Add vfs.FilesystemImplSaveRestoreExtension.PostSave.

This allows filesystem implementations to cleanup S/R state which is not needed
after resume. This state was being cleaned up on restore, but not resume.

Updates #11425

PiperOrigin-RevId: 728591228

pkg/sentry/fsimpl/gofer/save_restore.go

@@ -31,6 +31,8 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 )
 
+var _ vfs.FilesystemImplSaveRestoreExtension = (*filesystem)(nil)
+
 // +stateify savable
 type savedDentryRW struct {
 	read  bool
@@ -134,6 +136,11 @@ func (d *dentry) beforeSave() {
 	}
 }
 
+// PostSave implements vfs.FilesystemImplSaveRestoreExtension.PostSave.
+func (fs *filesystem) PostSave(ctx context.Context) {
+	fs.savedDentryRW = nil
+}
+
 // afterLoad is invoked by stateify.
 func (fs *filesystem) afterLoad(ctx goContext.Context) {
 	fs.mf = pgalloc.MemoryFileFromContext(ctx)

pkg/sentry/fsimpl/tmpfs/save_restore.go

@@ -23,6 +23,8 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 )
 
+var _ vfs.FilesystemImplSaveRestoreExtension = (*filesystem)(nil)
+
 // saveMf is called by stateify.
 func (fs *filesystem) saveMf() string {
 	if !fs.mf.IsSavable() {
@@ -75,6 +77,9 @@ func (fs *filesystem) PrepareSave(ctx context.Context) error {
 	return nil
 }
 
+// PostSave implements vfs.FilesystemImplSaveRestoreExtension.PostSave.
+func (fs *filesystem) PostSave(ctx context.Context) {}
+
 // CompleteRestore implements
 // vfs.FilesystemImplSaveRestoreExtension.CompleteRestore.
 func (fs *filesystem) CompleteRestore(ctx context.Context, opts vfs.CompleteRestoreOptions) error {

pkg/sentry/kernel/kernel.go

@@ -698,6 +698,8 @@ func (k *Kernel) SaveTo(ctx context.Context, w, pagesMetadata io.Writer, pagesFi
 		return mfSaveErr
 	}
 
+	k.vfs.PostSave(ctx)
+
 	log.Infof("Overall save took [%s].", time.Since(saveStart))
 	return nil
 }

pkg/sentry/vfs/save_restore.go

@@ -54,6 +54,10 @@ type FilesystemImplSaveRestoreExtension interface {
 	// PrepareSave prepares this filesystem for serialization.
 	PrepareSave(ctx context.Context) error
 
+	// PostSave is invoked after save. It can be used to clean up any state that
+	// should be discarded after save.
+	PostSave(ctx context.Context)
+
 	// CompleteRestore completes restoration from checkpoint for this
 	// filesystem after deserialization.
 	CompleteRestore(ctx context.Context, opts CompleteRestoreOptions) error
@@ -73,6 +77,16 @@ func (vfs *VirtualFilesystem) PrepareSave(ctx context.Context) error {
 	return nil
 }
 
+// PostSave is called after save and allows filesystems to clean up S/R state.
+func (vfs *VirtualFilesystem) PostSave(ctx context.Context) {
+	for fs := range vfs.getFilesystems() {
+		if ext, ok := fs.impl.(FilesystemImplSaveRestoreExtension); ok {
+			ext.PostSave(ctx)
+		}
+		fs.DecRef(ctx)
+	}
+}
+
 // CompleteRestore completes restoration from checkpoint for all filesystems
 // after deserialization.
 func (vfs *VirtualFilesystem) CompleteRestore(ctx context.Context, opts *CompleteRestoreOptions) error {