart))