overlay: check if metacopy is happening

nalind · nalind · commit 05c69f1b2a58 · 2019-01-17T14:14:10.000-05:00
In the overlay driver, check if metacopy is enabled, and report it along
with other status information.

Signed-off-by: Nalin Dahyabhai &lt;nalin@redhat.com&gt;
diff --git a/drivers/overlay/check.go b/drivers/overlay/check.go
@@ -10,6 +10,8 @@ import (
 	"path/filepath"
 	"syscall"
 
+	"github.com/containers/storage/pkg/ioutils"
+	"github.com/containers/storage/pkg/mount"
 	"github.com/containers/storage/pkg/system"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
@@ -57,10 +59,11 @@ func doesSupportNativeDiff(d, mountOpts string) error {
 	}
 
 	opts := fmt.Sprintf("lowerdir=%s:%s,upperdir=%s,workdir=%s", path.Join(td, "l2"), path.Join(td, "l1"), path.Join(td, "l3"), path.Join(td, "work"))
-	if mountOpts != "" {
-		opts = fmt.Sprintf("%s,%s", opts, mountOpts)
+	flags, data := mount.ParseOptions(mountOpts)
+	if data != "" {
+		opts = fmt.Sprintf("%s,%s", opts, data)
 	}
-	if err := unix.Mount("overlay", filepath.Join(td, "merged"), "overlay", 0, opts); err != nil {
+	if err := unix.Mount("overlay", filepath.Join(td, "merged"), "overlay", uintptr(flags), opts); err != nil {
 		return errors.Wrap(err, "failed to mount overlay")
 	}
 	defer func() {
@@ -103,3 +106,60 @@ func doesSupportNativeDiff(d, mountOpts string) error {
 
 	return nil
 }
+
+// doesMetacopy checks if the filesystem is going to optimize changes to
+// metadata by using nodes marked with an "overlay.metacopy" attribute to avoid
+// copying up a file from a lower layer unless/until its contents are being
+// modified
+func doesMetacopy(d, mountOpts string) (bool, error) {
+	td, err := ioutil.TempDir(d, "metacopy-check")
+	if err != nil {
+		return false, err
+	}
+	defer func() {
+		if err := os.RemoveAll(td); err != nil {
+			logrus.Warnf("Failed to remove check directory %v: %v", td, err)
+		}
+	}()
+
+	// Make directories l1, l2, work, merged
+	if err := os.MkdirAll(filepath.Join(td, "l1"), 0755); err != nil {
+		return false, err
+	}
+	if err := ioutils.AtomicWriteFile(filepath.Join(td, "l1", "f"), []byte{0xff}, 0700); err != nil {
+		return false, err
+	}
+	if err := os.MkdirAll(filepath.Join(td, "l2"), 0755); err != nil {
+		return false, err
+	}
+	if err := os.Mkdir(filepath.Join(td, "work"), 0755); err != nil {
+		return false, err
+	}
+	if err := os.Mkdir(filepath.Join(td, "merged"), 0755); err != nil {
+		return false, err
+	}
+	// Mount using the mandatory options and configured options
+	opts := fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", path.Join(td, "l1"), path.Join(td, "l2"), path.Join(td, "work"))
+	flags, data := mount.ParseOptions(mountOpts)
+	if data != "" {
+		opts = fmt.Sprintf("%s,%s", opts, data)
+	}
+	if err := unix.Mount("overlay", filepath.Join(td, "merged"), "overlay", uintptr(flags), opts); err != nil {
+		return false, errors.Wrap(err, "failed to mount overlay for metacopy check")
+	}
+	defer func() {
+		if err := unix.Unmount(filepath.Join(td, "merged"), 0); err != nil {
+			logrus.Warnf("Failed to unmount check directory %v: %v", filepath.Join(td, "merged"), err)
+		}
+	}()
+	// Make a change that only impacts the inode, and check if the pulled-up copy is marked
+	// as a metadata-only copy
+	if err := os.Chmod(filepath.Join(td, "merged", "f"), 0600); err != nil {
+		return false, errors.Wrap(err, "error changing permissions on file for metacopy check")
+	}
+	metacopy, err := system.Lgetxattr(filepath.Join(td, "l2", "f"), "trusted.overlay.metacopy")
+	if err != nil {
+		return false, errors.Wrap(err, "metacopy flag was not set on file in upper layer")
+	}
+	return metacopy != nil, nil
+}
diff --git a/drivers/overlay/overlay.go b/drivers/overlay/overlay.go
@@ -105,6 +105,7 @@ type Driver struct {
 	options       overlayOptions
 	naiveDiff     graphdriver.DiffDriver
 	supportsDType bool
+	usingMetacopy bool
 	locker        *locker.Locker
 	convert       map[string]bool
 }
@@ -174,6 +175,18 @@ func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (grap
 		}
 	}
 
+	usingMetacopy, err := doesMetacopy(home, opts.mountOptions)
+	if err == nil {
+		if usingMetacopy {
+			logrus.Debugf("overlay test mount indicated that metacopy is being used")
+		} else {
+			logrus.Debugf("overlay test mount indicated that metacopy is not being used")
+		}
+	} else {
+		logrus.Warnf("overlay test mount did not indicate whether or not metacopy is being used: %v", err)
+		return nil, err
+	}
+
 	if !opts.skipMountHome {
 		if err := mount.MakePrivate(home); err != nil {
 			return nil, err
@@ -193,6 +206,7 @@ func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (grap
 		gidMaps:       gidMaps,
 		ctr:           graphdriver.NewRefCounter(graphdriver.NewFsChecker(graphdriver.FsMagicOverlay)),
 		supportsDType: supportsDType,
+		usingMetacopy: usingMetacopy,
 		locker:        locker.New(),
 		options:       *opts,
 		convert:       make(map[string]bool),
@@ -212,7 +226,7 @@ func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (grap
 		return nil, fmt.Errorf("Storage option overlay.size only supported for backingFS XFS. Found %v", backingFs)
 	}
 
-	logrus.Debugf("backingFs=%s, projectQuotaSupported=%v, useNativeDiff=%v", backingFs, projectQuotaSupported, !d.useNaiveDiff())
+	logrus.Debugf("backingFs=%s, projectQuotaSupported=%v, useNativeDiff=%v, usingMetacopy=%v", backingFs, projectQuotaSupported, !d.useNaiveDiff(), d.usingMetacopy)
 
 	return d, nil
 }
@@ -375,6 +389,7 @@ func (d *Driver) Status() [][2]string {
 		{"Backing Filesystem", backingFs},
 		{"Supports d_type", strconv.FormatBool(d.supportsDType)},
 		{"Native Overlay Diff", strconv.FormatBool(!d.useNaiveDiff())},
+		{"Using metacopy", strconv.FormatBool(d.usingMetacopy)},
 	}
 }
 
