Enabled writeback caching by default.

This may bring significant performance improvements, but also comes with
some caveats. See the notes on MountConfig.DisableWritebackCaching for
details.

connection.go

@@ -57,6 +57,7 @@ const maxReadahead = 1 << 20
 
 // A connection to the fuse kernel process.
 type Connection struct {
+	cfg         MountConfig
 	debugLogger *log.Logger
 	errorLogger *log.Logger
 
@@ -65,9 +66,6 @@ type Connection struct {
 	dev      *os.File
 	protocol fusekernel.Protocol
 
-	// The context from which all op contexts inherit.
-	parentCtx context.Context
-
 	mu sync.Mutex
 
 	// A map from fuse "unique" request ID (*not* the op ID for logging used
@@ -94,15 +92,15 @@ type opState struct {
 //
 // The loggers may be nil.
 func newConnection(
-	parentCtx context.Context,
+	cfg MountConfig,
 	debugLogger *log.Logger,
 	errorLogger *log.Logger,
 	dev *os.File) (c *Connection, err error) {
 	c = &Connection{
+		cfg:         cfg,
 		debugLogger: debugLogger,
 		errorLogger: errorLogger,
 		dev:         dev,
-		parentCtx:   parentCtx,
 		cancelFuncs: make(map[uint64]func()),
 	}
 
@@ -165,6 +163,11 @@ func (c *Connection) Init() (err error) {
 	// Tell the kernel not to use pitifully small 4 KiB writes.
 	initOp.Flags |= fusekernel.InitBigWrites
 
+	// Enable writeback caching if the user hasn't asked us not to.
+	if !c.cfg.DisableWritebackCaching {
+		initOp.Flags |= fusekernel.InitWritebackCache
+	}
+
 	c.Reply(ctx, nil)
 	return
 }
@@ -227,7 +230,7 @@ func (c *Connection) beginOp(
 	opCode uint32,
 	fuseID uint64) (ctx context.Context) {
 	// Start with the parent context.
-	ctx = c.parentCtx
+	ctx = c.cfg.OpContext
 
 	// Set up a cancellation function.
 	//

debug.go

@@ -57,6 +57,23 @@ func describeRequest(op interface{}) (s string) {
 		addComponent("parent %d", typed.Parent)
 		addComponent("name %q", typed.Name)
 
+	case *fuseops.SetInodeAttributesOp:
+		if typed.Size != nil {
+			addComponent("size %d", *typed.Size)
+		}
+
+		if typed.Mode != nil {
+			addComponent("mode %v", *typed.Mode)
+		}
+
+		if typed.Atime != nil {
+			addComponent("atime %v", *typed.Atime)
+		}
+
+		if typed.Mtime != nil {
+			addComponent("mtime %v", *typed.Mtime)
+		}
+
 	case *fuseops.ReadFileOp:
 		addComponent("handle %d", typed.Handle)
 		addComponent("offset %d", typed.Offset)

fuseops/ops.go

@@ -509,17 +509,8 @@ type ReadFileOp struct {
 //
 // Note that the kernel *will* ensure that writes are received and acknowledged
 // by the file system before sending a FlushFileOp when closing the file
-// descriptor to which they were written:
+// descriptor to which they were written. Cf. the notes on
-//
+// fuse.MountConfig.DisableWritebackCaching.
-//  *  (http://goo.gl/PheZjf) fuse_flush calls write_inode_now, which appears
-//     to start a writeback in the background (it talks about a "flusher
-//     thread").
-//
-//  *  (http://goo.gl/1IiepM) fuse_flush then calls fuse_sync_writes, which
-//     "[waits] for all pending writepages on the inode to finish".
-//
-//  *  (http://goo.gl/zzvxWv) Only then does fuse_flush finally send the
-//     flush request.
 //
 // (See also http://goo.gl/ocdTdM, fuse-devel thread "Fuse guarantees on
 // concurrent requests".)

fusetesting/stat.go

@@ -28,28 +28,32 @@ import (
 // also that it matches.
 func MtimeIs(expected time.Time) oglematchers.Matcher {
 	return oglematchers.NewMatcher(
-		func(c interface{}) error { return mtimeIs(c, expected) },
+		func(c interface{}) error { return mtimeIsWithin(c, expected, 0) },
 		fmt.Sprintf("mtime is %v", expected))
 }
 
-func mtimeIs(c interface{}, expected time.Time) error {
+// Like MtimeIs, but allows for a tolerance.
+func MtimeIsWithin(expected time.Time, d time.Duration) oglematchers.Matcher {
+	return oglematchers.NewMatcher(
+		func(c interface{}) error { return mtimeIsWithin(c, expected, d) },
+		fmt.Sprintf("mtime is within %v of %v", d, expected))
+}
+
+func mtimeIsWithin(c interface{}, expected time.Time, d time.Duration) error {
 	fi, ok := c.(os.FileInfo)
 	if !ok {
 		return fmt.Errorf("which is of type %v", reflect.TypeOf(c))
 	}
 
 	// Check ModTime().
-	if fi.ModTime() != expected {
+	diff := fi.ModTime().Sub(expected)
-		d := fi.ModTime().Sub(expected)
+	absDiff := diff
-		return fmt.Errorf("which has mtime %v, off by %v", fi.ModTime(), d)
+	if absDiff < 0 {
+		absDiff = -absDiff
 	}
 
-	// Check Sys().
+	if !(absDiff < d) {
-	if sysMtime, ok := extractMtime(fi.Sys()); ok {
+		return fmt.Errorf("which has mtime %v, off by %v", fi.ModTime(), diff)
-		if sysMtime != expected {
-			d := sysMtime.Sub(expected)
-			return fmt.Errorf("which has Sys() mtime %v, off by %v", sysMtime, d)
-		}
 	}
 
 	return nil

mount.go

@@ -67,14 +67,14 @@ func Mount(
 	}
 
 	// Choose a parent context for ops.
-	opContext := config.OpContext
+	cfgCopy := *config
-	if opContext == nil {
+	if cfgCopy.OpContext == nil {
-		opContext = context.Background()
+		cfgCopy.OpContext = context.Background()
 	}
 
 	// Create a Connection object wrapping the device.
 	connection, err := newConnection(
-		opContext,
+		cfgCopy,
 		config.DebugLogger,
 		config.ErrorLogger,
 		dev)

mount_config.go

@@ -48,6 +48,71 @@ type MountConfig struct {
 	// performed.
 	DebugLogger *log.Logger
 
+	// Linux only. OS X always behaves as if writeback caching is disabled.
+	//
+	// By default on Linux we allow the kernel to perform writeback caching
+	// (cf. http://goo.gl/LdZzo1):
+	//
+	// *   When the user calls write(2), the kernel sticks the user's data into
+	//     its page cache. Only later does it call through to the file system,
+	//     potentially after coalescing multiple small user writes.
+	//
+	// *   The file system may receive multiple write ops from the kernel
+	//     concurrently if there is a lot of page cache data to flush.
+	//
+	// *   Write performance may be significantly improved due to the user and
+	//     the kernel not waiting for serial round trips to the file system. This
+	//     is especially true if the user makes tiny writes.
+	//
+	// *   close(2) (and anything else calling f_op->flush) causes all dirty
+	//     pages to be written out before it proceeds to send a FlushFileOp
+	//     (cf. https://goo.gl/TMrY6X).
+	//
+	// *   Similarly, close(2) causes the kernel to send a setattr request
+	//     filling in the mtime if any dirty pages were flushed, since the time
+	//     at which the pages were written to the file system can't be trusted.
+	//
+	// *   close(2) (and anything else calling f_op->flush) writes out all dirty
+	//     pages, then sends a setattr request with an appropriate mtime for
+	//     those writes if there were any, and only then proceeds to send a flush
+	//
+	//     Code walk:
+	//
+	//     *   (https://goo.gl/zTIZQ9) fuse_flush calls write_inode_now before
+	//         calling the file system. The latter eventually calls into
+	//         __writeback_single_inode.
+	//
+	//     *   (https://goo.gl/L7Z2w5) __writeback_single_inode calls
+	//         do_writepages, which writes out any dirty pages.
+	//
+	//     *   (https://goo.gl/DOPgla) __writeback_single_inode later calls
+	//         write_inode, which calls into the superblock op struct's write_inode
+	//         member. For fuse, this is fuse_write_inode
+	//         (cf. https://goo.gl/eDSKOX).
+	//
+	//     *   (https://goo.gl/PbkGA1) fuse_write_inode calls fuse_flush_times.
+	//
+	//     *   (https://goo.gl/ig8x9V) fuse_flush_times sends a setttr request
+	//         for setting the inode's mtime.
+	//
+	// However, this brings along some caveats:
+	//
+	// *   The file system must handle SetInodeAttributesOp or close(2) will fail,
+	//     due to the call chain into fuse_flush_times listed above.
+	//
+	// *   The kernel caches mtime and ctime regardless of whether the file
+	//     system tells it to do so, disregarding the result of further getattr
+	//     requests (cf. https://goo.gl/3ZZMUw, https://goo.gl/7WtQUp). It
+	//     appears this may be true of the file size, too. Writeback caching may
+	//     therefore not be suitable for file systems where these attributes can
+	//     spontaneously change for reasons the kernel doesn't observe. See
+	//     http://goo.gl/V5WQCN for more discussion.
+	//
+	// Setting DisableWritebackCaching disables this behavior. Instead the file
+	// system is called one or more times for each write(2), and the user's
+	// syscall doesn't return until the file system returns.
+	DisableWritebackCaching bool
+
 	// OS X only.
 	//
 	// Normally on OS X we mount with the novncache option

samples/cachingfs/caching_fs_test.go

@@ -53,6 +53,10 @@ func (t *cachingFSTest) setUp(
 	getattrTimeout time.Duration) {
 	var err error
 
+	// We assert things about whether or not mtimes are cached, but writeback
+	// caching causes them to always be cached. Turn it off.
+	t.MountConfig.DisableWritebackCaching = true
+
 	// Create the file system.
 	t.fs, err = cachingfs.NewCachingFS(lookupEntryTimeout, getattrTimeout)
 	AssertEq(nil, err)

samples/flushfs/flush_fs.go

@@ -90,6 +90,29 @@ func (fs *flushFS) barAttributes() fuseops.InodeAttributes {
 	}
 }
 
+// LOCKS_REQUIRED(fs.mu)
+func (fs *flushFS) getAttributes(id fuseops.InodeID) (
+	attrs fuseops.InodeAttributes,
+	err error) {
+	switch id {
+	case fuseops.RootInodeID:
+		attrs = fs.rootAttributes()
+		return
+
+	case fooID:
+		attrs = fs.fooAttributes()
+		return
+
+	case barID:
+		attrs = fs.barAttributes()
+		return
+
+	default:
+		err = fuse.ENOENT
+		return
+	}
+}
+
 ////////////////////////////////////////////////////////////////////////
 // FileSystem methods
 ////////////////////////////////////////////////////////////////////////
@@ -134,23 +157,20 @@ func (fs *flushFS) GetInodeAttributes(
 	fs.mu.Lock()
 	defer fs.mu.Unlock()
 
-	switch op.Inode {
+	op.Attributes, err = fs.getAttributes(op.Inode)
-	case fuseops.RootInodeID:
+	return
-		op.Attributes = fs.rootAttributes()
+}
-		return
 
-	case fooID:
+func (fs *flushFS) SetInodeAttributes(
-		op.Attributes = fs.fooAttributes()
+	ctx context.Context,
-		return
+	op *fuseops.SetInodeAttributesOp) (err error) {
+	fs.mu.Lock()
+	defer fs.mu.Unlock()
 
-	case barID:
+	// Ignore any changes and simply return existing attributes.
-		op.Attributes = fs.barAttributes()
+	op.Attributes, err = fs.getAttributes(op.Inode)
-		return
 
-	default:
+	return
-		err = fuse.ENOENT
-		return
-	}
 }
 
 func (fs *flushFS) OpenFile(

samples/flushfs/flush_fs_test.go

@@ -590,6 +590,8 @@ func (t *NoErrorsTest) Mmap_NoMsync_MunmapBeforeClose() {
 		syscall.MAP_SHARED)
 
 	AssertEq(nil, err)
+	defer syscall.Munmap(data)
+
 	AssertEq("taco", string(data))
 
 	// Modify the contents.
@@ -638,6 +640,8 @@ func (t *NoErrorsTest) Mmap_NoMsync_CloseBeforeMunmap() {
 		syscall.MAP_SHARED)
 
 	AssertEq(nil, err)
+	defer syscall.Munmap(data)
+
 	AssertEq("taco", string(data))
 
 	// Close the file. We should see a flush.
@@ -682,6 +686,8 @@ func (t *NoErrorsTest) Mmap_WithMsync_MunmapBeforeClose() {
 		syscall.MAP_SHARED)
 
 	AssertEq(nil, err)
+	defer syscall.Munmap(data)
+
 	AssertEq("taco", string(data))
 
 	// Modify the contents.
@@ -738,6 +744,8 @@ func (t *NoErrorsTest) Mmap_WithMsync_CloseBeforeMunmap() {
 		syscall.MAP_SHARED)
 
 	AssertEq(nil, err)
+	defer syscall.Munmap(data)
+
 	AssertEq("taco", string(data))
 
 	// Close the file. We should see a flush.
@@ -941,6 +949,7 @@ func (t *FsyncErrorTest) Msync() {
 		syscall.MAP_SHARED)
 
 	AssertEq(nil, err)
+	defer syscall.Munmap(data)
 
 	// msync the mapping.
 	err = msync(data)

samples/memfs/inode.go

@@ -22,19 +22,12 @@ import (
 
 	"github.com/jacobsa/fuse/fuseops"
 	"github.com/jacobsa/fuse/fuseutil"
-	"github.com/jacobsa/timeutil"
 )
 
 // Common attributes for files and directories.
 //
 // External synchronization is required.
 type inode struct {
-	/////////////////////////
-	// Dependencies
-	/////////////////////////
-
-	clock timeutil.Clock
-
 	/////////////////////////
 	// Mutable state
 	/////////////////////////
@@ -79,16 +72,14 @@ type inode struct {
 // Create a new inode with the supplied attributes, which need not contain
 // time-related information (the inode object will take care of that).
 func newInode(
-	clock timeutil.Clock,
 	attrs fuseops.InodeAttributes) (in *inode) {
 	// Update time info.
-	now := clock.Now()
+	now := time.Now()
 	attrs.Mtime = now
 	attrs.Crtime = now
 
 	// Create the object.
 	in = &inode{
-		clock: clock,
 		attrs: attrs,
 	}
 
@@ -226,7 +217,7 @@ func (in *inode) AddChild(
 	var index int
 
 	// Update the modification time.
-	in.attrs.Mtime = in.clock.Now()
+	in.attrs.Mtime = time.Now()
 
 	// No matter where we place the entry, make sure it has the correct Offset
 	// field.
@@ -260,7 +251,7 @@ func (in *inode) AddChild(
 // REQUIRES: An entry for the given name exists.
 func (in *inode) RemoveChild(name string) {
 	// Update the modification time.
-	in.attrs.Mtime = in.clock.Now()
+	in.attrs.Mtime = time.Now()
 
 	// Find the entry.
 	i, ok := in.findChild(name)
@@ -334,7 +325,7 @@ func (in *inode) WriteAt(p []byte, off int64) (n int, err error) {
 	}
 
 	// Update the modification time.
-	in.attrs.Mtime = in.clock.Now()
+	in.attrs.Mtime = time.Now()
 
 	// Ensure that the contents slice is long enough.
 	newLen := int(off) + len(p)
@@ -361,7 +352,7 @@ func (in *inode) SetAttributes(
 	mode *os.FileMode,
 	mtime *time.Time) {
 	// Update the modification time.
-	in.attrs.Mtime = in.clock.Now()
+	in.attrs.Mtime = time.Now()
 
 	// Truncate?
 	if size != nil {

samples/memfs/memfs.go

@@ -26,7 +26,6 @@ import (
 	"github.com/jacobsa/fuse/fuseops"
 	"github.com/jacobsa/fuse/fuseutil"
 	"github.com/jacobsa/syncutil"
-	"github.com/jacobsa/timeutil"
 )
 
 type memFS struct {
@@ -36,12 +35,6 @@ type memFS struct {
 	uid uint32
 	gid uint32
 
-	/////////////////////////
-	// Dependencies
-	/////////////////////////
-
-	clock timeutil.Clock
-
 	/////////////////////////
 	// Mutable state
 	/////////////////////////
@@ -74,11 +67,9 @@ type memFS struct {
 // default_permissions option.
 func NewMemFS(
 	uid uint32,
-	gid uint32,
+	gid uint32) fuse.Server {
-	clock timeutil.Clock) fuse.Server {
 	// Set up the basic struct.
 	fs := &memFS{
-		clock:  clock,
 		inodes: make([]*inode, fuseops.RootInodeID+1),
 		uid:    uid,
 		gid:    gid,
@@ -91,7 +82,7 @@ func NewMemFS(
 		Gid:  gid,
 	}
 
-	fs.inodes[fuseops.RootInodeID] = newInode(clock, rootAttrs)
+	fs.inodes[fuseops.RootInodeID] = newInode(rootAttrs)
 
 	// Set up invariant checking.
 	fs.mu = syncutil.NewInvariantMutex(fs.checkInvariants)
@@ -165,7 +156,7 @@ func (fs *memFS) getInodeOrDie(id fuseops.InodeID) (inode *inode) {
 func (fs *memFS) allocateInode(
 	attrs fuseops.InodeAttributes) (id fuseops.InodeID, inode *inode) {
 	// Create the inode.
-	inode = newInode(fs.clock, attrs)
+	inode = newInode(attrs)
 
 	// Re-use a free ID if possible. Otherwise mint a new one.
 	numFree := len(fs.freeInodes)
@@ -216,7 +207,7 @@ func (fs *memFS) LookUpInode(
 
 	// We don't spontaneously mutate, so the kernel can cache as long as it wants
 	// (since it also handles invalidation).
-	op.Entry.AttributesExpiration = fs.clock.Now().Add(365 * 24 * time.Hour)
+	op.Entry.AttributesExpiration = time.Now().Add(365 * 24 * time.Hour)
 	op.Entry.EntryExpiration = op.Entry.EntryExpiration
 
 	return
@@ -236,7 +227,7 @@ func (fs *memFS) GetInodeAttributes(
 
 	// We don't spontaneously mutate, so the kernel can cache as long as it wants
 	// (since it also handles invalidation).
-	op.AttributesExpiration = fs.clock.Now().Add(365 * 24 * time.Hour)
+	op.AttributesExpiration = time.Now().Add(365 * 24 * time.Hour)
 
 	return
 }
@@ -258,7 +249,7 @@ func (fs *memFS) SetInodeAttributes(
 
 	// We don't spontaneously mutate, so the kernel can cache as long as it wants
 	// (since it also handles invalidation).
-	op.AttributesExpiration = fs.clock.Now().Add(365 * 24 * time.Hour)
+	op.AttributesExpiration = time.Now().Add(365 * 24 * time.Hour)
 
 	return
 }
@@ -300,7 +291,7 @@ func (fs *memFS) MkDir(
 
 	// We don't spontaneously mutate, so the kernel can cache as long as it wants
 	// (since it also handles invalidation).
-	op.Entry.AttributesExpiration = fs.clock.Now().Add(365 * 24 * time.Hour)
+	op.Entry.AttributesExpiration = time.Now().Add(365 * 24 * time.Hour)
 	op.Entry.EntryExpiration = op.Entry.EntryExpiration
 
 	return
@@ -324,7 +315,7 @@ func (fs *memFS) CreateFile(
 	}
 
 	// Set up attributes from the child.
-	now := fs.clock.Now()
+	now := time.Now()
 	childAttrs := fuseops.InodeAttributes{
 		Nlink:  1,
 		Mode:   op.Mode,
@@ -348,7 +339,7 @@ func (fs *memFS) CreateFile(
 
 	// We don't spontaneously mutate, so the kernel can cache as long as it wants
 	// (since it also handles invalidation).
-	op.Entry.AttributesExpiration = fs.clock.Now().Add(365 * 24 * time.Hour)
+	op.Entry.AttributesExpiration = time.Now().Add(365 * 24 * time.Hour)
 	op.Entry.EntryExpiration = op.Entry.EntryExpiration
 
 	// We have nothing interesting to put in the Handle field.
@@ -374,7 +365,7 @@ func (fs *memFS) CreateSymlink(
 	}
 
 	// Set up attributes from the child.
-	now := fs.clock.Now()
+	now := time.Now()
 	childAttrs := fuseops.InodeAttributes{
 		Nlink:  1,
 		Mode:   0444 | os.ModeSymlink,
@@ -401,7 +392,7 @@ func (fs *memFS) CreateSymlink(
 
 	// We don't spontaneously mutate, so the kernel can cache as long as it wants
 	// (since it also handles invalidation).
-	op.Entry.AttributesExpiration = fs.clock.Now().Add(365 * 24 * time.Hour)
+	op.Entry.AttributesExpiration = time.Now().Add(365 * 24 * time.Hour)
 	op.Entry.EntryExpiration = op.Entry.EntryExpiration
 
 	return

samples/memfs/memfs_test.go

@@ -35,6 +35,11 @@ import (
 
 func TestMemFS(t *testing.T) { RunTests(t) }
 
+// The radius we use for "expect mtime is within"-style assertions. We can't
+// share a synchronized clock with the ultimate source of mtimes because with
+// writeback caching enabled the kernel manufactures them based on wall time.
+const timeSlop = 25 * time.Millisecond
+
 ////////////////////////////////////////////////////////////////////////
 // Helpers
 ////////////////////////////////////////////////////////////////////////
@@ -89,7 +94,7 @@ type MemFSTest struct {
 func init() { RegisterTestSuite(&MemFSTest{}) }
 
 func (t *MemFSTest) SetUp(ti *TestInfo) {
-	t.Server = memfs.NewMemFS(currentUid(), currentGid(), &t.Clock)
+	t.Server = memfs.NewMemFS(currentUid(), currentGid())
 	t.SampleTest.SetUp(ti)
 }
 
@@ -112,17 +117,11 @@ func (t *MemFSTest) Mkdir_OneLevel() {
 
 	dirName := path.Join(t.Dir, "dir")
 
-	// Simulate time advancing.
-	t.Clock.AdvanceTime(time.Second)
-
 	// Create a directory within the root.
-	createTime := t.Clock.Now()
+	createTime := time.Now()
 	err = os.Mkdir(dirName, 0754)
 	AssertEq(nil, err)
 
-	// Simulate time advancing.
-	t.Clock.AdvanceTime(time.Second)
-
 	// Stat the directory.
 	fi, err = os.Stat(dirName)
 	stat = fi.Sys().(*syscall.Stat_t)
@@ -131,7 +130,7 @@ func (t *MemFSTest) Mkdir_OneLevel() {
 	ExpectEq("dir", fi.Name())
 	ExpectEq(0, fi.Size())
 	ExpectEq(os.ModeDir|applyUmask(0754), fi.Mode())
-	ExpectThat(fi, fusetesting.MtimeIs(createTime))
+	ExpectThat(fi, fusetesting.MtimeIsWithin(createTime, timeSlop))
 	ExpectThat(fi, fusetesting.BirthtimeIs(createTime))
 	ExpectTrue(fi.IsDir())
 
@@ -145,7 +144,7 @@ func (t *MemFSTest) Mkdir_OneLevel() {
 	fi, err = os.Stat(t.Dir)
 
 	AssertEq(nil, err)
-	ExpectEq(0, fi.ModTime().Sub(createTime))
+	ExpectThat(fi, fusetesting.MtimeIsWithin(createTime, timeSlop))
 
 	// Read the directory.
 	entries, err = fusetesting.ReadDirPicky(dirName)
@@ -174,17 +173,11 @@ func (t *MemFSTest) Mkdir_TwoLevels() {
 	err = os.Mkdir(path.Join(t.Dir, "parent"), 0700)
 	AssertEq(nil, err)
 
-	// Simulate time advancing.
-	t.Clock.AdvanceTime(time.Second)
-
 	// Create a child of that directory.
-	createTime := t.Clock.Now()
+	createTime := time.Now()
 	err = os.Mkdir(path.Join(t.Dir, "parent/dir"), 0754)
 	AssertEq(nil, err)
 
-	// Simulate time advancing.
-	t.Clock.AdvanceTime(time.Second)
-
 	// Stat the directory.
 	fi, err = os.Stat(path.Join(t.Dir, "parent/dir"))
 	stat = fi.Sys().(*syscall.Stat_t)
@@ -193,7 +186,7 @@ func (t *MemFSTest) Mkdir_TwoLevels() {
 	ExpectEq("dir", fi.Name())
 	ExpectEq(0, fi.Size())
 	ExpectEq(os.ModeDir|applyUmask(0754), fi.Mode())
-	ExpectThat(fi, fusetesting.MtimeIs(createTime))
+	ExpectThat(fi, fusetesting.MtimeIsWithin(createTime, timeSlop))
 	ExpectThat(fi, fusetesting.BirthtimeIs(createTime))
 	ExpectTrue(fi.IsDir())
 
@@ -206,7 +199,7 @@ func (t *MemFSTest) Mkdir_TwoLevels() {
 	// Check the parent's mtime.
 	fi, err = os.Stat(path.Join(t.Dir, "parent"))
 	AssertEq(nil, err)
-	ExpectEq(0, fi.ModTime().Sub(createTime))
+	ExpectThat(fi, fusetesting.MtimeIsWithin(createTime, timeSlop))
 
 	// Read the directory.
 	entries, err = fusetesting.ReadDirPicky(path.Join(t.Dir, "parent/dir"))
@@ -290,13 +283,10 @@ func (t *MemFSTest) CreateNewFile_InRoot() {
 	fileName := path.Join(t.Dir, "foo")
 	const contents = "Hello\x00world"
 
-	createTime := t.Clock.Now()
+	createTime := time.Now()
 	err = ioutil.WriteFile(fileName, []byte(contents), 0400)
 	AssertEq(nil, err)
 
-	// Simulate time advancing.
-	t.Clock.AdvanceTime(time.Second)
-
 	// Stat it.
 	fi, err = os.Stat(fileName)
 	stat = fi.Sys().(*syscall.Stat_t)
@@ -305,7 +295,7 @@ func (t *MemFSTest) CreateNewFile_InRoot() {
 	ExpectEq("foo", fi.Name())
 	ExpectEq(len(contents), fi.Size())
 	ExpectEq(applyUmask(0400), fi.Mode())
-	ExpectThat(fi, fusetesting.MtimeIs(createTime))
+	ExpectThat(fi, fusetesting.MtimeIsWithin(createTime, timeSlop))
 	ExpectThat(fi, fusetesting.BirthtimeIs(createTime))
 	ExpectFalse(fi.IsDir())
 
@@ -335,13 +325,10 @@ func (t *MemFSTest) CreateNewFile_InSubDir() {
 	fileName := path.Join(dirName, "foo")
 	const contents = "Hello\x00world"
 
-	createTime := t.Clock.Now()
+	createTime := time.Now()
 	err = ioutil.WriteFile(fileName, []byte(contents), 0400)
 	AssertEq(nil, err)
 
-	// Simulate time advancing.
-	t.Clock.AdvanceTime(time.Second)
-
 	// Stat it.
 	fi, err = os.Stat(fileName)
 	stat = fi.Sys().(*syscall.Stat_t)
@@ -350,7 +337,7 @@ func (t *MemFSTest) CreateNewFile_InSubDir() {
 	ExpectEq("foo", fi.Name())
 	ExpectEq(len(contents), fi.Size())
 	ExpectEq(applyUmask(0400), fi.Mode())
-	ExpectThat(fi, fusetesting.MtimeIs(createTime))
+	ExpectThat(fi, fusetesting.MtimeIsWithin(createTime, timeSlop))
 	ExpectThat(fi, fusetesting.BirthtimeIs(createTime))
 	ExpectFalse(fi.IsDir())
 
@@ -375,26 +362,20 @@ func (t *MemFSTest) ModifyExistingFile_InRoot() {
 	// Write a file.
 	fileName := path.Join(t.Dir, "foo")
 
-	createTime := t.Clock.Now()
+	createTime := time.Now()
 	err = ioutil.WriteFile(fileName, []byte("Hello, world!"), 0600)
 	AssertEq(nil, err)
 
-	// Simulate time advancing.
-	t.Clock.AdvanceTime(time.Second)
-
 	// Open the file and modify it.
 	f, err := os.OpenFile(fileName, os.O_WRONLY, 0400)
 	t.ToClose = append(t.ToClose, f)
 	AssertEq(nil, err)
 
-	modifyTime := t.Clock.Now()
+	modifyTime := time.Now()
 	n, err = f.WriteAt([]byte("H"), 0)
 	AssertEq(nil, err)
 	AssertEq(1, n)
 
-	// Simulate time advancing.
-	t.Clock.AdvanceTime(time.Second)
-
 	// Stat the file.
 	fi, err = os.Stat(fileName)
 	stat = fi.Sys().(*syscall.Stat_t)
@@ -403,7 +384,7 @@ func (t *MemFSTest) ModifyExistingFile_InRoot() {
 	ExpectEq("foo", fi.Name())
 	ExpectEq(len("Hello, world!"), fi.Size())
 	ExpectEq(applyUmask(0600), fi.Mode())
-	ExpectThat(fi, fusetesting.MtimeIs(modifyTime))
+	ExpectThat(fi, fusetesting.MtimeIsWithin(modifyTime, timeSlop))
 	ExpectThat(fi, fusetesting.BirthtimeIs(createTime))
 	ExpectFalse(fi.IsDir())
 
@@ -433,26 +414,20 @@ func (t *MemFSTest) ModifyExistingFile_InSubDir() {
 	// Write a file.
 	fileName := path.Join(dirName, "foo")
 
-	createTime := t.Clock.Now()
+	createTime := time.Now()
 	err = ioutil.WriteFile(fileName, []byte("Hello, world!"), 0600)
 	AssertEq(nil, err)
 
-	// Simulate time advancing.
-	t.Clock.AdvanceTime(time.Second)
-
 	// Open the file and modify it.
 	f, err := os.OpenFile(fileName, os.O_WRONLY, 0400)
 	t.ToClose = append(t.ToClose, f)
 	AssertEq(nil, err)
 
-	modifyTime := t.Clock.Now()
+	modifyTime := time.Now()
 	n, err = f.WriteAt([]byte("H"), 0)
 	AssertEq(nil, err)
 	AssertEq(1, n)
 
-	// Simulate time advancing.
-	t.Clock.AdvanceTime(time.Second)
-
 	// Stat the file.
 	fi, err = os.Stat(fileName)
 	stat = fi.Sys().(*syscall.Stat_t)
@@ -461,7 +436,7 @@ func (t *MemFSTest) ModifyExistingFile_InSubDir() {
 	ExpectEq("foo", fi.Name())
 	ExpectEq(len("Hello, world!"), fi.Size())
 	ExpectEq(applyUmask(0600), fi.Mode())
-	ExpectThat(fi, fusetesting.MtimeIs(modifyTime))
+	ExpectThat(fi, fusetesting.MtimeIsWithin(modifyTime, timeSlop))
 	ExpectThat(fi, fusetesting.BirthtimeIs(createTime))
 	ExpectFalse(fi.IsDir())
 
@@ -574,17 +549,11 @@ func (t *MemFSTest) Rmdir_Empty() {
 	err = os.MkdirAll(path.Join(t.Dir, "foo/bar"), 0754)
 	AssertEq(nil, err)
 
-	// Simulate time advancing.
-	t.Clock.AdvanceTime(time.Second)
-
 	// Remove the leaf.
-	rmTime := t.Clock.Now()
+	rmTime := time.Now()
 	err = os.Remove(path.Join(t.Dir, "foo/bar"))
 	AssertEq(nil, err)
 
-	// Simulate time advancing.
-	t.Clock.AdvanceTime(time.Second)
-
 	// There should be nothing left in the parent.
 	entries, err = fusetesting.ReadDirPicky(path.Join(t.Dir, "foo"))
 
@@ -594,7 +563,7 @@ func (t *MemFSTest) Rmdir_Empty() {
 	// Check the parent's mtime.
 	fi, err := os.Stat(path.Join(t.Dir, "foo"))
 	AssertEq(nil, err)
-	ExpectEq(0, fi.ModTime().Sub(rmTime))
+	ExpectThat(fi, fusetesting.MtimeIsWithin(rmTime, timeSlop))
 
 	// Remove the parent.
 	err = os.Remove(path.Join(t.Dir, "foo"))
@@ -618,13 +587,10 @@ func (t *MemFSTest) Rmdir_OpenedForReading() {
 	var err error
 
 	// Create a directory.
-	createTime := t.Clock.Now()
+	createTime := time.Now()
 	err = os.Mkdir(path.Join(t.Dir, "dir"), 0700)
 	AssertEq(nil, err)
 
-	// Simulate time advancing.
-	t.Clock.AdvanceTime(time.Second)
-
 	// Open the directory for reading.
 	f, err := os.Open(path.Join(t.Dir, "dir"))
 	defer func() {
@@ -655,7 +621,7 @@ func (t *MemFSTest) Rmdir_OpenedForReading() {
 	fi, err := f.Stat()
 
 	ExpectEq("dir", fi.Name())
-	ExpectEq(0, fi.ModTime().Sub(createTime))
+	ExpectThat(fi, fusetesting.MtimeIsWithin(createTime, timeSlop))
 	ExpectEq(0, fi.Sys().(*syscall.Stat_t).Nlink)
 
 	// Attempt to read from the directory. This shouldn't see any junk from the
@@ -1054,7 +1020,7 @@ func (t *MemFSTest) Chtimes() {
 	// Stat it.
 	fi, err := os.Stat(fileName)
 	AssertEq(nil, err)
-	ExpectEq(0, fi.ModTime().Sub(expectedMtime))
+	ExpectThat(fi, fusetesting.MtimeIsWithin(expectedMtime, timeSlop))
 }
 
 func (t *MemFSTest) ReadDirWhileModifying() {