Skip atime and mtime update when file is backed by host FD

When file is backed by host FD, atime and mtime for the host file and the
cached attributes in the Sentry must be close together. In this case,
the call to update atime and mtime can be skipped. This is important when
host filesystem is using overlay because updating atime and mtime explicitly
forces a copy up for every file that is touched.

PiperOrigin-RevId: 196176413
Change-Id: I3933ea91637a071ba2ea9db9d8ac7cdba5dc0482

pkg/sentry/fs/gofer/inode.go

@@ -206,7 +206,7 @@ func (i *inodeFileState) WriteFromBlocksAt(ctx context.Context, srcs safemem.Blo
 
 // SetMaskedAttributes implements fsutil.CachedFileObject.SetMaskedAttributes.
 func (i *inodeFileState) SetMaskedAttributes(ctx context.Context, mask fs.AttrMask, attr fs.UnstableAttr) error {
-	if mask.Empty() {
+	if i.skipSetAttr(mask) {
 		return nil
 	}
 	as, ans := attr.AccessTime.Unix()
@@ -237,6 +237,35 @@ func (i *inodeFileState) SetMaskedAttributes(ctx context.Context, mask fs.AttrMa
 		})
 }
 
+// skipSetAttr checks if attribute change can be skipped. It can be skipped
+// when:
+//   - Mask is empty
+//   - Mask contains only atime and/or mtime, and host FD exists
+//
+// Updates to atime and mtime can be skipped because cached value will be
+// "close enough" to host value, given that operation went directly to host FD.
+// Skipping atime updates is particularly important to reduce the number of
+// operations sent to the Gofer for readonly files.
+func (i *inodeFileState) skipSetAttr(mask fs.AttrMask) bool {
+	if mask.Empty() {
+		return true
+	}
+
+	cpy := mask
+	cpy.AccessTime = false
+	cpy.ModificationTime = false
+	if !cpy.Empty() {
+		// More than just atime and mtime is being set.
+		return false
+	}
+
+	i.handlesMu.RLock()
+	defer i.handlesMu.RUnlock()
+	return (i.readonly != nil && i.readonly.Host != nil) ||
+		(i.readthrough != nil && i.readthrough.Host != nil) ||
+		(i.writeback != nil && i.writeback.Host != nil)
+}
+
 // Sync implements fsutil.CachedFileObject.Sync.
 func (i *inodeFileState) Sync(ctx context.Context) error {
 	i.handlesMu.RLock()

runsc/boot/fs.go

@@ -141,10 +141,7 @@ func createMountNamespace(ctx context.Context, spec *specs.Spec, conf *Config, i
 // createRootMount creates the root filesystem.
 func createRootMount(ctx context.Context, spec *specs.Spec, conf *Config, fds *fdDispenser) (*fs.Inode, error) {
 	// First construct the filesystem from the spec.Root.
-	mf := fs.MountSourceFlags{
-		ReadOnly: spec.Root.Readonly,
-		NoAtime:  true,
-	}
+	mf := fs.MountSourceFlags{ReadOnly: spec.Root.Readonly}
 
 	var (
 		rootInode *fs.Inode
@@ -261,7 +258,6 @@ func mountSubmount(ctx context.Context, spec *specs.Spec, conf *Config, mns *fs.
 		// All writes go to upper, be paranoid and make lower readonly.
 		mf.ReadOnly = true
 	}
-	mf.NoAtime = true
 
 	inode, err := filesystem.Mount(ctx, m.Type, mf, strings.Join(data, ","))
 	if err != nil {