Add support to retrieve kernel cache (retrieve notify)

This patch continues bdca0e6a (Add support for store notify) and adds
corresponding support for counterpart to cache-store - to retrieve an inode
data from kernel cache.

As it was already noted in bdca0e6a, FUSE protocol provides primitives for
pagecache control: to invalidate a data region for inode (notify_inval_inode),
to store data into inode kernel's cache (notify_store), and to retrieve data
from inode kernel's cache (notify_retrieve). For the latter 2 FUSE protocol
messages and brief documentation about semantic can be seen here:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/fuse.h?id=v4.19-rc6-177-gcec4de302c5f#n68
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/fuse.h?id=v4.19-rc6-177-gcec4de302c5f#n756

https://git.kernel.org/linus/a1d75f2582
https://git.kernel.org/linus/2d45ba381a

In short, to retrieve data from kernel cache, filesystem server sends

	S > C	NOTIFY_RETRIEVE_CACHE{notifyUnique, inode, offset, size}

and if that message was sent correctly, the kernel sends back another
write-style message with unique=notifyUnique

	S < C	NOTIFY_REPLY{inode, offset, size, data}

Since so far there were no cases when a server was querying the kernel,
and the reply comes as separate kernel "request", we have to add
infrastructure for tracking such in-flight queries. This is done by
adding Server.retrieveTab and friends.

Otherwise the implementation is straightforward.

A particular note is that from a user-level API point of view we are not
following e.g. libfuse to register a callback to be invoked upon reply, but
instead provide {Inode,File}RetrieveCache that synchronously send notify query
and wait for kernel's reply.

This fits more naturally to Go and is easier to use.

fuse/nodefs/fsconnector.go

@@ -407,6 +407,32 @@ func (c *FileSystemConnector) FileNotifyStoreCache(node *Inode, off int64, data
 	return c.server.InodeNotifyStoreCache(nID, off, data)
 }
 
+// FileRetrieveCache retrieves data from kernel's inode cache.
+//
+// This call retrieves data from kernel's inode cache @ offset and up to
+// len(dest) bytes. If kernel cache has fewer consecutive data starting at
+// offset, that fewer amount is returned. In particular if inode data at offset
+// is not cached (0, OK) is returned.
+//
+// If the kernel does not currently have entry for this inode in its dentry
+// cache (0, OK) is still returned, pretending that the inode could be known to
+// the kernel, but kernel's inode cache is empty.
+func (c *FileSystemConnector) FileRetrieveCache(node *Inode, off int64, dest []byte) (n int, st fuse.Status) {
+	var nID uint64
+	if node == c.rootNode {
+		nID = fuse.FUSE_ROOT_ID
+	} else {
+		nID = c.inodeMap.Handle(&node.handled)
+	}
+
+	if nID == 0 {
+		// the kernel does not currently know about this inode.
+		// -> we can pretend that its cache for the inode is empty.
+		return 0, fuse.OK
+	}
+	return c.server.InodeRetrieveCache(nID, off, dest)
+}
+
 // EntryNotify makes the kernel forget the entry data from the given
 // name from a directory.  After this call, the kernel will issue a
 // new lookup request for the given name when necessary. No filesystem

fuse/opcode.go

@@ -58,12 +58,13 @@ const (
 	_OP_FUSE_RENAME2 = int32(45) // protocol version 23.
 
 	// The following entries don't have to be compatible across Go-FUSE versions.
-	_OP_NOTIFY_INVAL_ENTRY = int32(100)
-	_OP_NOTIFY_INVAL_INODE = int32(101)
-	_OP_NOTIFY_STORE_CACHE = int32(102)
-	_OP_NOTIFY_DELETE      = int32(103) // protocol version 18
+	_OP_NOTIFY_INVAL_ENTRY    = int32(100)
+	_OP_NOTIFY_INVAL_INODE    = int32(101)
+	_OP_NOTIFY_STORE_CACHE    = int32(102)
+	_OP_NOTIFY_RETRIEVE_CACHE = int32(103)
+	_OP_NOTIFY_DELETE         = int32(104) // protocol version 18
 
-	_OPCODE_COUNT = int32(104)
+	_OPCODE_COUNT = int32(105)
 )
 
 ////////////////////////////////////////////////////////////////
@@ -177,6 +178,44 @@ func doWrite(server *Server, req *request) {
 	req.status = status
 }
 
+func doNotifyReply(server *Server, req *request) {
+	reply := (*NotifyRetrieveIn)(req.inData)
+	server.retrieveMu.Lock()
+	reading := server.retrieveTab[reply.Unique]
+	delete(server.retrieveTab, reply.Unique)
+	server.retrieveMu.Unlock()
+
+	badf := func(format string, argv ...interface{}) {
+		log.Printf("notify reply: "+format, argv...)
+	}
+
+	if reading == nil {
+		badf("unexpected unique - ignoring")
+		return
+	}
+
+	reading.n = 0
+	reading.st = EIO
+	defer close(reading.ready)
+
+	if reading.nodeid != reply.NodeId {
+		badf("inode mismatch: expected %s, got %s", reading.nodeid, reply.NodeId)
+		return
+	}
+
+	if reading.offset != reply.Offset {
+		badf("offset mismatch: expected @%d, got @%d", reading.offset, reply.Offset)
+		return
+	}
+
+	if len(reading.dest) < len(req.arg) {
+		badf("too much data: requested %db, got %db (will use only %db)", len(reading.dest), len(req.arg), len(reading.dest))
+	}
+
+	reading.n = copy(reading.dest, req.arg)
+	reading.st = OK
+}
+
 const _SECURITY_CAPABILITY = "security.capability"
 const _SECURITY_ACL = "system.posix_acl_access"
 const _SECURITY_ACL_DEFAULT = "system.posix_acl_default"
@@ -483,6 +522,7 @@ func init() {
 		_OP_BMAP:         unsafe.Sizeof(_BmapIn{}),
 		_OP_IOCTL:        unsafe.Sizeof(_IoctlIn{}),
 		_OP_POLL:         unsafe.Sizeof(_PollIn{}),
+		_OP_NOTIFY_REPLY: unsafe.Sizeof(NotifyRetrieveIn{}),
 		_OP_FALLOCATE:    unsafe.Sizeof(FallocateIn{}),
 		_OP_READDIRPLUS:  unsafe.Sizeof(ReadIn{}),
 	} {
@@ -490,79 +530,82 @@ func init() {
 	}
 
 	for op, sz := range map[int32]uintptr{
-		_OP_LOOKUP:             unsafe.Sizeof(EntryOut{}),
-		_OP_GETATTR:            unsafe.Sizeof(AttrOut{}),
-		_OP_SETATTR:            unsafe.Sizeof(AttrOut{}),
-		_OP_SYMLINK:            unsafe.Sizeof(EntryOut{}),
-		_OP_MKNOD:              unsafe.Sizeof(EntryOut{}),
-		_OP_MKDIR:              unsafe.Sizeof(EntryOut{}),
-		_OP_LINK:               unsafe.Sizeof(EntryOut{}),
-		_OP_OPEN:               unsafe.Sizeof(OpenOut{}),
-		_OP_WRITE:              unsafe.Sizeof(WriteOut{}),
-		_OP_STATFS:             unsafe.Sizeof(StatfsOut{}),
-		_OP_GETXATTR:           unsafe.Sizeof(GetXAttrOut{}),
-		_OP_LISTXATTR:          unsafe.Sizeof(GetXAttrOut{}),
-		_OP_INIT:               unsafe.Sizeof(InitOut{}),
-		_OP_OPENDIR:            unsafe.Sizeof(OpenOut{}),
-		_OP_GETLK:              unsafe.Sizeof(LkOut{}),
-		_OP_CREATE:             unsafe.Sizeof(CreateOut{}),
-		_OP_BMAP:               unsafe.Sizeof(_BmapOut{}),
-		_OP_IOCTL:              unsafe.Sizeof(_IoctlOut{}),
-		_OP_POLL:               unsafe.Sizeof(_PollOut{}),
-		_OP_NOTIFY_INVAL_ENTRY: unsafe.Sizeof(NotifyInvalEntryOut{}),
-		_OP_NOTIFY_INVAL_INODE: unsafe.Sizeof(NotifyInvalInodeOut{}),
-		_OP_NOTIFY_STORE_CACHE: unsafe.Sizeof(NotifyStoreOut{}),
-		_OP_NOTIFY_DELETE:      unsafe.Sizeof(NotifyInvalDeleteOut{}),
+		_OP_LOOKUP:                unsafe.Sizeof(EntryOut{}),
+		_OP_GETATTR:               unsafe.Sizeof(AttrOut{}),
+		_OP_SETATTR:               unsafe.Sizeof(AttrOut{}),
+		_OP_SYMLINK:               unsafe.Sizeof(EntryOut{}),
+		_OP_MKNOD:                 unsafe.Sizeof(EntryOut{}),
+		_OP_MKDIR:                 unsafe.Sizeof(EntryOut{}),
+		_OP_LINK:                  unsafe.Sizeof(EntryOut{}),
+		_OP_OPEN:                  unsafe.Sizeof(OpenOut{}),
+		_OP_WRITE:                 unsafe.Sizeof(WriteOut{}),
+		_OP_STATFS:                unsafe.Sizeof(StatfsOut{}),
+		_OP_GETXATTR:              unsafe.Sizeof(GetXAttrOut{}),
+		_OP_LISTXATTR:             unsafe.Sizeof(GetXAttrOut{}),
+		_OP_INIT:                  unsafe.Sizeof(InitOut{}),
+		_OP_OPENDIR:               unsafe.Sizeof(OpenOut{}),
+		_OP_GETLK:                 unsafe.Sizeof(LkOut{}),
+		_OP_CREATE:                unsafe.Sizeof(CreateOut{}),
+		_OP_BMAP:                  unsafe.Sizeof(_BmapOut{}),
+		_OP_IOCTL:                 unsafe.Sizeof(_IoctlOut{}),
+		_OP_POLL:                  unsafe.Sizeof(_PollOut{}),
+		_OP_NOTIFY_INVAL_ENTRY:    unsafe.Sizeof(NotifyInvalEntryOut{}),
+		_OP_NOTIFY_INVAL_INODE:    unsafe.Sizeof(NotifyInvalInodeOut{}),
+		_OP_NOTIFY_STORE_CACHE:    unsafe.Sizeof(NotifyStoreOut{}),
+		_OP_NOTIFY_RETRIEVE_CACHE: unsafe.Sizeof(NotifyRetrieveOut{}),
+		_OP_NOTIFY_DELETE:         unsafe.Sizeof(NotifyInvalDeleteOut{}),
 	} {
 		operationHandlers[op].OutputSize = sz
 	}
 
 	for op, v := range map[int32]string{
-		_OP_LOOKUP:             "LOOKUP",
-		_OP_FORGET:             "FORGET",
-		_OP_BATCH_FORGET:       "BATCH_FORGET",
-		_OP_GETATTR:            "GETATTR",
-		_OP_SETATTR:            "SETATTR",
-		_OP_READLINK:           "READLINK",
-		_OP_SYMLINK:            "SYMLINK",
-		_OP_MKNOD:              "MKNOD",
-		_OP_MKDIR:              "MKDIR",
-		_OP_UNLINK:             "UNLINK",
-		_OP_RMDIR:              "RMDIR",
-		_OP_RENAME:             "RENAME",
-		_OP_LINK:               "LINK",
-		_OP_OPEN:               "OPEN",
-		_OP_READ:               "READ",
-		_OP_WRITE:              "WRITE",
-		_OP_STATFS:             "STATFS",
-		_OP_RELEASE:            "RELEASE",
-		_OP_FSYNC:              "FSYNC",
-		_OP_SETXATTR:           "SETXATTR",
-		_OP_GETXATTR:           "GETXATTR",
-		_OP_LISTXATTR:          "LISTXATTR",
-		_OP_REMOVEXATTR:        "REMOVEXATTR",
-		_OP_FLUSH:              "FLUSH",
-		_OP_INIT:               "INIT",
-		_OP_OPENDIR:            "OPENDIR",
-		_OP_READDIR:            "READDIR",
-		_OP_RELEASEDIR:         "RELEASEDIR",
-		_OP_FSYNCDIR:           "FSYNCDIR",
-		_OP_GETLK:              "GETLK",
-		_OP_SETLK:              "SETLK",
-		_OP_SETLKW:             "SETLKW",
-		_OP_ACCESS:             "ACCESS",
-		_OP_CREATE:             "CREATE",
-		_OP_INTERRUPT:          "INTERRUPT",
-		_OP_BMAP:               "BMAP",
-		_OP_DESTROY:            "DESTROY",
-		_OP_IOCTL:              "IOCTL",
-		_OP_POLL:               "POLL",
-		_OP_NOTIFY_INVAL_ENTRY: "NOTIFY_INVAL_ENTRY",
-		_OP_NOTIFY_INVAL_INODE: "NOTIFY_INVAL_INODE",
-		_OP_NOTIFY_STORE_CACHE: "NOTIFY_STORE",
-		_OP_NOTIFY_DELETE:      "NOTIFY_DELETE",
-		_OP_FALLOCATE:          "FALLOCATE",
-		_OP_READDIRPLUS:        "READDIRPLUS",
+		_OP_LOOKUP:                "LOOKUP",
+		_OP_FORGET:                "FORGET",
+		_OP_BATCH_FORGET:          "BATCH_FORGET",
+		_OP_GETATTR:               "GETATTR",
+		_OP_SETATTR:               "SETATTR",
+		_OP_READLINK:              "READLINK",
+		_OP_SYMLINK:               "SYMLINK",
+		_OP_MKNOD:                 "MKNOD",
+		_OP_MKDIR:                 "MKDIR",
+		_OP_UNLINK:                "UNLINK",
+		_OP_RMDIR:                 "RMDIR",
+		_OP_RENAME:                "RENAME",
+		_OP_LINK:                  "LINK",
+		_OP_OPEN:                  "OPEN",
+		_OP_READ:                  "READ",
+		_OP_WRITE:                 "WRITE",
+		_OP_STATFS:                "STATFS",
+		_OP_RELEASE:               "RELEASE",
+		_OP_FSYNC:                 "FSYNC",
+		_OP_SETXATTR:              "SETXATTR",
+		_OP_GETXATTR:              "GETXATTR",
+		_OP_LISTXATTR:             "LISTXATTR",
+		_OP_REMOVEXATTR:           "REMOVEXATTR",
+		_OP_FLUSH:                 "FLUSH",
+		_OP_INIT:                  "INIT",
+		_OP_OPENDIR:               "OPENDIR",
+		_OP_READDIR:               "READDIR",
+		_OP_RELEASEDIR:            "RELEASEDIR",
+		_OP_FSYNCDIR:              "FSYNCDIR",
+		_OP_GETLK:                 "GETLK",
+		_OP_SETLK:                 "SETLK",
+		_OP_SETLKW:                "SETLKW",
+		_OP_ACCESS:                "ACCESS",
+		_OP_CREATE:                "CREATE",
+		_OP_INTERRUPT:             "INTERRUPT",
+		_OP_BMAP:                  "BMAP",
+		_OP_DESTROY:               "DESTROY",
+		_OP_IOCTL:                 "IOCTL",
+		_OP_POLL:                  "POLL",
+		_OP_NOTIFY_REPLY:          "NOTIFY_REPLY",
+		_OP_NOTIFY_INVAL_ENTRY:    "NOTIFY_INVAL_ENTRY",
+		_OP_NOTIFY_INVAL_INODE:    "NOTIFY_INVAL_INODE",
+		_OP_NOTIFY_STORE_CACHE:    "NOTIFY_STORE",
+		_OP_NOTIFY_RETRIEVE_CACHE: "NOTIFY_RETRIEVE",
+		_OP_NOTIFY_DELETE:         "NOTIFY_DELETE",
+		_OP_FALLOCATE:             "FALLOCATE",
+		_OP_READDIRPLUS:           "READDIRPLUS",
 	} {
 		operationHandlers[op].Name = v
 	}
@@ -604,6 +647,7 @@ func init() {
 		_OP_STATFS:       doStatFs,
 		_OP_IOCTL:        doIoctl,
 		_OP_DESTROY:      doDestroy,
+		_OP_NOTIFY_REPLY: doNotifyReply,
 		_OP_FALLOCATE:    doFallocate,
 		_OP_READDIRPLUS:  doReadDirPlus,
 	} {
@@ -612,22 +656,23 @@ func init() {
 
 	// Outputs.
 	for op, f := range map[int32]castPointerFunc{
-		_OP_LOOKUP:             func(ptr unsafe.Pointer) interface{} { return (*EntryOut)(ptr) },
-		_OP_OPEN:               func(ptr unsafe.Pointer) interface{} { return (*OpenOut)(ptr) },
-		_OP_OPENDIR:            func(ptr unsafe.Pointer) interface{} { return (*OpenOut)(ptr) },
-		_OP_GETATTR:            func(ptr unsafe.Pointer) interface{} { return (*AttrOut)(ptr) },
-		_OP_CREATE:             func(ptr unsafe.Pointer) interface{} { return (*CreateOut)(ptr) },
-		_OP_LINK:               func(ptr unsafe.Pointer) interface{} { return (*EntryOut)(ptr) },
-		_OP_SETATTR:            func(ptr unsafe.Pointer) interface{} { return (*AttrOut)(ptr) },
-		_OP_INIT:               func(ptr unsafe.Pointer) interface{} { return (*InitOut)(ptr) },
-		_OP_MKDIR:              func(ptr unsafe.Pointer) interface{} { return (*EntryOut)(ptr) },
-		_OP_NOTIFY_INVAL_ENTRY: func(ptr unsafe.Pointer) interface{} { return (*NotifyInvalEntryOut)(ptr) },
-		_OP_NOTIFY_INVAL_INODE: func(ptr unsafe.Pointer) interface{} { return (*NotifyInvalInodeOut)(ptr) },
-		_OP_NOTIFY_STORE_CACHE: func(ptr unsafe.Pointer) interface{} { return (*NotifyStoreOut)(ptr) },
-		_OP_NOTIFY_DELETE:      func(ptr unsafe.Pointer) interface{} { return (*NotifyInvalDeleteOut)(ptr) },
-		_OP_STATFS:             func(ptr unsafe.Pointer) interface{} { return (*StatfsOut)(ptr) },
-		_OP_SYMLINK:            func(ptr unsafe.Pointer) interface{} { return (*EntryOut)(ptr) },
-		_OP_GETLK:              func(ptr unsafe.Pointer) interface{} { return (*LkOut)(ptr) },
+		_OP_LOOKUP:                func(ptr unsafe.Pointer) interface{} { return (*EntryOut)(ptr) },
+		_OP_OPEN:                  func(ptr unsafe.Pointer) interface{} { return (*OpenOut)(ptr) },
+		_OP_OPENDIR:               func(ptr unsafe.Pointer) interface{} { return (*OpenOut)(ptr) },
+		_OP_GETATTR:               func(ptr unsafe.Pointer) interface{} { return (*AttrOut)(ptr) },
+		_OP_CREATE:                func(ptr unsafe.Pointer) interface{} { return (*CreateOut)(ptr) },
+		_OP_LINK:                  func(ptr unsafe.Pointer) interface{} { return (*EntryOut)(ptr) },
+		_OP_SETATTR:               func(ptr unsafe.Pointer) interface{} { return (*AttrOut)(ptr) },
+		_OP_INIT:                  func(ptr unsafe.Pointer) interface{} { return (*InitOut)(ptr) },
+		_OP_MKDIR:                 func(ptr unsafe.Pointer) interface{} { return (*EntryOut)(ptr) },
+		_OP_NOTIFY_INVAL_ENTRY:    func(ptr unsafe.Pointer) interface{} { return (*NotifyInvalEntryOut)(ptr) },
+		_OP_NOTIFY_INVAL_INODE:    func(ptr unsafe.Pointer) interface{} { return (*NotifyInvalInodeOut)(ptr) },
+		_OP_NOTIFY_STORE_CACHE:    func(ptr unsafe.Pointer) interface{} { return (*NotifyStoreOut)(ptr) },
+		_OP_NOTIFY_RETRIEVE_CACHE: func(ptr unsafe.Pointer) interface{} { return (*NotifyRetrieveOut)(ptr) },
+		_OP_NOTIFY_DELETE:         func(ptr unsafe.Pointer) interface{} { return (*NotifyInvalDeleteOut)(ptr) },
+		_OP_STATFS:                func(ptr unsafe.Pointer) interface{} { return (*StatfsOut)(ptr) },
+		_OP_SYMLINK:               func(ptr unsafe.Pointer) interface{} { return (*EntryOut)(ptr) },
+		_OP_GETLK:                 func(ptr unsafe.Pointer) interface{} { return (*LkOut)(ptr) },
 	} {
 		operationHandlers[op].DecodeOut = f
 	}
@@ -655,6 +700,7 @@ func init() {
 		_OP_RELEASE:      func(ptr unsafe.Pointer) interface{} { return (*ReleaseIn)(ptr) },
 		_OP_RELEASEDIR:   func(ptr unsafe.Pointer) interface{} { return (*ReleaseIn)(ptr) },
 		_OP_FALLOCATE:    func(ptr unsafe.Pointer) interface{} { return (*FallocateIn)(ptr) },
+		_OP_NOTIFY_REPLY: func(ptr unsafe.Pointer) interface{} { return (*NotifyRetrieveIn)(ptr) },
 		_OP_READDIRPLUS:  func(ptr unsafe.Pointer) interface{} { return (*ReadIn)(ptr) },
 		_OP_RENAME:       func(ptr unsafe.Pointer) interface{} { return (*RenameIn)(ptr) },
 		_OP_GETLK:        func(ptr unsafe.Pointer) interface{} { return (*LkIn)(ptr) },

fuse/print.go

@@ -236,6 +236,14 @@ func (o *NotifyStoreOut) string() string {
 	return fmt.Sprintf("{nodeid %d off %d sz %d}", o.Nodeid, o.Offset, o.Size)
 }
 
+func (o *NotifyRetrieveOut) string() string {
+	return fmt.Sprintf("{notifyUnique %d nodeid %d off %d sz %d}", o.NotifyUnique, o.Nodeid, o.Offset, o.Size)
+}
+
+func (i *NotifyRetrieveIn) string() string {
+	return fmt.Sprintf("{off %d sz %d}", i.Offset, i.Size)
+}
+
 func (f *FallocateIn) string() string {
 	return fmt.Sprintf("{Fh %d off %d sz %d mod 0%o}",
 		f.Fh, f.Offset, f.Length, f.Mode)

fuse/server.go

@@ -48,6 +48,11 @@ type Server struct {
 	reqReaders     int
 	kernelSettings InitIn
 
+	// in-flight notify-retrieve queries
+	retrieveMu   sync.Mutex
+	retrieveNext uint64
+	retrieveTab  map[uint64]*retrieveCacheRequest // notifyUnique -> retrieve request
+
 	singleReader bool
 	canSplice    bool
 	loops        sync.WaitGroup
@@ -155,8 +160,9 @@ func NewServer(fs RawFileSystem, mountPoint string, opts *MountOptions) (*Server
 	}
 
 	ms := &Server{
-		fileSystem: fs,
-		opts:       &o,
+		fileSystem:  fs,
+		opts:        &o,
+		retrieveTab: make(map[uint64]*retrieveCacheRequest),
 		// OSX has races when multiple routines read from the
 		// FUSE device: on unmount, sometime some reads do not
 		// error-out, meaning that unmount will hang.
@@ -328,6 +334,23 @@ func (ms *Server) Serve() {
 	ms.writeMu.Lock()
 	syscall.Close(ms.mountFd)
 	ms.writeMu.Unlock()
+
+	// shutdown in-flight cache retrieves.
+	//
+	// It is possible that umount comes in the middle - after retrieve
+	// request was sent to kernel, but corresponding kernel reply has not
+	// yet been read. We unblock all such readers and wake them up with ENODEV.
+	ms.retrieveMu.Lock()
+	rtab := ms.retrieveTab
+	// retrieve attempts might be erroneously tried even after close
+	// we have to keep retrieveTab !nil not to panic.
+	ms.retrieveTab = make(map[uint64]*retrieveCacheRequest)
+	ms.retrieveMu.Unlock()
+	for _, reading := range rtab {
+		reading.n = 0
+		reading.st = ENODEV
+		close(reading.ready)
+	}
 }
 
 func (ms *Server) handleInit() Status {
@@ -427,8 +450,9 @@ func (ms *Server) allocOut(req *request, size uint32) []byte {
 }
 
 func (ms *Server) write(req *request) Status {
-	// Forget does not wait for reply.
-	if req.inHeader.Opcode == _OP_FORGET || req.inHeader.Opcode == _OP_BATCH_FORGET {
+	// Forget/NotifyReply do not wait for reply from filesystem server.
+	switch req.inHeader.Opcode {
+	case _OP_FORGET, _OP_BATCH_FORGET, _OP_NOTIFY_REPLY:
 		return OK
 	}
 
@@ -535,6 +559,96 @@ func (ms *Server) inodeNotifyStoreCache32(node uint64, offset int64, data []byte
 	return result
 }
 
+// InodeRetrieveCache retrieves data from kernel's inode cache.
+//
+// InodeRetrieveCache asks kernel to return data from its cache for inode at
+// [offset:offset+len(dest)) and waits for corresponding reply. If kernel cache
+// has fewer consecutive data starting at offset, that fewer amount is returned.
+// In particular if inode data at offset is not cached (0, OK) is returned.
+func (ms *Server) InodeRetrieveCache(node uint64, offset int64, dest []byte) (n int, st Status) {
+	if !ms.kernelSettings.SupportsNotify(NOTIFY_RETRIEVE_CACHE) {
+		return 0, ENOSYS
+	}
+
+	req := request{
+		inHeader: &InHeader{
+			Opcode: _OP_NOTIFY_RETRIEVE_CACHE,
+		},
+		handler: operationHandlers[_OP_NOTIFY_RETRIEVE_CACHE],
+		status:  NOTIFY_RETRIEVE_CACHE,
+	}
+
+	// retrieve up to 2GB not to overflow uint32 size in NotifyRetrieveOut.
+	// see InodeNotifyStoreCache in similar place for why it is only 2GB, not 4GB.
+	size := len(dest)
+	if size > math.MaxInt32 {
+		size = math.MaxInt32
+	}
+	dest = dest[:size]
+
+	q := (*NotifyRetrieveOut)(req.outData())
+	q.Nodeid = node
+	q.Offset = uint64(offset) // not int64, as it is e.g. in NotifyInvalInodeOut
+	q.Size = uint32(len(dest))
+
+	reading := &retrieveCacheRequest{
+		nodeid: q.Nodeid,
+		offset: q.Offset,
+		dest:   dest,
+		ready:  make(chan struct{}),
+	}
+
+	ms.retrieveMu.Lock()
+	q.NotifyUnique = ms.retrieveNext
+	ms.retrieveNext++
+	ms.retrieveTab[q.NotifyUnique] = reading
+	ms.retrieveMu.Unlock()
+
+	// Protect against concurrent close.
+	ms.writeMu.Lock()
+	result := ms.write(&req)
+	ms.writeMu.Unlock()
+
+	if ms.opts.Debug {
+		log.Printf("Response: NOTIFY_RETRIEVE_CACHE: %v", result)
+	}
+	if result != OK {
+		ms.retrieveMu.Lock()
+		r := ms.retrieveTab[q.NotifyUnique]
+		if r == reading {
+			delete(ms.retrieveTab, q.NotifyUnique)
+		} else if r == nil {
+			// ok - might be dequeued by umount
+		} else {
+			// although very unlikely, it is possible that kernel sends
+			// unexpected NotifyReply with our notifyUnique, then
+			// retrieveNext wraps, makes full cycle, and another
+			// retrieve request is made with the same notifyUnique.
+			log.Printf("W: INODE_RETRIEVE_CACHE: request with notifyUnique=%d mutated", q.NotifyUnique)
+		}
+		ms.retrieveMu.Unlock()
+		return 0, result
+	}
+
+	// NotifyRetrieveOut sent to the kernel successfully. Now the kernel
+	// have to return data in a separate write-style NotifyReply request.
+	// Wait for the result.
+	<-reading.ready
+	return reading.n, reading.st
+}
+
+// retrieveCacheRequest represents in-flight cache retrieve request.
+type retrieveCacheRequest struct {
+	nodeid uint64
+	offset uint64
+	dest   []byte
+
+	// reply status
+	n     int
+	st    Status
+	ready chan struct{}
+}
+
 // DeleteNotify notifies the kernel that an entry is removed from a
 // directory.  In many cases, this is equivalent to EntryNotify,
 // except when the directory is in use, eg. as working directory of
@@ -626,7 +740,7 @@ func (in *InitIn) SupportsNotify(notifyType int) bool {
 		return in.SupportsVersion(7, 12)
 	case NOTIFY_INVAL_INODE:
 		return in.SupportsVersion(7, 12)
-	case NOTIFY_STORE_CACHE:
+	case NOTIFY_STORE_CACHE, NOTIFY_RETRIEVE_CACHE:
 		return in.SupportsVersion(7, 15)
 	case NOTIFY_DELETE:
 		return in.SupportsVersion(7, 18)

fuse/test/cachecontrol_test.go

@@ -95,6 +95,42 @@ func TestCacheControl(t *testing.T) {
 		}
 	}
 
+	// assertCacheRead asserts that file's kernel cache is retrieved as dataOK.
+	assertCacheRead := func(subj, dataOK string) {
+		t.Helper()
+
+		assertCacheReadAt := func(offset int64, size int, dataOK string) {
+			t.Helper()
+			buf := make([]byte, size)
+			n, st := fsconn.FileRetrieveCache(file.Inode(), offset, buf)
+			if st != fuse.OK {
+				t.Fatalf("%s: retrieve cache @%d [%d]: %s", subj, offset, size, st)
+			}
+			if got := buf[:n]; string(got) != dataOK {
+				t.Fatalf("%s: retrieve cache @%d [%d]: have %q; want %q", subj, offset, size, got, dataOK)
+			}
+		}
+
+		// retrieve [1:len - 1]   (also verifying that offset/size are handled correctly)
+		l := len(dataOK)
+		if l >= 2 {
+			assertCacheReadAt(1, l-2, dataOK[1:l-1])
+		}
+
+		// retrieve [:∞]
+		assertCacheReadAt(0, l+10000, dataOK)
+	}
+
+	// before the kernel has entry for file in its dentry cache, the cache
+	// should read as empty.
+	assertCacheRead("before lookup", "")
+
+	// lookup on the file - forces to assign inode ID to it
+	os.Stat(dir + "/hello.txt")
+
+	// cache should be initially empty
+	assertCacheRead("initial", "")
+
 	// make sure the file reads correctly
 	assertFileRead("original", data0)
 
@@ -132,6 +168,7 @@ func TestCacheControl(t *testing.T) {
 
 	// make sure the cache has original data
 	assertMmapRead("original", data0)
+	assertCacheRead("original", data0)
 
 	// store changed data into OS cache
 	st := fsconn.FileNotifyStoreCache(file.Inode(), 7, []byte("123"))
@@ -143,8 +180,7 @@ func TestCacheControl(t *testing.T) {
 	data1 := "hello w123d"
 	assertMmapRead("after storecache", data1)
 	assertFileRead("after storecache", data1)
-
-	// TODO verify retrieve cache
+	assertCacheRead("after storecache", data1)
 
 	// invalidate cache
 	st = fsconn.FileNotify(file.Inode(), 0, 0)
@@ -152,7 +188,11 @@ func TestCacheControl(t *testing.T) {
 		t.Fatalf("invalidate cache: %s", st)
 	}
 
-	// make sure mmapped data and file read as original data
+	// make sure cache reads as empty right after invalidation
+	assertCacheRead("after invalcache", "")
+
+	// make sure mmapped data, file and cache read as original data
 	assertMmapRead("after invalcache", data0)
 	assertFileRead("after invalcache", data0)
+	assertCacheRead("after invalcache + refill", data0)
 }

fuse/types.go

@@ -386,13 +386,31 @@ type NotifyStoreOut struct {
 	Padding uint32
 }
 
+type NotifyRetrieveOut struct {
+	NotifyUnique uint64
+	Nodeid       uint64
+	Offset       uint64
+	Size         uint32
+	Padding      uint32
+}
+
+type NotifyRetrieveIn struct {
+	InHeader
+	Dummy1 uint64
+	Offset uint64
+	Size   uint32
+	Dummy2 uint32
+	Dummy3 uint64
+	Dummy4 uint64
+}
+
 const (
 	//	NOTIFY_POLL         = -1 // notify kernel that a poll waiting for IO on a file handle should wake up
-	NOTIFY_INVAL_INODE = -2 // notify kernel that an inode should be invalidated
-	NOTIFY_INVAL_ENTRY = -3 // notify kernel that a directory entry should be invalidated
-	NOTIFY_STORE_CACHE = -4 // store data into kernel cache of an inode
-	//	NOTIFY_RETRIEVE_CACHE  = -5 // retrieve data from kernel cache of an inode
-	NOTIFY_DELETE = -6 // notify kernel that a directory entry has been deleted
+	NOTIFY_INVAL_INODE    = -2 // notify kernel that an inode should be invalidated
+	NOTIFY_INVAL_ENTRY    = -3 // notify kernel that a directory entry should be invalidated
+	NOTIFY_STORE_CACHE    = -4 // store data into kernel cache of an inode
+	NOTIFY_RETRIEVE_CACHE = -5 // retrieve data from kernel cache of an inode
+	NOTIFY_DELETE         = -6 // notify kernel that a directory entry has been deleted
 
 //	NOTIFY_CODE_MAX     = -6
 )