Speed up O_APPEND with remote revalidating

Remote revalidating requires to update file size on every write
on a file opened with O_APPEND. If host FD exists, it can be
used to update the size and skip round trip to the gofer. With
this change, O_APPEND writes with remote revalidating is almost
as fast as exclusive mode:

              BM_Append
VFS1           60.7us
VFS2           56.8us
VFS2 exclusive 14.2us
This change    15.8us

Updates #1792

PiperOrigin-RevId: 369486801

2 files changed, 47 insertions, 19 deletions

diff --git a/pkg/sentry/fsimpl/gofer/gofer.go b/pkg/sentry/fsimpl/gofer/gofer.go
index 966f01698..f491aa641 100644
--- a/pkg/sentry/fsimpl/gofer/gofer.go
+++ b/pkg/sentry/fsimpl/gofer/gofer.go
@@ -987,36 +987,63 @@ func (d *dentry) updateFromP9AttrsLocked(mask p9.AttrMask, attr *p9.Attr) {
 }
 
 // Preconditions: !d.isSynthetic().
+// Preconditions: d.metadataMu is locked.
+func (d *dentry) refreshSizeLocked(ctx context.Context) error {
+	d.handleMu.RLock()
+
+	if d.writeFD < 0 {
+		d.handleMu.RUnlock()
+		// Ask the gofer if we don't have a host FD.
+		return d.updateFromGetattrLocked(ctx)
+	}
+
+	var stat unix.Statx_t
+	err := unix.Statx(int(d.writeFD), "", unix.AT_EMPTY_PATH, unix.STATX_SIZE, &stat)
+	d.handleMu.RUnlock() // must be released before updateSizeLocked()
+	if err != nil {
+		return err
+	}
+	d.updateSizeLocked(stat.Size)
+	return nil
+}
+
+// Preconditions: !d.isSynthetic().
 func (d *dentry) updateFromGetattr(ctx context.Context) error {
-	// Use d.readFile or d.writeFile, which represent 9P fids that have been
+	// d.metadataMu must be locked *before* we getAttr so that we do not end up
+	// updating stale attributes in d.updateFromP9AttrsLocked().
+	d.metadataMu.Lock()
+	defer d.metadataMu.Unlock()
+	return d.updateFromGetattrLocked(ctx)
+}
+
+// Preconditions:
+// * !d.isSynthetic().
+// * d.metadataMu is locked.
+func (d *dentry) updateFromGetattrLocked(ctx context.Context) error {
+	// Use d.readFile or d.writeFile, which represent 9P FIDs that have been
 	// opened, in preference to d.file, which represents a 9P fid that has not.
 	// This may be significantly more efficient in some implementations. Prefer
 	// d.writeFile over d.readFile since some filesystem implementations may
 	// update a writable handle's metadata after writes to that handle, without
 	// making metadata updates immediately visible to read-only handles
 	// representing the same file.
-	var (
-		file            p9file
-		handleMuRLocked bool
-	)
-	// d.metadataMu must be locked *before* we getAttr so that we do not end up
-	// updating stale attributes in d.updateFromP9AttrsLocked().
-	d.metadataMu.Lock()
-	defer d.metadataMu.Unlock()
 	d.handleMu.RLock()
-	if !d.writeFile.isNil() {
+	handleMuRLocked := true
+	var file p9file
+	switch {
+	case !d.writeFile.isNil():
 		file = d.writeFile
-		handleMuRLocked = true
-	} else if !d.readFile.isNil() {
+	case !d.readFile.isNil():
 		file = d.readFile
-		handleMuRLocked = true
-	} else {
+	default:
 		file = d.file
 		d.handleMu.RUnlock()
+		handleMuRLocked = false
 	}
+
 	_, attrMask, attr, err := file.getAttr(ctx, dentryAttrMask())
 	if handleMuRLocked {
-		d.handleMu.RUnlock()
+		d.handleMu.RUnlock() // must be released before updateFromP9AttrsLocked()
 	}
 	if err != nil {
 		return err
diff --git a/pkg/sentry/fsimpl/gofer/regular_file.go b/pkg/sentry/fsimpl/gofer/regular_file.go
index 713f0a480..f0e7bbaf7 100644
--- a/pkg/sentry/fsimpl/gofer/regular_file.go
+++ b/pkg/sentry/fsimpl/gofer/regular_file.go
@@ -204,18 +204,19 @@ func (fd *regularFileFD) pwrite(ctx context.Context, src usermem.IOSequence, off
 	}
 
 	d := fd.dentry()
+
+	d.metadataMu.Lock()
+	defer d.metadataMu.Unlock()
+
 	// If the fd was opened with O_APPEND, make sure the file size is updated.
 	// There is a possible race here if size is modified externally after
 	// metadata cache is updated.
 	if fd.vfsfd.StatusFlags()&linux.O_APPEND != 0 && !d.cachedMetadataAuthoritative() {
-		if err := d.updateFromGetattr(ctx); err != nil {
+		if err := d.refreshSizeLocked(ctx); err != nil {
 			return 0, offset, err
 		}
 	}
 
-	d.metadataMu.Lock()
-	defer d.metadataMu.Unlock()
-
 	// Set offset to file size if the fd was opened with O_APPEND.
 	if fd.vfsfd.StatusFlags()&linux.O_APPEND != 0 {
 		// Holding d.metadataMu is sufficient for reading d.size.