Merge release-20210503.0-47-g07e32fa69 (automated)

28 files changed, 224 insertions, 0 deletions

diff --git a/pkg/sentry/fsimpl/cgroupfs/dir_refs.go b/pkg/sentry/fsimpl/cgroupfs/dir_refs.go
index 741d0cddf..3351de386 100644
--- a/pkg/sentry/fsimpl/cgroupfs/dir_refs.go
+++ b/pkg/sentry/fsimpl/cgroupfs/dir_refs.go
@@ -20,6 +20,14 @@ var dirobj *dir
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type dirRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/fsimpl/devpts/root_inode_refs.go b/pkg/sentry/fsimpl/devpts/root_inode_refs.go
index 155e7abcb..4456ef708 100644
--- a/pkg/sentry/fsimpl/devpts/root_inode_refs.go
+++ b/pkg/sentry/fsimpl/devpts/root_inode_refs.go
@@ -20,6 +20,14 @@ var rootInodeobj *rootInode
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type rootInodeRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/fsimpl/fuse/inode_refs.go b/pkg/sentry/fsimpl/fuse/inode_refs.go
index e221f3b41..ad79c4a56 100644
--- a/pkg/sentry/fsimpl/fuse/inode_refs.go
+++ b/pkg/sentry/fsimpl/fuse/inode_refs.go
@@ -20,6 +20,14 @@ var inodeobj *inode
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type inodeRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/fsimpl/host/connected_endpoint_refs.go b/pkg/sentry/fsimpl/host/connected_endpoint_refs.go
index d784166ae..4bc9294bf 100644
--- a/pkg/sentry/fsimpl/host/connected_endpoint_refs.go
+++ b/pkg/sentry/fsimpl/host/connected_endpoint_refs.go
@@ -20,6 +20,14 @@ var ConnectedEndpointobj *ConnectedEndpoint
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type ConnectedEndpointRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/fsimpl/host/inode_refs.go b/pkg/sentry/fsimpl/host/inode_refs.go
index 8862c4a5e..1ca9a7868 100644
--- a/pkg/sentry/fsimpl/host/inode_refs.go
+++ b/pkg/sentry/fsimpl/host/inode_refs.go
@@ -20,6 +20,14 @@ var inodeobj *inode
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type inodeRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/fsimpl/kernfs/static_directory_refs.go b/pkg/sentry/fsimpl/kernfs/static_directory_refs.go
index 88cba9456..d6180daa3 100644
--- a/pkg/sentry/fsimpl/kernfs/static_directory_refs.go
+++ b/pkg/sentry/fsimpl/kernfs/static_directory_refs.go
@@ -20,6 +20,14 @@ var StaticDirectoryobj *StaticDirectory
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type StaticDirectoryRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/fsimpl/kernfs/synthetic_directory_refs.go b/pkg/sentry/fsimpl/kernfs/synthetic_directory_refs.go
index 4409edd35..c428cdead 100644
--- a/pkg/sentry/fsimpl/kernfs/synthetic_directory_refs.go
+++ b/pkg/sentry/fsimpl/kernfs/synthetic_directory_refs.go
@@ -20,6 +20,14 @@ var syntheticDirectoryobj *syntheticDirectory
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type syntheticDirectoryRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/fsimpl/proc/fd_dir_inode_refs.go b/pkg/sentry/fsimpl/proc/fd_dir_inode_refs.go
index 390927c63..bda0589f8 100644
--- a/pkg/sentry/fsimpl/proc/fd_dir_inode_refs.go
+++ b/pkg/sentry/fsimpl/proc/fd_dir_inode_refs.go
@@ -20,6 +20,14 @@ var fdDirInodeobj *fdDirInode
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type fdDirInodeRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/fsimpl/proc/fd_info_dir_inode_refs.go b/pkg/sentry/fsimpl/proc/fd_info_dir_inode_refs.go
index 5c44e617f..f000064ef 100644
--- a/pkg/sentry/fsimpl/proc/fd_info_dir_inode_refs.go
+++ b/pkg/sentry/fsimpl/proc/fd_info_dir_inode_refs.go
@@ -20,6 +20,14 @@ var fdInfoDirInodeobj *fdInfoDirInode
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type fdInfoDirInodeRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/fsimpl/proc/subtasks_inode_refs.go b/pkg/sentry/fsimpl/proc/subtasks_inode_refs.go
index 4b655a383..fb3517437 100644
--- a/pkg/sentry/fsimpl/proc/subtasks_inode_refs.go
+++ b/pkg/sentry/fsimpl/proc/subtasks_inode_refs.go
@@ -20,6 +20,14 @@ var subtasksInodeobj *subtasksInode
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type subtasksInodeRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/fsimpl/proc/task_inode_refs.go b/pkg/sentry/fsimpl/proc/task_inode_refs.go
index 1bc355b12..5d08a2992 100644
--- a/pkg/sentry/fsimpl/proc/task_inode_refs.go
+++ b/pkg/sentry/fsimpl/proc/task_inode_refs.go
@@ -20,6 +20,14 @@ var taskInodeobj *taskInode
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type taskInodeRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/fsimpl/proc/tasks_inode_refs.go b/pkg/sentry/fsimpl/proc/tasks_inode_refs.go
index af0c4f025..2bfadc272 100644
--- a/pkg/sentry/fsimpl/proc/tasks_inode_refs.go
+++ b/pkg/sentry/fsimpl/proc/tasks_inode_refs.go
@@ -20,6 +20,14 @@ var tasksInodeobj *tasksInode
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type tasksInodeRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/fsimpl/sys/dir_refs.go b/pkg/sentry/fsimpl/sys/dir_refs.go
index 0f8a6e0f2..3b5335671 100644
--- a/pkg/sentry/fsimpl/sys/dir_refs.go
+++ b/pkg/sentry/fsimpl/sys/dir_refs.go
@@ -20,6 +20,14 @@ var dirobj *dir
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type dirRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/fsimpl/tmpfs/inode_refs.go b/pkg/sentry/fsimpl/tmpfs/inode_refs.go
index a30d79603..d333bb742 100644
--- a/pkg/sentry/fsimpl/tmpfs/inode_refs.go
+++ b/pkg/sentry/fsimpl/tmpfs/inode_refs.go
@@ -20,6 +20,14 @@ var inodeobj *inode
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type inodeRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/kernel/fd_table_refs.go b/pkg/sentry/kernel/fd_table_refs.go
index ddbe50261..9b0b1628e 100644
--- a/pkg/sentry/kernel/fd_table_refs.go
+++ b/pkg/sentry/kernel/fd_table_refs.go
@@ -20,6 +20,14 @@ var FDTableobj *FDTable
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type FDTableRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/kernel/fs_context_refs.go b/pkg/sentry/kernel/fs_context_refs.go
index d97c82fc9..484c20683 100644
--- a/pkg/sentry/kernel/fs_context_refs.go
+++ b/pkg/sentry/kernel/fs_context_refs.go
@@ -20,6 +20,14 @@ var FSContextobj *FSContext
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type FSContextRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/kernel/ipc_namespace_refs.go b/pkg/sentry/kernel/ipc_namespace_refs.go
index a2a2d3458..dd399aebd 100644
--- a/pkg/sentry/kernel/ipc_namespace_refs.go
+++ b/pkg/sentry/kernel/ipc_namespace_refs.go
@@ -20,6 +20,14 @@ var IPCNamespaceobj *IPCNamespace
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type IPCNamespaceRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/kernel/process_group_refs.go b/pkg/sentry/kernel/process_group_refs.go
index 51b0fcd34..e29b2ea3a 100644
--- a/pkg/sentry/kernel/process_group_refs.go
+++ b/pkg/sentry/kernel/process_group_refs.go
@@ -20,6 +20,14 @@ var ProcessGroupobj *ProcessGroup
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type ProcessGroupRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/kernel/session_refs.go b/pkg/sentry/kernel/session_refs.go
index 84919943d..0186a0e6d 100644
--- a/pkg/sentry/kernel/session_refs.go
+++ b/pkg/sentry/kernel/session_refs.go
@@ -20,6 +20,14 @@ var Sessionobj *Session
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type SessionRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/kernel/shm/shm_refs.go b/pkg/sentry/kernel/shm/shm_refs.go
index e1c9167ad..f832c1a99 100644
--- a/pkg/sentry/kernel/shm/shm_refs.go
+++ b/pkg/sentry/kernel/shm/shm_refs.go
@@ -20,6 +20,14 @@ var Shmobj *Shm
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type ShmRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/mm/aio_mappable_refs.go b/pkg/sentry/mm/aio_mappable_refs.go
index 4b87070bf..9bf463256 100644
--- a/pkg/sentry/mm/aio_mappable_refs.go
+++ b/pkg/sentry/mm/aio_mappable_refs.go
@@ -20,6 +20,14 @@ var aioMappableobj *aioMappable
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type aioMappableRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/mm/special_mappable_refs.go b/pkg/sentry/mm/special_mappable_refs.go
index 6c89e0f7e..b6d20ede5 100644
--- a/pkg/sentry/mm/special_mappable_refs.go
+++ b/pkg/sentry/mm/special_mappable_refs.go
@@ -20,6 +20,14 @@ var SpecialMappableobj *SpecialMappable
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type SpecialMappableRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/socket/unix/socket_refs.go b/pkg/sentry/socket/unix/socket_refs.go
index 2a7fcb253..6a76efe24 100644
--- a/pkg/sentry/socket/unix/socket_refs.go
+++ b/pkg/sentry/socket/unix/socket_refs.go
@@ -20,6 +20,14 @@ var socketOperationsobj *SocketOperations
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type socketOperationsRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/socket/unix/socket_vfs2_refs.go b/pkg/sentry/socket/unix/socket_vfs2_refs.go
index f10033260..fe05721cf 100644
--- a/pkg/sentry/socket/unix/socket_vfs2_refs.go
+++ b/pkg/sentry/socket/unix/socket_vfs2_refs.go
@@ -20,6 +20,14 @@ var socketVFS2obj *SocketVFS2
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type socketVFS2Refs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/socket/unix/transport/queue_refs.go b/pkg/sentry/socket/unix/transport/queue_refs.go
index 42c5b7ce0..0db027c09 100644
--- a/pkg/sentry/socket/unix/transport/queue_refs.go
+++ b/pkg/sentry/socket/unix/transport/queue_refs.go
@@ -20,6 +20,14 @@ var queueobj *queue
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type queueRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/vfs/file_description_refs.go b/pkg/sentry/vfs/file_description_refs.go
index 5d4634ba7..4f3531d75 100644
--- a/pkg/sentry/vfs/file_description_refs.go
+++ b/pkg/sentry/vfs/file_description_refs.go
@@ -20,6 +20,14 @@ var FileDescriptionobj *FileDescription
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type FileDescriptionRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/vfs/filesystem_refs.go b/pkg/sentry/vfs/filesystem_refs.go
index 2a98a8ead..a16815fda 100644
--- a/pkg/sentry/vfs/filesystem_refs.go
+++ b/pkg/sentry/vfs/filesystem_refs.go
@@ -20,6 +20,14 @@ var Filesystemobj *Filesystem
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type FilesystemRefs struct {
 	// refCount is composed of two fields:
diff --git a/pkg/sentry/vfs/mount_namespace_refs.go b/pkg/sentry/vfs/mount_namespace_refs.go
index d5d48a7cd..a44e73aed 100644
--- a/pkg/sentry/vfs/mount_namespace_refs.go
+++ b/pkg/sentry/vfs/mount_namespace_refs.go
@@ -20,6 +20,14 @@ var MountNamespaceobj *MountNamespace
 // Refs implements refs.RefCounter. It keeps a reference count using atomic
 // operations and calls the destructor when the count reaches zero.
 //
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
 // +stateify savable
 type MountNamespaceRefs struct {
 	// refCount is composed of two fields: