apply bitmap for fd_table

Apply bitmap in fd_table to record open file fd. It can accelerate the speed of allocating or removing fd from fdtable. Signed-off-by: Howard Zhang <howard.zhang@arm.com>
2026-05-22 17:12:49 -07:00 · 2021-07-13 14:16:07 +08:00
parent 79b7fb3348
commit c8d252466f
3 changed files with 118 additions and 108 deletions
@@ -220,6 +220,7 @@ go_library(
        "//pkg/abi/linux",
        "//pkg/abi/linux/errno",
        "//pkg/amutex",
+        "//pkg/bitmap",
        "//pkg/bits",
        "//pkg/bpf",
        "//pkg/cleanup",
@@ -18,10 +18,10 @@ import (
 	"fmt"
 	"math"
 	"strings"
-	"sync/atomic"

 	"golang.org/x/sys/unix"
 	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/bitmap"
 	"gvisor.dev/gvisor/pkg/context"
 	"gvisor.dev/gvisor/pkg/errors/linuxerr"
 	"gvisor.dev/gvisor/pkg/sentry/fs"
@@ -84,13 +84,8 @@ type FDTable struct {
 	// mu protects below.
 	mu sync.Mutex `state:"nosave"`

-	// next is start position to find fd.
-	next int32
-
-	// used contains the number of non-nil entries. It must be accessed
-	// atomically. It may be read atomically without holding mu (but not
-	// written).
-	used int32
+	// fdBitmap shows which fds are already in use.
+	fdBitmap bitmap.Bitmap `state:"nosave"`

 	// descriptorTable holds descriptors.
 	descriptorTable `state:".(map[int32]descriptor)"`
@@ -98,6 +93,8 @@ type FDTable struct {

 func (f *FDTable) saveDescriptorTable() map[int32]descriptor {
 	m := make(map[int32]descriptor)
+	f.mu.Lock()
+	defer f.mu.Unlock()
 	f.forEach(context.Background(), func(fd int32, file *fs.File, fileVFS2 *vfs.FileDescription, flags FDFlags) {
 		m[fd] = descriptor{
 			file:     file,
@@ -111,12 +108,16 @@ func (f *FDTable) saveDescriptorTable() map[int32]descriptor {
 func (f *FDTable) loadDescriptorTable(m map[int32]descriptor) {
 	ctx := context.Background()
 	f.initNoLeakCheck() // Initialize table.
-	f.used = 0
+	f.fdBitmap = bitmap.BitmapWithSize(uint32(math.MaxUint16))
 	for fd, d := range m {
+		if fd < 0 {
+			panic(fmt.Sprintf("FD is not supposed to be negative. FD: %d", fd))
+		}
+
 		if file, fileVFS2 := f.setAll(ctx, fd, d.file, d.fileVFS2, d.flags); file != nil || fileVFS2 != nil {
 			panic("VFS1 or VFS2 files set")
 		}
-
+		f.fdBitmap.Add(uint32(fd))
 		// Note that we do _not_ need to acquire a extra table reference here. The
 		// table reference will already be accounted for in the file, so we drop the
 		// reference taken by set above.
@@ -189,8 +190,10 @@ func (f *FDTable) DecRef(ctx context.Context) {
 func (f *FDTable) forEach(ctx context.Context, fn func(fd int32, file *fs.File, fileVFS2 *vfs.FileDescription, flags FDFlags)) {
 	// retries tracks the number of failed TryIncRef attempts for the same FD.
 	retries := 0
-	fd := int32(0)
-	for {
+	fds := f.fdBitmap.ToSlice()
+	// Iterate through the fdBitmap.
+	for _, ufd := range fds {
+		fd := int32(ufd)
 		file, fileVFS2, flags, ok := f.getAll(fd)
 		if !ok {
 			break
@@ -218,7 +221,6 @@ func (f *FDTable) forEach(ctx context.Context, fn func(fd int32, file *fs.File,
 			fileVFS2.DecRef(ctx)
 		}
 		retries = 0
-		fd++
 	}
 }

@@ -226,6 +228,8 @@ func (f *FDTable) forEach(ctx context.Context, fn func(fd int32, file *fs.File,
 func (f *FDTable) String() string {
 	var buf strings.Builder
 	ctx := context.Background()
+	f.mu.Lock()
+	defer f.mu.Unlock()
 	f.forEach(ctx, func(fd int32, file *fs.File, fileVFS2 *vfs.FileDescription, flags FDFlags) {
 		switch {
 		case file != nil:
@@ -250,10 +254,10 @@ func (f *FDTable) String() string {
 }

 // NewFDs allocates new FDs guaranteed to be the lowest number available
-// greater than or equal to the fd parameter. All files will share the set
+// greater than or equal to the minFD parameter. All files will share the set
 // flags. Success is guaranteed to be all or none.
-func (f *FDTable) NewFDs(ctx context.Context, fd int32, files []*fs.File, flags FDFlags) (fds []int32, err error) {
-	if fd < 0 {
+func (f *FDTable) NewFDs(ctx context.Context, minFD int32, files []*fs.File, flags FDFlags) (fds []int32, err error) {
+	if minFD < 0 {
 		// Don't accept negative FDs.
 		return nil, unix.EINVAL
 	}
@@ -267,24 +271,49 @@ func (f *FDTable) NewFDs(ctx context.Context, fd int32, files []*fs.File, flags
 		if lim.Cur != limits.Infinity {
 			end = int32(lim.Cur)
 		}
-		if fd >= end {
+		if minFD >= end {
 			return nil, unix.EMFILE
 		}
 	}

 	f.mu.Lock()

-	// From f.next to find available fd.
-	if fd < f.next {
-		fd = f.next
+	// max is used as the largest number in fdBitmap + 1.
+	max := int32(0)
+	// flipFdBitmap is flip of fdBitmap which is used to find free fd in fdBitmap.
+	flipFdBitmap := f.fdBitmap.Clone()
+
+	if !f.fdBitmap.IsEmpty() {
+		max = int32(f.fdBitmap.Maximum())
+		max++
+		flipFdBitmap.FlipRange(uint32(0), uint32(max))
+		// Clear 0 to (minFD-1) bit in flipFdBitmap, thus it will not use fd that is less than minFD.
+		if minFD > 0 {
+			flipFdBitmap.ClearRange(uint32(0), uint32(minFD))
+		}
 	}

+	// Adjust max in case it is less than minFD.
+	if max < minFD {
+		max = minFD
+	}
 	// Install all entries.
-	for i := fd; i < end && len(fds) < len(files); i++ {
-		if d, _, _ := f.get(i); d == nil {
-			// Set the descriptor.
-			f.set(ctx, i, files[len(fds)], flags)
-			fds = append(fds, i) // Record the file descriptor.
+	for len(fds) < len(files) {
+		// Try to use free bit in fdBitmap.
+		// If all bits in fdBitmap are used, expand fd to the max.
+		if !flipFdBitmap.IsEmpty() {
+			fd := flipFdBitmap.Minimum()
+			f.fdBitmap.Add(fd)
+			flipFdBitmap.Remove(fd)
+			f.set(ctx, int32(fd), files[len(fds)], flags)
+			fds = append(fds, int32(fd))
+		} else if max < end {
+			f.fdBitmap.Add(uint32(max))
+			f.set(ctx, max, files[len(fds)], flags)
+			fds = append(fds, max)
+			max++
+		} else {
+			break
 		}
 	}

@@ -292,6 +321,7 @@ func (f *FDTable) NewFDs(ctx context.Context, fd int32, files []*fs.File, flags
 	if len(fds) < len(files) {
 		for _, i := range fds {
 			f.set(ctx, i, nil, FDFlags{})
+			f.fdBitmap.Remove(uint32(i))
 		}
 		f.mu.Unlock()

@@ -305,20 +335,15 @@ func (f *FDTable) NewFDs(ctx context.Context, fd int32, files []*fs.File, flags
 		return nil, unix.EMFILE
 	}

-	if fd == f.next {
-		// Update next search start position.
-		f.next = fds[len(fds)-1] + 1
-	}
-
 	f.mu.Unlock()
 	return fds, nil
 }

 // NewFDsVFS2 allocates new FDs guaranteed to be the lowest number available
-// greater than or equal to the fd parameter. All files will share the set
+// greater than or equal to the minFD parameter. All files will share the set
 // flags. Success is guaranteed to be all or none.
-func (f *FDTable) NewFDsVFS2(ctx context.Context, fd int32, files []*vfs.FileDescription, flags FDFlags) (fds []int32, err error) {
-	if fd < 0 {
+func (f *FDTable) NewFDsVFS2(ctx context.Context, minFD int32, files []*vfs.FileDescription, flags FDFlags) (fds []int32, err error) {
+	if minFD < 0 {
 		// Don't accept negative FDs.
 		return nil, unix.EINVAL
 	}
@@ -332,31 +357,56 @@ func (f *FDTable) NewFDsVFS2(ctx context.Context, fd int32, files []*vfs.FileDes
 		if lim.Cur != limits.Infinity {
 			end = int32(lim.Cur)
 		}
-		if fd >= end {
+		if minFD >= end {
 			return nil, unix.EMFILE
 		}
 	}

 	f.mu.Lock()

-	// From f.next to find available fd.
-	if fd < f.next {
-		fd = f.next
-	}
+	// max is used as the largest number in fdBitmap + 1.
+	max := int32(0)
+	// flipFdBitmap is flip of fdBitmap which is used to find free fd in fdBitmap.
+	flipFdBitmap := f.fdBitmap.Clone()

-	// Install all entries.
-	for i := fd; i < end && len(fds) < len(files); i++ {
-		if d, _, _ := f.getVFS2(i); d == nil {
-			// Set the descriptor.
-			f.setVFS2(ctx, i, files[len(fds)], flags)
-			fds = append(fds, i) // Record the file descriptor.
+	if !f.fdBitmap.IsEmpty() {
+		max = int32(f.fdBitmap.Maximum())
+		max++
+		flipFdBitmap.FlipRange(uint32(0), uint32(max))
+		// Clear 0 to (minFD-1) bit in flipFdBitmap, thus it will not use fd that is less than minFD.
+		if minFD > 0 {
+			flipFdBitmap.ClearRange(uint32(0), uint32(minFD))
 		}
 	}

+	// Adjust max in case it is less than minFD.
+	if max < minFD {
+		max = minFD
+	}
+
+	for len(fds) < len(files) {
+		// Try to use free bit in fdBitmap.
+		// If all bits in fdBitmap are used, expand fd to the max.
+		if !flipFdBitmap.IsEmpty() {
+			fd := flipFdBitmap.Minimum()
+			f.fdBitmap.Add(fd)
+			flipFdBitmap.Remove(fd)
+			f.setVFS2(ctx, int32(fd), files[len(fds)], flags)
+			fds = append(fds, int32(fd))
+		} else if max < end {
+			f.fdBitmap.Add(uint32(max))
+			f.setVFS2(ctx, max, files[len(fds)], flags)
+			fds = append(fds, max)
+			max++
+		} else {
+			break
+		}
+	}
 	// Failure? Unwind existing FDs.
 	if len(fds) < len(files) {
 		for _, i := range fds {
 			f.setVFS2(ctx, i, nil, FDFlags{})
+			f.fdBitmap.Remove(uint32(i))
 		}
 		f.mu.Unlock()

@@ -370,57 +420,19 @@ func (f *FDTable) NewFDsVFS2(ctx context.Context, fd int32, files []*vfs.FileDes
 		return nil, unix.EMFILE
 	}

-	if fd == f.next {
-		// Update next search start position.
-		f.next = fds[len(fds)-1] + 1
-	}
-
 	f.mu.Unlock()
 	return fds, nil
 }

-// NewFDVFS2 allocates a file descriptor greater than or equal to minfd for
+// NewFDVFS2 allocates a file descriptor greater than or equal to minFD for
 // the given file description. If it succeeds, it takes a reference on file.
-func (f *FDTable) NewFDVFS2(ctx context.Context, minfd int32, file *vfs.FileDescription, flags FDFlags) (int32, error) {
-	if minfd < 0 {
-		// Don't accept negative FDs.
-		return -1, unix.EINVAL
+func (f *FDTable) NewFDVFS2(ctx context.Context, minFD int32, file *vfs.FileDescription, flags FDFlags) (int32, error) {
+	files := []*vfs.FileDescription{file}
+	fileSlice, error := f.NewFDsVFS2(ctx, minFD, files, flags)
+	if error != nil {
+		return -1, error
 	}
-
-	// Default limit.
-	end := int32(math.MaxInt32)
-
-	// Ensure we don't get past the provided limit.
-	if limitSet := limits.FromContext(ctx); limitSet != nil {
-		lim := limitSet.Get(limits.NumberOfFiles)
-		if lim.Cur != limits.Infinity {
-			end = int32(lim.Cur)
-		}
-		if minfd >= end {
-			return -1, unix.EMFILE
-		}
-	}
-
-	f.mu.Lock()
-	defer f.mu.Unlock()
-
-	// From f.next to find available fd.
-	fd := minfd
-	if fd < f.next {
-		fd = f.next
-	}
-	for fd < end {
-		if d, _, _ := f.getVFS2(fd); d == nil {
-			f.setVFS2(ctx, fd, file, flags)
-			if fd == f.next {
-				// Update next search start position.
-				f.next = fd + 1
-			}
-			return fd, nil
-		}
-		fd++
-	}
-	return -1, unix.EMFILE
+	return fileSlice[0], nil
 }

 // NewFDAt sets the file reference for the given FD. If there is an active
@@ -469,6 +481,11 @@ func (f *FDTable) newFDAt(ctx context.Context, fd int32, file *fs.File, fileVFS2
 	defer f.mu.Unlock()

 	df, dfVFS2 := f.setAll(ctx, fd, file, fileVFS2, flags)
+	// Add fd to fdBitmap.
+	if file != nil || fileVFS2 != nil {
+		f.fdBitmap.Add(uint32(fd))
+	}
+
 	return df, dfVFS2, nil
 }

@@ -573,7 +590,9 @@ func (f *FDTable) GetVFS2(fd int32) (*vfs.FileDescription, FDFlags) {
 // Precondition: The caller must be running on the task goroutine, or Task.mu
 // must be locked.
 func (f *FDTable) GetFDs(ctx context.Context) []int32 {
-	fds := make([]int32, 0, int(atomic.LoadInt32(&f.used)))
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	fds := make([]int32, 0, int(f.fdBitmap.GetNumOnes()))
 	f.forEach(ctx, func(fd int32, _ *fs.File, _ *vfs.FileDescription, _ FDFlags) {
 		fds = append(fds, fd)
 	})
@@ -583,13 +602,15 @@ func (f *FDTable) GetFDs(ctx context.Context) []int32 {
 // Fork returns an independent FDTable.
 func (f *FDTable) Fork(ctx context.Context) *FDTable {
 	clone := f.k.NewFDTable()
-
+	f.mu.Lock()
+	defer f.mu.Unlock()
 	f.forEach(ctx, func(fd int32, file *fs.File, fileVFS2 *vfs.FileDescription, flags FDFlags) {
 		// The set function here will acquire an appropriate table
 		// reference for the clone. We don't need anything else.
 		if df, dfVFS2 := clone.setAll(ctx, fd, file, fileVFS2, flags); df != nil || dfVFS2 != nil {
 			panic("VFS1 or VFS2 files set")
 		}
+		clone.fdBitmap.Add(uint32(fd))
 	})
 	return clone
 }
@@ -604,11 +625,6 @@ func (f *FDTable) Remove(ctx context.Context, fd int32) (*fs.File, *vfs.FileDesc

 	f.mu.Lock()

-	// Update current available position.
-	if fd < f.next {
-		f.next = fd
-	}
-
 	orig, orig2, _, _ := f.getAll(fd)

 	// Add reference for caller.
@@ -621,6 +637,7 @@ func (f *FDTable) Remove(ctx context.Context, fd int32) (*fs.File, *vfs.FileDesc

 	if orig != nil || orig2 != nil {
 		orig, orig2 = f.setAll(ctx, fd, nil, nil, FDFlags{}) // Zap entry.
+		f.fdBitmap.Remove(uint32(fd))
 	}
 	f.mu.Unlock()

@@ -644,16 +661,13 @@ func (f *FDTable) RemoveIf(ctx context.Context, cond func(*fs.File, *vfs.FileDes
 	f.forEach(ctx, func(fd int32, file *fs.File, fileVFS2 *vfs.FileDescription, flags FDFlags) {
 		if cond(file, fileVFS2, flags) {
 			df, dfVFS2 := f.setAll(ctx, fd, nil, nil, FDFlags{}) // Clear from table.
+			f.fdBitmap.Remove(uint32(fd))
 			if df != nil {
 				files = append(files, df)
 			}
 			if dfVFS2 != nil {
 				filesVFS2 = append(filesVFS2, dfVFS2)
 			}
-			// Update current available position.
-			if fd < f.next {
-				f.next = fd
-			}
 		}
 	})
 	f.mu.Unlock()
@@ -15,9 +15,11 @@
 package kernel

 import (
+	"math"
 	"sync/atomic"
 	"unsafe"

+	"gvisor.dev/gvisor/pkg/bitmap"
 	"gvisor.dev/gvisor/pkg/context"
 	"gvisor.dev/gvisor/pkg/sentry/fs"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
@@ -44,6 +46,7 @@ func (f *FDTable) initNoLeakCheck() {
 func (f *FDTable) init() {
 	f.initNoLeakCheck()
 	f.InitRefs()
+	f.fdBitmap = bitmap.BitmapWithSize(uint32(math.MaxUint16))
 }

 // get gets a file entry.
@@ -162,14 +165,6 @@ func (f *FDTable) setAll(ctx context.Context, fd int32, file *fs.File, fileVFS2
 		}
 	}

-	// Adjust used.
-	switch {
-	case orig == nil && desc != nil:
-		atomic.AddInt32(&f.used, 1)
-	case orig != nil && desc == nil:
-		atomic.AddInt32(&f.used, -1)
-	}
-
 	if orig != nil {
 		switch {
 		case orig.file != nil: