Implement vectored read support

Read requests can now take vectored responses from the filesystem
implementation and send them to FUSE device via the writev() system call.

This allows file systems to send data without copying it into the
library-provided buffer if the data is already in memory.

connection.go

@@ -338,7 +338,7 @@ func (c *Connection) readMessage() (*buffer.InMessage, error) {
 
 	// Loop past transient errors.
 	for {
-		// Attempt a reaed.
+		// Attempt a read.
 		err := m.Init(c.dev)
 
 		// Special cases:
@@ -405,7 +405,7 @@ func (c *Connection) ReadOp() (_ context.Context, op interface{}, _ error) {
 
 		// Convert the message to an op.
 		outMsg := c.getOutMessage()
-		op, err = convertInMessage(inMsg, outMsg, c.protocol)
+		op, err = convertInMessage(&c.cfg, inMsg, outMsg, c.protocol)
 		if err != nil {
 			c.putOutMessage(outMsg)
 			return nil, nil, fmt.Errorf("convertInMessage: %v", err)
@@ -510,10 +510,16 @@ func (c *Connection) Reply(ctx context.Context, opErr error) {
 	noResponse := c.kernelResponse(outMsg, inMsg.Header().Unique, op, opErr)
 
 	if !noResponse {
-		err := c.writeMessage(outMsg.Bytes())
+		var err error
-		if err != nil && c.errorLogger != nil {
+		if outMsg.Sglist != nil {
-			c.errorLogger.Printf("writeMessage: %v %v", err, outMsg.Bytes())
+			_, err = writev(int(c.dev.Fd()), outMsg.Sglist)
+		} else {
+			err = c.writeMessage(outMsg.OutHeaderBytes())
 		}
+		if err != nil && c.errorLogger != nil {
+			c.errorLogger.Printf("writeMessage: %v %v", err, outMsg.OutHeaderBytes())
+		}
+		outMsg.Sglist = nil
 	}
 }
 

conversions.go

@@ -38,6 +38,7 @@ import (
 //
 // The caller is responsible for arranging for the message to be destroyed.
 func convertInMessage(
+	config *MountConfig,
 	inMsg *buffer.InMessage,
 	outMsg *buffer.OutMessage,
 	protocol fusekernel.Protocol) (o interface{}, err error) {
@@ -288,20 +289,15 @@ func convertInMessage(
 			Inode:     fuseops.InodeID(inMsg.Header().Nodeid),
 			Handle:    fuseops.HandleID(in.Fh),
 			Offset:    int64(in.Offset),
+			Size:      int64(in.Size),
 			OpContext: fuseops.OpContext{Pid: inMsg.Header().Pid},
 		}
-		o = to
+		if !config.UseVectoredRead {
-
+			// Use part of the incoming message storage as the read buffer
-		readSize := int(in.Size)
+			// For vectored zero-copy reads, don't allocate any buffers
-		p := outMsg.GrowNoZero(readSize)
+			to.Dst = inMsg.GetFree(int(in.Size))
-		if p == nil {
-			return nil, fmt.Errorf("Can't grow for %d-byte read", readSize)
 		}
-
+		o = to
-		sh := (*reflect.SliceHeader)(unsafe.Pointer(&to.Dst))
-		sh.Data = uintptr(p)
-		sh.Len = readSize
-		sh.Cap = readSize
 
 	case fusekernel.OpReaddir:
 		in := (*fusekernel.ReadIn)(inMsg.Consume(fusekernel.ReadInSize(protocol)))
@@ -318,7 +314,7 @@ func convertInMessage(
 		o = to
 
 		readSize := int(in.Size)
-		p := outMsg.GrowNoZero(readSize)
+		p := outMsg.Grow(readSize)
 		if p == nil {
 			return nil, fmt.Errorf("Can't grow for %d-byte read", readSize)
 		}
@@ -489,15 +485,19 @@ func convertInMessage(
 		o = to
 
 		readSize := int(in.Size)
-		p := outMsg.GrowNoZero(readSize)
+		if readSize > 0 {
-		if p == nil {
+			p := outMsg.Grow(readSize)
-			return nil, fmt.Errorf("Can't grow for %d-byte read", readSize)
+			if p == nil {
-		}
+				return nil, fmt.Errorf("Can't grow for %d-byte read", readSize)
+			}
 
-		sh := (*reflect.SliceHeader)(unsafe.Pointer(&to.Dst))
+			sh := (*reflect.SliceHeader)(unsafe.Pointer(&to.Dst))
-		sh.Data = uintptr(p)
+			sh.Data = uintptr(p)
-		sh.Len = readSize
+			sh.Len = readSize
-		sh.Cap = readSize
+			sh.Cap = readSize
+		} else {
+			to.Dst = nil
+		}
 
 	case fusekernel.OpListxattr:
 		type input fusekernel.ListxattrIn
@@ -514,7 +514,7 @@ func convertInMessage(
 
 		readSize := int(in.Size)
 		if readSize != 0 {
-			p := outMsg.GrowNoZero(readSize)
+			p := outMsg.Grow(readSize)
 			if p == nil {
 				return nil, fmt.Errorf("Can't grow for %d-byte read", readSize)
 			}
@@ -718,9 +718,11 @@ func (c *Connection) kernelResponseForOp(
 		}
 
 	case *fuseops.ReadFileOp:
-		// convertInMessage already set up the destination buffer to be at the end
+		if o.Dst != nil {
-		// of the out message. We need only shrink to the right size based on how
+			m.Append(o.Dst)
-		// much the user read.
+		} else {
+			m.Append(o.Data...)
+		}
 		m.ShrinkTo(buffer.OutMessageHeaderSize + o.BytesRead)
 
 	case *fuseops.WriteFileOp:

debug.go

@@ -95,7 +95,7 @@ func describeRequest(op interface{}) (s string) {
 	case *fuseops.ReadFileOp:
 		addComponent("handle %d", typed.Handle)
 		addComponent("offset %d", typed.Offset)
-		addComponent("%d bytes", len(typed.Dst))
+		addComponent("%d bytes", typed.Size)
 
 	case *fuseops.WriteFileOp:
 		addComponent("handle %d", typed.Handle)

fuseops/ops.go

@@ -637,9 +637,17 @@ type ReadFileOp struct {
 	// The offset within the file at which to read.
 	Offset int64
 
+	// The size of the read.
+	Size int64
+
 	// The destination buffer, whose length gives the size of the read.
+	// For vectored reads, this field is always nil as the buffer is not provided.
 	Dst []byte
 
+	// Set by the file system:
+	// A list of slices of data to send back to the client for vectored reads.
+	Data [][]byte
+
 	// Set by the file system: the number of bytes read.
 	//
 	// The FUSE documentation requires that exactly the requested number of bytes

internal/buffer/in_message.go

@@ -42,6 +42,7 @@ func init() {
 type InMessage struct {
 	remaining []byte
 	storage   []byte
+	size      int
 }
 
 // NewInMessage creates a new InMessage with its storage initialized.
@@ -66,6 +67,7 @@ func (m *InMessage) Init(r io.Reader) error {
 		return fmt.Errorf("Unexpectedly read only %d bytes.", n)
 	}
 
+	m.size = n
 	m.remaining = m.storage[headerSize:n]
 
 	// Check the header's length.
@@ -114,3 +116,11 @@ func (m *InMessage) ConsumeBytes(n uintptr) []byte {
 
 	return b
 }
+
+// Get the next n bytes after the message to use them as a temporary buffer
+func (m *InMessage) GetFree(n int) []byte {
+	if n <= 0 || n > len(m.storage)-m.size {
+		return nil
+	}
+	return m.storage[m.size : m.size+n]
+}

internal/buffer/out_message.go

@@ -16,7 +16,6 @@ package buffer
 
 import (
 	"fmt"
-	"log"
 	"reflect"
 	"unsafe"
 
@@ -33,30 +32,15 @@ const OutMessageHeaderSize = int(unsafe.Sizeof(fusekernel.OutHeader{}))
 //
 // Must be initialized with Reset.
 type OutMessage struct {
-	// The offset into payload to which we're currently writing.
+	header fusekernel.OutHeader
-	payloadOffset int
+	Sglist [][]byte
-
-	header  fusekernel.OutHeader
-	payload [MaxReadSize]byte
-}
-
-// Make sure that the header and payload are contiguous.
-func init() {
-	a := unsafe.Offsetof(OutMessage{}.header) + uintptr(OutMessageHeaderSize)
-	b := unsafe.Offsetof(OutMessage{}.payload)
-
-	if a != b {
-		log.Panicf(
-			"header ends at offset %d, but payload starts at offset %d",
-			a, b)
-	}
 }
 
 // Reset resets m so that it's ready to be used again. Afterward, the contents
 // are solely a zeroed fusekernel.OutHeader struct.
 func (m *OutMessage) Reset() {
-	m.payloadOffset = 0
 	m.header = fusekernel.OutHeader{}
+	m.Sglist = nil
 }
 
 // OutHeader returns a pointer to the header at the start of the message.
@@ -64,30 +48,12 @@ func (m *OutMessage) OutHeader() *fusekernel.OutHeader {
 	return &m.header
 }
 
-// Grow grows m's buffer by the given number of bytes, returning a pointer to
+// Grow adds a new buffer of <n> bytes to the message, returning a pointer to
-// the start of the new segment, which is guaranteed to be zeroed. If there is
+// the start of the new segment, which is guaranteed to be zeroed.
-// insufficient space, it returns nil.
 func (m *OutMessage) Grow(n int) unsafe.Pointer {
-	p := m.GrowNoZero(n)
+	b := make([]byte, n)
-	if p != nil {
+	m.Append(b)
-		jacobsa_fuse_memclr(p, uintptr(n))
+	p := unsafe.Pointer(&b[0])
-	}
-
-	return p
-}
-
-// GrowNoZero is equivalent to Grow, except the new segment is not zeroed. Use
-// with caution!
-func (m *OutMessage) GrowNoZero(n int) unsafe.Pointer {
-	// Will we overflow the buffer?
-	o := m.payloadOffset
-	if len(m.payload)-o < n {
-		return nil
-	}
-
-	p := unsafe.Pointer(uintptr(unsafe.Pointer(&m.payload)) + uintptr(o))
-	m.payloadOffset = o + n
-
 	return p
 }
 
@@ -100,51 +66,62 @@ func (m *OutMessage) ShrinkTo(n int) {
 			n,
 			m.Len()))
 	}
-
+	if n == OutMessageHeaderSize {
-	m.payloadOffset = n - OutMessageHeaderSize
+		m.Sglist = nil
+	} else {
+		i := 1
+		n -= OutMessageHeaderSize
+		for len(m.Sglist) > i && n >= len(m.Sglist[i]) {
+			n -= len(m.Sglist[i])
+			i++
+		}
+		if n > 0 {
+			m.Sglist[i] = m.Sglist[i][0:n]
+			i++
+		}
+		m.Sglist = m.Sglist[0:i]
+	}
 }
 
 // Append is equivalent to growing by len(src), then copying src over the new
 // segment. Int panics if there is not enough room available.
-func (m *OutMessage) Append(src []byte) {
+func (m *OutMessage) Append(src ...[]byte) {
-	p := m.GrowNoZero(len(src))
+	if m.Sglist == nil {
-	if p == nil {
+		// First element of Sglist is pre-filled with a pointer to the header
-		panic(fmt.Sprintf("Can't grow %d bytes", len(src)))
+		// to allow sending it with a single writev() call without copying the
+		// slice again
+		m.Sglist = append(m.Sglist, m.OutHeaderBytes())
 	}
-
+	m.Sglist = append(m.Sglist, src...)
-	sh := (*reflect.SliceHeader)(unsafe.Pointer(&src))
-	jacobsa_fuse_memmove(p, unsafe.Pointer(sh.Data), uintptr(sh.Len))
-
 	return
 }
 
 // AppendString is like Append, but accepts string input.
 func (m *OutMessage) AppendString(src string) {
-	p := m.GrowNoZero(len(src))
+	m.Append([]byte(src))
-	if p == nil {
-		panic(fmt.Sprintf("Can't grow %d bytes", len(src)))
-	}
-
-	sh := (*reflect.StringHeader)(unsafe.Pointer(&src))
-	jacobsa_fuse_memmove(p, unsafe.Pointer(sh.Data), uintptr(sh.Len))
-
 	return
 }
 
 // Len returns the current size of the message, including the leading header.
 func (m *OutMessage) Len() int {
-	return OutMessageHeaderSize + m.payloadOffset
+	if m.Sglist == nil {
+		return OutMessageHeaderSize
+	}
+	// First element of Sglist is the header, so we don't need to count it here
+	r := 0
+	for _, b := range m.Sglist {
+		r += len(b)
+	}
+	return r
 }
 
-// Bytes returns a reference to the current contents of the buffer, including
+// OutHeaderBytes returns a byte slice containing the current header.
-// the leading header.
+func (m *OutMessage) OutHeaderBytes() []byte {
-func (m *OutMessage) Bytes() []byte {
+	l := OutMessageHeaderSize
-	l := m.Len()
 	sh := reflect.SliceHeader{
 		Data: uintptr(unsafe.Pointer(&m.header)),
 		Len:  l,
 		Cap:  l,
 	}
-
 	return *(*[]byte)(unsafe.Pointer(&sh))
 }

internal/buffer/out_message_test.go

@@ -107,9 +107,12 @@ func TestOutMessageAppend(t *testing.T) {
 		t.Errorf("om.Len() = %d, want %d", got, want)
 	}
 
-	b := om.Bytes()
+	b := []byte(nil)
+	for i := 0; i < len(om.Sglist); i++ {
+		b = append(b, om.Sglist[i]...)
+	}
 	if got, want := len(b), wantLen; got != want {
-		t.Fatalf("len(om.Bytes()) = %d, want %d", got, want)
+		t.Fatalf("len(om.OutHeaderBytes()) = %d, want %d", got, want)
 	}
 
 	want := append(
@@ -137,9 +140,12 @@ func TestOutMessageAppendString(t *testing.T) {
 		t.Errorf("om.Len() = %d, want %d", got, want)
 	}
 
-	b := om.Bytes()
+	b := []byte(nil)
+	for i := 0; i < len(om.Sglist); i++ {
+		b = append(b, om.Sglist[i]...)
+	}
 	if got, want := len(b), wantLen; got != want {
-		t.Fatalf("len(om.Bytes()) = %d, want %d", got, want)
+		t.Fatalf("len(om.OutHeaderBytes()) = %d, want %d", got, want)
 	}
 
 	want := append(
@@ -168,9 +174,12 @@ func TestOutMessageShrinkTo(t *testing.T) {
 		t.Errorf("om.Len() = %d, want %d", got, want)
 	}
 
-	b := om.Bytes()
+	b := []byte(nil)
+	for i := 0; i < len(om.Sglist); i++ {
+		b = append(b, om.Sglist[i]...)
+	}
 	if got, want := len(b), wantLen; got != want {
-		t.Fatalf("len(om.Bytes()) = %d, want %d", got, want)
+		t.Fatalf("len(om.OutHeaderBytes()) = %d, want %d", got, want)
 	}
 
 	want := append(
@@ -201,7 +210,7 @@ func TestOutMessageHeader(t *testing.T) {
 	*h = want
 
 	// Check that the result is as expected.
-	b := om.Bytes()
+	b := om.OutHeaderBytes()
 	if len(b) != int(unsafe.Sizeof(want)) {
 		t.Fatalf("unexpected length %d; want %d", len(b), unsafe.Sizeof(want))
 	}
@@ -225,9 +234,7 @@ func TestOutMessageReset(t *testing.T) {
 		}
 
 		// Ensure a non-zero payload length.
-		if p := om.GrowNoZero(128); p == nil {
+		p := om.Grow(128)
-			t.Fatal("GrowNoZero failed")
-		}
 
 		// Reset.
 		om.Reset()
@@ -259,10 +266,7 @@ func TestOutMessageGrow(t *testing.T) {
 	// Set up garbage where the payload will soon be.
 	const payloadSize = 1234
 	{
-		p := om.GrowNoZero(payloadSize)
+		p := om.Grow(payloadSize)
-		if p == nil {
-			t.Fatal("GrowNoZero failed")
-		}
 
 		err := fillWithGarbage(p, payloadSize)
 		if err != nil {
@@ -283,7 +287,10 @@ func TestOutMessageGrow(t *testing.T) {
 		t.Errorf("om.Len() = %d, want %d", got, want)
 	}
 
-	b := om.Bytes()
+	b := []byte(nil)
+	for i := 0; i < len(om.Sglist); i++ {
+		b = append(b, om.Sglist[i]...)
+	}
 	if got, want := len(b), wantLen; got != want {
 		t.Fatalf("len(om.Len()) = %d, want %d", got, want)
 	}
@@ -304,7 +311,7 @@ func BenchmarkOutMessageReset(b *testing.B) {
 			om.Reset()
 		}
 
-		b.SetBytes(int64(unsafe.Offsetof(om.payload)))
+		b.SetBytes(int64(om.Len()))
 	})
 
 	// Many megabytes worth of buffers, which should defeat the CPU cache.
@@ -321,7 +328,7 @@ func BenchmarkOutMessageReset(b *testing.B) {
 			oms[i%numMessages].Reset()
 		}
 
-		b.SetBytes(int64(unsafe.Offsetof(oms[0].payload)))
+		b.SetBytes(int64(oms[0].Len()))
 	})
 }
 

mount_config.go

@@ -156,6 +156,13 @@ type MountConfig struct {
 	// actually utilise any form of qualifiable UNIX permissions.
 	DisableDefaultPermissions bool
 
+	// Use vectored reads.
+	// Vectored read allows file systems to avoid memory copying overhead if
+	// the data is already in memory when they return it to FUSE.
+	// When turned on, ReadFileOp.Dst is always nil and the FS must return data
+	// being read from the file as a list of slices in ReadFileOp.Data.
+	UseVectoredRead bool
+
 	// OS X only.
 	//
 	// The name of the mounted volume, as displayed in the Finder. If empty, a

writev.go

@@ -0,0 +1,29 @@
+package fuse
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func writev(fd int, packet [][]byte) (n int, err error) {
+	iovecs := make([]syscall.Iovec, 0, len(packet))
+	for _, v := range packet {
+		if len(v) == 0 {
+			continue
+		}
+		vec := syscall.Iovec{
+			Base: &v[0],
+		}
+		vec.SetLen(len(v))
+		iovecs = append(iovecs, vec)
+	}
+	n1, _, e1 := syscall.Syscall(
+		syscall.SYS_WRITEV,
+		uintptr(fd), uintptr(unsafe.Pointer(&iovecs[0])), uintptr(len(iovecs)),
+	)
+	n = int(n1)
+	if e1 != 0 {
+		err = syscall.Errno(e1)
+	}
+	return
+}