Support >= 128 layers in overlayfs snapshots

Auto-detect longest common dir in lowerdir option and compact it if the
option size is hitting one page size. If does, Use chdir + CLONE to do
mount thing to avoid hitting one page argument buffer in linux kernel
mount.

Signed-off-by: Wei Fu <fhfuwei@163.com>

mount/mount_linux.go

@@ -17,16 +17,41 @@
 package mount
 
 import (
+	"fmt"
+	"os"
+	"path"
 	"strings"
 	"time"
 
+	"github.com/containerd/containerd/sys"
 	"github.com/pkg/errors"
 	"golang.org/x/sys/unix"
 )
 
+var pagesize = 4096
+
+func init() {
+	pagesize = os.Getpagesize()
+}
+
 // Mount to the provided target path
 func (m *Mount) Mount(target string) error {
-	flags, data := parseMountOptions(m.Options)
+	var (
+		chdir   string
+		options = m.Options
+	)
+
+	// avoid hitting one page limit of mount argument buffer
+	//
+	// NOTE: 512 is a buffer during pagesize check.
+	if m.Type == "overlay" && optionsSize(options) >= pagesize-512 {
+		chdir, options = compactLowerdirOption(options)
+	}
+
+	flags, data := parseMountOptions(options)
+	if len(data) > pagesize {
+		return errors.Errorf("mount options is too long")
+	}
 
 	// propagation types.
 	const ptypes = unix.MS_SHARED | unix.MS_PRIVATE | unix.MS_SLAVE | unix.MS_UNBINDABLE
@@ -38,7 +63,7 @@ func (m *Mount) Mount(target string) error {
 	if flags&unix.MS_REMOUNT == 0 || data != "" {
 		// Initial call applying all non-propagation flags for mount
 		// or remount with changed data
-		if err := unix.Mount(m.Source, target, m.Type, uintptr(oflags), data); err != nil {
+		if err := mountAt(chdir, m.Source, target, m.Type, uintptr(oflags), data); err != nil {
 			return err
 		}
 	}
@@ -155,3 +180,129 @@ func parseMountOptions(options []string) (int, string) {
 	}
 	return flag, strings.Join(data, ",")
 }
+
+// compactLowerdirOption updates overlay lowdir option and returns the common
+// dir among all the lowdirs.
+func compactLowerdirOption(opts []string) (string, []string) {
+	idx, dirs := findOverlayLowerdirs(opts)
+	if idx == -1 || len(dirs) == 1 {
+		// no need to compact if there is only one lowerdir
+		return "", opts
+	}
+
+	// find out common dir
+	commondir := longestCommonPrefix(dirs)
+	if commondir == "" {
+		return "", opts
+	}
+
+	// NOTE: the snapshot id is based on digits.
+	// in order to avoid to get snapshots/x, should be back to parent dir.
+	// however, there is assumption that the common dir is ${root}/io.containerd.v1.overlayfs/snapshots.
+	commondir = path.Dir(commondir)
+	if commondir == "/" {
+		return "", opts
+	}
+	commondir = commondir + "/"
+
+	newdirs := make([]string, 0, len(dirs))
+	for _, dir := range dirs {
+		newdirs = append(newdirs, dir[len(commondir):])
+	}
+
+	newopts := copyOptions(opts)
+	newopts = append(newopts[:idx], newopts[idx+1:]...)
+	newopts = append(newopts, fmt.Sprintf("lowerdir=%s", strings.Join(newdirs, ":")))
+	return commondir, newopts
+}
+
+// findOverlayLowerdirs returns the index of lowerdir in mount's options and
+// all the lowerdir target.
+func findOverlayLowerdirs(opts []string) (int, []string) {
+	var (
+		idx    = -1
+		prefix = "lowerdir="
+	)
+
+	for i, opt := range opts {
+		if strings.HasPrefix(opt, prefix) {
+			idx = i
+			break
+		}
+	}
+
+	if idx == -1 {
+		return -1, nil
+	}
+	return idx, strings.Split(opts[idx][len(prefix):], ":")
+}
+
+// longestCommonPrefix finds the longest common prefix in the string slice.
+func longestCommonPrefix(strs []string) string {
+	if len(strs) == 0 {
+		return ""
+	} else if len(strs) == 1 {
+		return strs[0]
+	}
+
+	// find out the min/max value by alphabetical order
+	min, max := strs[0], strs[0]
+	for _, str := range strs[1:] {
+		if min > str {
+			min = str
+		}
+		if max < str {
+			max = str
+		}
+	}
+
+	// find out the common part between min and max
+	for i := 0; i < len(min) && i < len(max); i++ {
+		if min[i] != max[i] {
+			return min[:i]
+		}
+	}
+	return min
+}
+
+// copyOptions copies the options.
+func copyOptions(opts []string) []string {
+	if len(opts) == 0 {
+		return nil
+	}
+
+	acopy := make([]string, len(opts))
+	copy(acopy, opts)
+	return acopy
+}
+
+// optionsSize returns the byte size of options of mount.
+func optionsSize(opts []string) int {
+	size := 0
+	for _, opt := range opts {
+		size += len(opt)
+	}
+	return size
+}
+
+func mountAt(chdir string, source, target, fstype string, flags uintptr, data string) error {
+	if chdir == "" {
+		return unix.Mount(source, target, fstype, flags, data)
+	}
+
+	f, err := os.Open(chdir)
+	if err != nil {
+		return errors.Wrap(err, "failed to mountat")
+	}
+	defer f.Close()
+
+	fs, err := f.Stat()
+	if err != nil {
+		return errors.Wrap(err, "failed to mountat")
+	}
+
+	if !fs.IsDir() {
+		return errors.Wrap(errors.Errorf("%s is not dir", chdir), "failed to mountat")
+	}
+	return errors.Wrap(sys.FMountat(f.Fd(), source, target, fstype, flags, data), "failed to mountat")
+}

mount/mount_linux_test.go

@@ -0,0 +1,94 @@
+// +build linux
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package mount
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestLongestCommonPrefix(t *testing.T) {
+	tcases := []struct {
+		in       []string
+		expected string
+	}{
+		{[]string{}, ""},
+		{[]string{"foo"}, "foo"},
+		{[]string{"foo", "bar"}, ""},
+		{[]string{"foo", "foo"}, "foo"},
+		{[]string{"foo", "foobar"}, "foo"},
+		{[]string{"foo", "", "foobar"}, ""},
+	}
+
+	for i, tc := range tcases {
+		if got := longestCommonPrefix(tc.in); got != tc.expected {
+			t.Fatalf("[%d case] expected (%s), but got (%s)", i+1, tc.expected, got)
+		}
+	}
+}
+
+func TestCompactLowerdirOption(t *testing.T) {
+	tcases := []struct {
+		opts      []string
+		commondir string
+		newopts   []string
+	}{
+		// no lowerdir or only one
+		{
+			[]string{"workdir=a"},
+			"",
+			[]string{"workdir=a"},
+		},
+		{
+			[]string{"workdir=a", "lowerdir=b"},
+			"",
+			[]string{"workdir=a", "lowerdir=b"},
+		},
+
+		// >= 2 lowerdir
+		{
+			[]string{"lowerdir=/snapshots/1/fs:/snapshots/10/fs"},
+			"/snapshots/",
+			[]string{"lowerdir=1/fs:10/fs"},
+		},
+		{
+			[]string{"lowerdir=/snapshots/1/fs:/snapshots/10/fs:/snapshots/2/fs"},
+			"/snapshots/",
+			[]string{"lowerdir=1/fs:10/fs:2/fs"},
+		},
+
+		// if common dir is /
+		{
+			[]string{"lowerdir=/snapshots/1/fs:/other_snapshots/1/fs"},
+			"",
+			[]string{"lowerdir=/snapshots/1/fs:/other_snapshots/1/fs"},
+		},
+	}
+
+	for i, tc := range tcases {
+		dir, opts := compactLowerdirOption(tc.opts)
+		if dir != tc.commondir {
+			t.Fatalf("[%d case] expected common dir (%s), but got (%s)", i+1, tc.commondir, dir)
+		}
+
+		if !reflect.DeepEqual(opts, tc.newopts) {
+			t.Fatalf("[%d case] expected options (%v), but got (%v)", i+1, tc.newopts, opts)
+		}
+	}
+}

snapshots/testsuite/testsuite.go

@@ -63,6 +63,8 @@ func SnapshotterSuite(t *testing.T, name string, snapshotterFn func(ctx context.
 	t.Run("StatInWalk", makeTest(name, snapshotterFn, checkStatInWalk))
 	t.Run("CloseTwice", makeTest(name, snapshotterFn, closeTwice))
 	t.Run("RootPermission", makeTest(name, snapshotterFn, checkRootPermission))
+
+	t.Run("128LayersMount", makeTest(name, snapshotterFn, check128LayersMount))
 }
 
 func makeTest(name string, snapshotterFn func(ctx context.Context, root string) (snapshots.Snapshotter, func() error, error), fn func(ctx context.Context, t *testing.T, snapshotter snapshots.Snapshotter, work string)) func(t *testing.T) {
@@ -858,3 +860,94 @@ func checkRootPermission(ctx context.Context, t *testing.T, snapshotter snapshot
 		t.Fatalf("expected 0755, got 0%o", mode)
 	}
 }
+
+func check128LayersMount(ctx context.Context, t *testing.T, snapshotter snapshots.Snapshotter, work string) {
+	lowestApply := fstest.Apply(
+		fstest.CreateFile("/bottom", []byte("way at the bottom\n"), 0777),
+		fstest.CreateFile("/overwriteme", []byte("FIRST!\n"), 0777),
+		fstest.CreateDir("/ADDHERE", 0755),
+		fstest.CreateDir("/ONLYME", 0755),
+		fstest.CreateFile("/ONLYME/bottom", []byte("bye!\n"), 0777),
+	)
+
+	appliers := []fstest.Applier{lowestApply}
+	for i := 1; i <= 127; i++ {
+		appliers = append(appliers, fstest.Apply(
+			fstest.CreateFile("/overwriteme", []byte(fmt.Sprintf("%d WAS HERE!\n", i)), 0777),
+			fstest.CreateFile(fmt.Sprintf("/ADDHERE/file-%d", i), []byte("same\n"), 0755),
+			fstest.RemoveAll("/ONLYME"),
+			fstest.CreateDir("/ONLYME", 0755),
+			fstest.CreateFile(fmt.Sprintf("/ONLYME/file-%d", i), []byte("only me!\n"), 0777),
+		))
+	}
+
+	flat := filepath.Join(work, "flat")
+	if err := os.MkdirAll(flat, 0777); err != nil {
+		t.Fatalf("failed to create flat dir(%s): %+v", flat, err)
+	}
+
+	// NOTE: add gc labels to avoid snapshots get removed by gc...
+	parent := ""
+	for i, applier := range appliers {
+		preparing := filepath.Join(work, fmt.Sprintf("prepare-layer-%d", i))
+		if err := os.MkdirAll(preparing, 0777); err != nil {
+			t.Fatalf("[layer %d] failed to create preparing dir(%s): %+v", i, preparing, err)
+		}
+
+		mounts, err := snapshotter.Prepare(ctx, preparing, parent, opt)
+		if err != nil {
+			t.Fatalf("[layer %d] failed to get mount info: %+v", i, err)
+		}
+
+		if err := mount.All(mounts, preparing); err != nil {
+			t.Fatalf("[layer %d] failed to mount on the target(%s): %+v", i, preparing, err)
+		}
+
+		if err := fstest.CheckDirectoryEqual(preparing, flat); err != nil {
+			testutil.Unmount(t, preparing)
+			t.Fatalf("[layer %d] preparing doesn't equal to flat before apply: %+v", i, err)
+		}
+
+		if err := applier.Apply(flat); err != nil {
+			testutil.Unmount(t, preparing)
+			t.Fatalf("[layer %d] failed to apply on flat dir: %+v", i, err)
+		}
+
+		if err = applier.Apply(preparing); err != nil {
+			testutil.Unmount(t, preparing)
+			t.Fatalf("[layer %d] failed to apply on preparing dir: %+v", i, err)
+		}
+
+		if err := fstest.CheckDirectoryEqual(preparing, flat); err != nil {
+			testutil.Unmount(t, preparing)
+			t.Fatalf("[layer %d] preparing doesn't equal to flat after apply: %+v", i, err)
+		}
+
+		testutil.Unmount(t, preparing)
+
+		parent = filepath.Join(work, fmt.Sprintf("committed-%d", i))
+		if err := snapshotter.Commit(ctx, parent, preparing, opt); err != nil {
+			t.Fatalf("[layer %d] failed to commit the preparing: %+v", i, err)
+		}
+
+	}
+
+	view := filepath.Join(work, "fullview")
+	if err := os.MkdirAll(view, 0777); err != nil {
+		t.Fatalf("failed to create fullview dir(%s): %+v", view, err)
+	}
+
+	mounts, err := snapshotter.View(ctx, view, parent, opt)
+	if err != nil {
+		t.Fatalf("failed to get view's mount info: %+v", err)
+	}
+
+	if err := mount.All(mounts, view); err != nil {
+		t.Fatalf("failed to mount on the target(%s): %+v", view, err)
+	}
+	defer testutil.Unmount(t, view)
+
+	if err := fstest.CheckDirectoryEqual(view, flat); err != nil {
+		t.Fatalf("fullview should equal to flat: %+v", err)
+	}
+}