Cache directory entries in the overlay

Currently, the overlay dirCache is only used for a single logical use of
getdents. i.e., it is discard when the FD is closed or seeked back to
the beginning.

But the initial work of getting the directory contents can be quite
expensive (particularly sorting large directories), so we should keep it
as long as possible.

This is very similar to the readdirCache in fs/gofer.

Since the upper filesystem does not have to allow caching readdir
entries, the new CacheReaddir MountSourceOperations method controls this
behavior.

This caching should be trivially movable to all Inodes if desired,
though that adds an additional copy step for non-overlay Inodes.
(Overlay Inodes already do the extra copy).

PiperOrigin-RevId: 255477592

1 files changed, 18 insertions, 0 deletions

diff --git a/pkg/sentry/fs/overlay.go b/pkg/sentry/fs/overlay.go
index c87f24997..1d3ff39e0 100644
--- a/pkg/sentry/fs/overlay.go
+++ b/pkg/sentry/fs/overlay.go
@@ -24,6 +24,7 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/memmap"
 	"gvisor.dev/gvisor/pkg/sentry/usermem"
 	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/third_party/gvsync"
 )
 
 // The virtual filesystem implements an overlay configuration. For a high-level
@@ -196,6 +197,12 @@ type overlayEntry struct {
 	// these locks is sufficient to read upper; holding all three for writing
 	// is required to mutate it.
 	upper *Inode
+
+	// dirCacheMu protects dirCache.
+	dirCacheMu gvsync.DowngradableRWMutex `state:"nosave"`
+
+	// dirCache is cache of DentAttrs from upper and lower Inodes.
+	dirCache *SortedDentryMap
 }
 
 // newOverlayEntry returns a new overlayEntry.
@@ -258,6 +265,17 @@ func (o *overlayEntry) isMappableLocked() bool {
 	return o.inodeLocked().Mappable() != nil
 }
 
+// markDirectoryDirty marks any cached data dirty for this directory. This is
+// necessary in order to ensure that this node does not retain stale state
+// throughout its lifetime across multiple open directory handles.
+//
+// Currently this means invalidating any readdir caches.
+func (o *overlayEntry) markDirectoryDirty() {
+	o.dirCacheMu.Lock()
+	o.dirCache = nil
+	o.dirCacheMu.Unlock()
+}
+
 // AddMapping implements memmap.Mappable.AddMapping.
 func (o *overlayEntry) AddMapping(ctx context.Context, ms memmap.MappingSpace, ar usermem.AddrRange, offset uint64, writable bool) error {
 	o.mapsMu.Lock()