dmz: use sendfile(2) when cloning /proc/self/exe

This results in a 5-20% speedup of dmz.CloneBinary(), depending on the machine. io.Copy: goos: linux goarch: amd64 pkg: github.com/opencontainers/runc/libcontainer/dmz cpu: Intel(R) Core(TM) i5-10210U CPU @ 1.60GHz BenchmarkCloneBinary BenchmarkCloneBinary-8 139 8075074 ns/op PASS ok github.com/opencontainers/runc/libcontainer/dmz 2.286s unix.Sendfile: goos: linux goarch: amd64 pkg: github.com/opencontainers/runc/libcontainer/dmz cpu: Intel(R) Core(TM) i5-10210U CPU @ 1.60GHz BenchmarkCloneBinary BenchmarkCloneBinary-8 192 6382121 ns/op PASS ok github.com/opencontainers/runc/libcontainer/dmz 2.415s Signed-off-by: Aleksa Sarai <[email protected]>
opencontainers · Sep 22, 2023 · 90c8d36 · 90c8d36
1 parent f8348f6
commit 90c8d36
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Showing 2 changed files with 41 additions and 1 deletion.
diff --git a/libcontainer/dmz/cloned_binary_linux.go b/libcontainer/dmz/cloned_binary_linux.go
@@ -179,7 +179,7 @@ func CloneBinary(src io.Reader, size int64, name, tmpDir string) (*os.File, erro
 	if err != nil {
 		return nil, err
 	}
-	copied, err := io.Copy(file, src)
+	copied, err := system.Copy(file, src)
 	if err != nil {
 		file.Close()
 		return nil, fmt.Errorf("copy binary: %w", err)

diff --git a/libcontainer/system/linux.go b/libcontainer/system/linux.go
@@ -5,6 +5,7 @@ package system
 
 import (
 	"fmt"
+	"io"
 	"os"
 	"os/exec"
 	"strconv"
@@ -174,3 +175,42 @@ func ExecutableMemfd(comment string, flags int) (*os.File, error) {
 	}
 	return os.NewFile(uintptr(memfd), "/memfd:"+comment), nil
 }
+
+// Copy is like io.Copy except it uses sendfile(2) if the source and sink are
+// both (*os.File) as an optimisation to make copies faster.
+func Copy(dst io.Writer, src io.Reader) (copied int64, err error) {
+	dstFile, _ := dst.(*os.File)
+	srcFile, _ := src.(*os.File)
+
+	if dstFile != nil && srcFile != nil {
+		fi, err := srcFile.Stat()
+		if err != nil {
+			goto fallback
+		}
+		size := fi.Size()
+		for size > 0 {
+			n, err := unix.Sendfile(int(dstFile.Fd()), int(srcFile.Fd()), nil, int(size))
+			if n > 0 {
+				size -= int64(n)
+				copied += int64(n)
+			}
+			if err == unix.EINTR {
+				continue
+			}
+			if err != nil {
+				if copied == 0 {
+					// If we haven't copied anything so far, we can safely just
+					// fallback to io.Copy. We could always do the fallback but
+					// it's safer to error out in the case of a partial copy
+					// followed by an error (which should never happen).
+					goto fallback
+				}
+				return copied, fmt.Errorf("partial sendfile copy: %w", err)
+			}
+		}
+		return copied, nil
+	}
+
+fallback:
+	return io.Copy(dst, src)
+}