Support inotify in vfs2 gofer fs.

Because there is no inode structure stored in the sandbox, inotify watches
must be held on the dentry. This would be an issue in the presence of hard
links, where multiple dentries would need to share the same set of watches,
but in VFS2, we do not support the internal creation of hard links on gofer
fs. As a result, we make the assumption that every dentry corresponds to a
unique inode.

Furthermore, dentries can be cached and then evicted, even if the underlying
file has not be deleted. We must prevent this from occurring if there are any
watches that would be lost. Note that if the dentry was deleted or invalidated
(d.vfsd.IsDead()), we should still destroy it along with its watches.

Additionally, when a dentry’s last watch is removed, we cache it if it also
has zero references. This way, the dentry can eventually be evicted from
memory if it is no longer needed. This is accomplished with a new dentry
method, OnZeroWatches(), which is called by Inotify.RmWatch and
Inotify.Release. Note that it must be called after all inotify locks are
released to avoid violating lock order. Stress tests are added to make sure
that inotify operations don't deadlock with gofer.OnZeroWatches.

Updates #1479.

PiperOrigin-RevId: 317958034

9 files changed, 156 insertions, 35 deletions

diff --git a/pkg/sentry/fsimpl/ext/dentry.go b/pkg/sentry/fsimpl/ext/dentry.go
index 6bd1a9fc6..55902322a 100644
--- a/pkg/sentry/fsimpl/ext/dentry.go
+++ b/pkg/sentry/fsimpl/ext/dentry.go
@@ -63,12 +63,17 @@ func (d *dentry) DecRef() {
 
 // InotifyWithParent implements vfs.DentryImpl.InotifyWithParent.
 //
-// TODO(gvisor.dev/issue/1479): Implement inotify.
-func (d *dentry) InotifyWithParent(events uint32, cookie uint32, et vfs.EventType) {}
+// TODO(b/134676337): Implement inotify.
+func (d *dentry) InotifyWithParent(events, cookie uint32, et vfs.EventType) {}
 
 // Watches implements vfs.DentryImpl.Watches.
 //
-// TODO(gvisor.dev/issue/1479): Implement inotify.
+// TODO(b/134676337): Implement inotify.
 func (d *dentry) Watches() *vfs.Watches {
 	return nil
 }
+
+// OnZeroWatches implements vfs.Dentry.OnZeroWatches.
+//
+// TODO(b/134676337): Implement inotify.
+func (d *dentry) OnZeroWatches() {}
diff --git a/pkg/sentry/fsimpl/gofer/directory.go b/pkg/sentry/fsimpl/gofer/directory.go
index 480510b2a..5d83fe363 100644
--- a/pkg/sentry/fsimpl/gofer/directory.go
+++ b/pkg/sentry/fsimpl/gofer/directory.go
@@ -138,6 +138,7 @@ func (fd *directoryFD) IterDirents(ctx context.Context, cb vfs.IterDirentsCallba
 		fd.dirents = ds
 	}
 
+	d.InotifyWithParent(linux.IN_ACCESS, 0, vfs.PathEvent)
 	if d.cachedMetadataAuthoritative() {
 		d.touchAtime(fd.vfsfd.Mount())
 	}
diff --git a/pkg/sentry/fsimpl/gofer/filesystem.go b/pkg/sentry/fsimpl/gofer/filesystem.go
index f065c4bad..0321fd384 100644
--- a/pkg/sentry/fsimpl/gofer/filesystem.go
+++ b/pkg/sentry/fsimpl/gofer/filesystem.go
@@ -371,6 +371,11 @@ func (fs *filesystem) doCreateAt(ctx context.Context, rp *vfs.ResolvingPath, dir
 		}
 		parent.touchCMtime()
 		parent.dirents = nil
+		ev := linux.IN_CREATE
+		if dir {
+			ev |= linux.IN_ISDIR
+		}
+		parent.watches.Notify(name, uint32(ev), 0, vfs.InodeEvent)
 		return nil
 	}
 	if fs.opts.interop == InteropModeShared {
@@ -400,6 +405,11 @@ func (fs *filesystem) doCreateAt(ctx context.Context, rp *vfs.ResolvingPath, dir
 	}
 	parent.touchCMtime()
 	parent.dirents = nil
+	ev := linux.IN_CREATE
+	if dir {
+		ev |= linux.IN_ISDIR
+	}
+	parent.watches.Notify(name, uint32(ev), 0, vfs.InodeEvent)
 	return nil
 }
 
@@ -530,6 +540,18 @@ func (fs *filesystem) unlinkAt(ctx context.Context, rp *vfs.ResolvingPath, dir b
 			return err
 		}
 	}
+
+	// Generate inotify events for rmdir or unlink.
+	if dir {
+		parent.watches.Notify(name, linux.IN_DELETE|linux.IN_ISDIR, 0, vfs.InodeEvent)
+	} else {
+		var cw *vfs.Watches
+		if child != nil {
+			cw = &child.watches
+		}
+		vfs.InotifyRemoveChild(cw, &parent.watches, name)
+	}
+
 	if child != nil {
 		vfsObj.CommitDeleteDentry(&child.vfsd)
 		child.setDeleted()
@@ -1018,6 +1040,7 @@ func (d *dentry) createAndOpenChildLocked(ctx context.Context, rp *vfs.Resolving
 		}
 		childVFSFD = &fd.vfsfd
 	}
+	d.watches.Notify(name, linux.IN_CREATE, 0, vfs.PathEvent)
 	return childVFSFD, nil
 }
 
@@ -1203,6 +1226,7 @@ func (fs *filesystem) RenameAt(ctx context.Context, rp *vfs.ResolvingPath, oldPa
 			newParent.incLinks()
 		}
 	}
+	vfs.InotifyRename(ctx, &renamed.watches, &oldParent.watches, &newParent.watches, oldName, newName, renamed.isDir())
 	return nil
 }
 
@@ -1215,12 +1239,21 @@ func (fs *filesystem) RmdirAt(ctx context.Context, rp *vfs.ResolvingPath) error
 func (fs *filesystem) SetStatAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.SetStatOptions) error {
 	var ds *[]*dentry
 	fs.renameMu.RLock()
-	defer fs.renameMuRUnlockAndCheckCaching(&ds)
 	d, err := fs.resolveLocked(ctx, rp, &ds)
 	if err != nil {
+		fs.renameMuRUnlockAndCheckCaching(&ds)
+		return err
+	}
+	if err := d.setStat(ctx, rp.Credentials(), &opts.Stat, rp.Mount()); err != nil {
+		fs.renameMuRUnlockAndCheckCaching(&ds)
 		return err
 	}
-	return d.setStat(ctx, rp.Credentials(), &opts.Stat, rp.Mount())
+	fs.renameMuRUnlockAndCheckCaching(&ds)
+
+	if ev := vfs.InotifyEventFromStatMask(opts.Stat.Mask); ev != 0 {
+		d.InotifyWithParent(ev, 0, vfs.InodeEvent)
+	}
+	return nil
 }
 
 // StatAt implements vfs.FilesystemImpl.StatAt.
@@ -1344,24 +1377,38 @@ func (fs *filesystem) GetxattrAt(ctx context.Context, rp *vfs.ResolvingPath, opt
 func (fs *filesystem) SetxattrAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.SetxattrOptions) error {
 	var ds *[]*dentry
 	fs.renameMu.RLock()
-	defer fs.renameMuRUnlockAndCheckCaching(&ds)
 	d, err := fs.resolveLocked(ctx, rp, &ds)
 	if err != nil {
+		fs.renameMuRUnlockAndCheckCaching(&ds)
 		return err
 	}
-	return d.setxattr(ctx, rp.Credentials(), &opts)
+	if err := d.setxattr(ctx, rp.Credentials(), &opts); err != nil {
+		fs.renameMuRUnlockAndCheckCaching(&ds)
+		return err
+	}
+	fs.renameMuRUnlockAndCheckCaching(&ds)
+
+	d.InotifyWithParent(linux.IN_ATTRIB, 0, vfs.InodeEvent)
+	return nil
 }
 
 // RemovexattrAt implements vfs.FilesystemImpl.RemovexattrAt.
 func (fs *filesystem) RemovexattrAt(ctx context.Context, rp *vfs.ResolvingPath, name string) error {
 	var ds *[]*dentry
 	fs.renameMu.RLock()
-	defer fs.renameMuRUnlockAndCheckCaching(&ds)
 	d, err := fs.resolveLocked(ctx, rp, &ds)
 	if err != nil {
+		fs.renameMuRUnlockAndCheckCaching(&ds)
+		return err
+	}
+	if err := d.removexattr(ctx, rp.Credentials(), name); err != nil {
+		fs.renameMuRUnlockAndCheckCaching(&ds)
 		return err
 	}
-	return d.removexattr(ctx, rp.Credentials(), name)
+	fs.renameMuRUnlockAndCheckCaching(&ds)
+
+	d.InotifyWithParent(linux.IN_ATTRIB, 0, vfs.InodeEvent)
+	return nil
 }
 
 // PrependPath implements vfs.FilesystemImpl.PrependPath.
diff --git a/pkg/sentry/fsimpl/gofer/gofer.go b/pkg/sentry/fsimpl/gofer/gofer.go
index 43c8153a4..c6c284ff3 100644
--- a/pkg/sentry/fsimpl/gofer/gofer.go
+++ b/pkg/sentry/fsimpl/gofer/gofer.go
@@ -665,6 +665,9 @@ type dentry struct {
 	pipe *pipe.VFSPipe
 
 	locks vfs.FileLocks
+
+	// Inotify watches for this dentry.
+	watches vfs.Watches
 }
 
 // dentryAttrMask returns a p9.AttrMask enabling all attributes used by the
@@ -947,6 +950,8 @@ func (d *dentry) setStat(ctx context.Context, creds *auth.Credentials, stat *lin
 		} else {
 			atomic.StoreInt64(&d.atime, dentryTimestampFromStatx(stat.Atime))
 		}
+		// Restore mask bits that we cleared earlier.
+		stat.Mask |= linux.STATX_ATIME
 	}
 	if setLocalMtime {
 		if stat.Mtime.Nsec == linux.UTIME_NOW {
@@ -954,6 +959,8 @@ func (d *dentry) setStat(ctx context.Context, creds *auth.Credentials, stat *lin
 		} else {
 			atomic.StoreInt64(&d.mtime, dentryTimestampFromStatx(stat.Mtime))
 		}
+		// Restore mask bits that we cleared earlier.
+		stat.Mask |= linux.STATX_MTIME
 	}
 	atomic.StoreInt64(&d.ctime, now)
 	if stat.Mask&linux.STATX_SIZE != 0 {
@@ -1051,15 +1058,34 @@ func (d *dentry) decRefLocked() {
 }
 
 // InotifyWithParent implements vfs.DentryImpl.InotifyWithParent.
-//
-// TODO(gvisor.dev/issue/1479): Implement inotify.
-func (d *dentry) InotifyWithParent(events uint32, cookie uint32, et vfs.EventType) {}
+func (d *dentry) InotifyWithParent(events, cookie uint32, et vfs.EventType) {
+	if d.isDir() {
+		events |= linux.IN_ISDIR
+	}
+
+	d.fs.renameMu.RLock()
+	// The ordering below is important, Linux always notifies the parent first.
+	if d.parent != nil {
+		d.parent.watches.NotifyWithExclusions(d.name, events, cookie, et, d.isDeleted())
+	}
+	d.watches.Notify("", events, cookie, et)
+	d.fs.renameMu.RUnlock()
+}
 
 // Watches implements vfs.DentryImpl.Watches.
-//
-// TODO(gvisor.dev/issue/1479): Implement inotify.
 func (d *dentry) Watches() *vfs.Watches {
-	return nil
+	return &d.watches
+}
+
+// OnZeroWatches implements vfs.DentryImpl.OnZeroWatches.
+//
+// If no watches are left on this dentry and it has no references, cache it.
+func (d *dentry) OnZeroWatches() {
+	if atomic.LoadInt64(&d.refs) == 0 {
+		d.fs.renameMu.Lock()
+		d.checkCachingLocked()
+		d.fs.renameMu.Unlock()
+	}
 }
 
 // checkCachingLocked should be called after d's reference count becomes 0 or it
@@ -1093,6 +1119,9 @@ func (d *dentry) checkCachingLocked() {
 	// Deleted and invalidated dentries with zero references are no longer
 	// reachable by path resolution and should be dropped immediately.
 	if d.vfsd.IsDead() {
+		if d.isDeleted() {
+			d.watches.HandleDeletion()
+		}
 		if d.cached {
 			d.fs.cachedDentries.Remove(d)
 			d.fs.cachedDentriesLen--
@@ -1101,6 +1130,14 @@ func (d *dentry) checkCachingLocked() {
 		d.destroyLocked()
 		return
 	}
+	// If d still has inotify watches and it is not deleted or invalidated, we
+	// cannot cache it and allow it to be evicted. Otherwise, we will lose its
+	// watches, even if a new dentry is created for the same file in the future.
+	// Note that the size of d.watches cannot concurrently transition from zero
+	// to non-zero, because adding a watch requires holding a reference on d.
+	if d.watches.Size() > 0 {
+		return
+	}
 	// If d is already cached, just move it to the front of the LRU.
 	if d.cached {
 		d.fs.cachedDentries.Remove(d)
@@ -1277,7 +1314,7 @@ func (d *dentry) userXattrSupported() bool {
 	return filetype == linux.S_IFREG || filetype == linux.S_IFDIR
 }
 
-// Preconditions: !d.isSynthetic(). d.isRegularFile() || d.isDirectory().
+// Preconditions: !d.isSynthetic(). d.isRegularFile() || d.isDir().
 func (d *dentry) ensureSharedHandle(ctx context.Context, read, write, trunc bool) error {
 	// O_TRUNC unconditionally requires us to obtain a new handle (opened with
 	// O_TRUNC).
@@ -1422,7 +1459,13 @@ func (fd *fileDescription) Stat(ctx context.Context, opts vfs.StatOptions) (linu
 
 // SetStat implements vfs.FileDescriptionImpl.SetStat.
 func (fd *fileDescription) SetStat(ctx context.Context, opts vfs.SetStatOptions) error {
-	return fd.dentry().setStat(ctx, auth.CredentialsFromContext(ctx), &opts.Stat, fd.vfsfd.Mount())
+	if err := fd.dentry().setStat(ctx, auth.CredentialsFromContext(ctx), &opts.Stat, fd.vfsfd.Mount()); err != nil {
+		return err
+	}
+	if ev := vfs.InotifyEventFromStatMask(opts.Stat.Mask); ev != 0 {
+		fd.dentry().InotifyWithParent(ev, 0, vfs.InodeEvent)
+	}
+	return nil
 }
 
 // Listxattr implements vfs.FileDescriptionImpl.Listxattr.
@@ -1437,12 +1480,22 @@ func (fd *fileDescription) Getxattr(ctx context.Context, opts vfs.GetxattrOption
 
 // Setxattr implements vfs.FileDescriptionImpl.Setxattr.
 func (fd *fileDescription) Setxattr(ctx context.Context, opts vfs.SetxattrOptions) error {
-	return fd.dentry().setxattr(ctx, auth.CredentialsFromContext(ctx), &opts)
+	d := fd.dentry()
+	if err := d.setxattr(ctx, auth.CredentialsFromContext(ctx), &opts); err != nil {
+		return err
+	}
+	d.InotifyWithParent(linux.IN_ATTRIB, 0, vfs.InodeEvent)
+	return nil
 }
 
 // Removexattr implements vfs.FileDescriptionImpl.Removexattr.
 func (fd *fileDescription) Removexattr(ctx context.Context, name string) error {
-	return fd.dentry().removexattr(ctx, auth.CredentialsFromContext(ctx), name)
+	d := fd.dentry()
+	if err := d.removexattr(ctx, auth.CredentialsFromContext(ctx), name); err != nil {
+		return err
+	}
+	d.InotifyWithParent(linux.IN_ATTRIB, 0, vfs.InodeEvent)
+	return nil
 }
 
 // LockBSD implements vfs.FileDescriptionImpl.LockBSD.
diff --git a/pkg/sentry/fsimpl/kernfs/kernfs.go b/pkg/sentry/fsimpl/kernfs/kernfs.go
index 07a9dc830..55349f2a3 100644
--- a/pkg/sentry/fsimpl/kernfs/kernfs.go
+++ b/pkg/sentry/fsimpl/kernfs/kernfs.go
@@ -228,7 +228,7 @@ func (d *Dentry) destroy() {
 // InotifyWithParent implements vfs.DentryImpl.InotifyWithParent.
 //
 // TODO(gvisor.dev/issue/1479): Implement inotify.
-func (d *Dentry) InotifyWithParent(events uint32, cookie uint32, et vfs.EventType) {}
+func (d *Dentry) InotifyWithParent(events, cookie uint32, et vfs.EventType) {}
 
 // Watches implements vfs.DentryImpl.Watches.
 //
@@ -237,6 +237,11 @@ func (d *Dentry) Watches() *vfs.Watches {
 	return nil
 }
 
+// OnZeroWatches implements vfs.Dentry.OnZeroWatches.
+//
+// TODO(gvisor.dev/issue/1479): Implement inotify.
+func (d *Dentry) OnZeroWatches() {}
+
 // InsertChild inserts child into the vfs dentry cache with the given name under
 // this dentry. This does not update the directory inode, so calling this on
 // its own isn't sufficient to insert a child into a directory. InsertChild
diff --git a/pkg/sentry/fsimpl/overlay/overlay.go b/pkg/sentry/fsimpl/overlay/overlay.go
index e11a3ff19..e720d4825 100644
--- a/pkg/sentry/fsimpl/overlay/overlay.go
+++ b/pkg/sentry/fsimpl/overlay/overlay.go
@@ -528,6 +528,11 @@ func (d *dentry) Watches() *vfs.Watches {
 	return nil
 }
 
+// OnZeroWatches implements vfs.DentryImpl.OnZeroWatches.
+//
+// TODO(gvisor.dev/issue/1479): Implement inotify.
+func (d *dentry) OnZeroWatches() {}
+
 // iterLayers invokes yield on each layer comprising d, from top to bottom. If
 // any call to yield returns false, iterLayer stops iteration.
 func (d *dentry) iterLayers(yield func(vd vfs.VirtualDentry, isUpper bool) bool) {
diff --git a/pkg/sentry/fsimpl/tmpfs/directory.go b/pkg/sentry/fsimpl/tmpfs/directory.go
index 913b8a6c5..b172abc15 100644
--- a/pkg/sentry/fsimpl/tmpfs/directory.go
+++ b/pkg/sentry/fsimpl/tmpfs/directory.go
@@ -79,7 +79,6 @@ func (dir *directory) removeChildLocked(child *dentry) {
 	dir.iterMu.Lock()
 	dir.childList.Remove(child)
 	dir.iterMu.Unlock()
-	child.unlinked = true
 }
 
 type directoryFD struct {
@@ -107,13 +106,14 @@ func (fd *directoryFD) IterDirents(ctx context.Context, cb vfs.IterDirentsCallba
 	fs := fd.filesystem()
 	dir := fd.inode().impl.(*directory)
 
+	defer fd.dentry().InotifyWithParent(linux.IN_ACCESS, 0, vfs.PathEvent)
+
 	// fs.mu is required to read d.parent and dentry.name.
 	fs.mu.RLock()
 	defer fs.mu.RUnlock()
 	dir.iterMu.Lock()
 	defer dir.iterMu.Unlock()
 
-	fd.dentry().InotifyWithParent(linux.IN_ACCESS, 0, vfs.PathEvent)
 	fd.inode().touchAtime(fd.vfsfd.Mount())
 
 	if fd.off == 0 {
diff --git a/pkg/sentry/fsimpl/tmpfs/filesystem.go b/pkg/sentry/fsimpl/tmpfs/filesystem.go
index 637d84e04..85d8e37b2 100644
--- a/pkg/sentry/fsimpl/tmpfs/filesystem.go
+++ b/pkg/sentry/fsimpl/tmpfs/filesystem.go
@@ -638,14 +638,16 @@ func (fs *filesystem) RmdirAt(ctx context.Context, rp *vfs.ResolvingPath) error
 // SetStatAt implements vfs.FilesystemImpl.SetStatAt.
 func (fs *filesystem) SetStatAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.SetStatOptions) error {
 	fs.mu.RLock()
-	defer fs.mu.RUnlock()
 	d, err := resolveLocked(rp)
 	if err != nil {
+		fs.mu.RUnlock()
 		return err
 	}
 	if err := d.inode.setStat(ctx, rp.Credentials(), &opts.Stat); err != nil {
+		fs.mu.RUnlock()
 		return err
 	}
+	fs.mu.RUnlock()
 
 	if ev := vfs.InotifyEventFromStatMask(opts.Stat.Mask); ev != 0 {
 		d.InotifyWithParent(ev, 0, vfs.InodeEvent)
@@ -788,14 +790,16 @@ func (fs *filesystem) GetxattrAt(ctx context.Context, rp *vfs.ResolvingPath, opt
 // SetxattrAt implements vfs.FilesystemImpl.SetxattrAt.
 func (fs *filesystem) SetxattrAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.SetxattrOptions) error {
 	fs.mu.RLock()
-	defer fs.mu.RUnlock()
 	d, err := resolveLocked(rp)
 	if err != nil {
+		fs.mu.RUnlock()
 		return err
 	}
 	if err := d.inode.setxattr(rp.Credentials(), &opts); err != nil {
+		fs.mu.RUnlock()
 		return err
 	}
+	fs.mu.RUnlock()
 
 	d.InotifyWithParent(linux.IN_ATTRIB, 0, vfs.InodeEvent)
 	return nil
@@ -804,14 +808,16 @@ func (fs *filesystem) SetxattrAt(ctx context.Context, rp *vfs.ResolvingPath, opt
 // RemovexattrAt implements vfs.FilesystemImpl.RemovexattrAt.
 func (fs *filesystem) RemovexattrAt(ctx context.Context, rp *vfs.ResolvingPath, name string) error {
 	fs.mu.RLock()
-	defer fs.mu.RUnlock()
 	d, err := resolveLocked(rp)
 	if err != nil {
+		fs.mu.RUnlock()
 		return err
 	}
 	if err := d.inode.removexattr(rp.Credentials(), name); err != nil {
+		fs.mu.RUnlock()
 		return err
 	}
+	fs.mu.RUnlock()
 
 	d.InotifyWithParent(linux.IN_ATTRIB, 0, vfs.InodeEvent)
 	return nil
diff --git a/pkg/sentry/fsimpl/tmpfs/tmpfs.go b/pkg/sentry/fsimpl/tmpfs/tmpfs.go
index a94333ee0..a85bfc968 100644
--- a/pkg/sentry/fsimpl/tmpfs/tmpfs.go
+++ b/pkg/sentry/fsimpl/tmpfs/tmpfs.go
@@ -215,11 +215,6 @@ type dentry struct {
 	// filesystem.mu.
 	name string
 
-	// unlinked indicates whether this dentry has been unlinked from its parent.
-	// It is only set to true on an unlink operation, and never set from true to
-	// false. unlinked is protected by filesystem.mu.
-	unlinked bool
-
 	// dentryEntry (ugh) links dentries into their parent directory.childList.
 	dentryEntry
 
@@ -259,18 +254,20 @@ func (d *dentry) DecRef() {
 }
 
 // InotifyWithParent implements vfs.DentryImpl.InotifyWithParent.
-func (d *dentry) InotifyWithParent(events uint32, cookie uint32, et vfs.EventType) {
+func (d *dentry) InotifyWithParent(events, cookie uint32, et vfs.EventType) {
 	if d.inode.isDir() {
 		events |= linux.IN_ISDIR
 	}
 
+	d.inode.fs.mu.RLock()
 	// The ordering below is important, Linux always notifies the parent first.
 	if d.parent != nil {
-		// Note that d.parent or d.name may be stale if there is a concurrent
-		// rename operation. Inotify does not provide consistency guarantees.
-		d.parent.inode.watches.NotifyWithExclusions(d.name, events, cookie, et, d.unlinked)
+		// tmpfs never calls VFS.InvalidateDentry(), so d.vfsd.IsDead() indicates
+		// that d was deleted.
+		d.parent.inode.watches.NotifyWithExclusions(d.name, events, cookie, et, d.vfsd.IsDead())
 	}
 	d.inode.watches.Notify("", events, cookie, et)
+	d.inode.fs.mu.RUnlock()
 }
 
 // Watches implements vfs.DentryImpl.Watches.
@@ -278,6 +275,9 @@ func (d *dentry) Watches() *vfs.Watches {
 	return &d.inode.watches
 }
 
+// OnZeroWatches implements vfs.Dentry.OnZeroWatches.
+func (d *dentry) OnZeroWatches() {}
+
 // inode represents a filesystem object.
 type inode struct {
 	// fs is the owning filesystem. fs is immutable.
@@ -336,7 +336,6 @@ func (i *inode) init(impl interface{}, fs *filesystem, kuid auth.KUID, kgid auth
 	i.ctime = now
 	i.mtime = now
 	// i.nlink initialized by caller
-	i.watches = vfs.Watches{}
 	i.impl = impl
 }