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, 6 insertions, 0 deletions

diff --git a/pkg/sentry/fs/mock.go b/pkg/sentry/fs/mock.go
index b3c4ad80b..7a24c6f1b 100644
--- a/pkg/sentry/fs/mock.go
+++ b/pkg/sentry/fs/mock.go
@@ -75,6 +75,12 @@ func (n *MockMountSourceOps) Keep(dirent *Dirent) bool {
 	return n.keep
 }
 
+// CacheReaddir implements fs.MountSourceOperations.CacheReaddir.
+func (n *MockMountSourceOps) CacheReaddir() bool {
+	// Common case: cache readdir results if there is a dirent cache.
+	return n.keep
+}
+
 // WriteOut implements fs.InodeOperations.WriteOut.
 func (n *MockInodeOperations) WriteOut(context.Context, *Inode) error {
 	return nil