8265175: (fs) Files.copy(Path,Path,CopyOption...) should use sendfile…

… on Linux

Reviewed-by: alanb

src/java.base/unix/native/libnio/fs/UnixCopyFile.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008, 2009, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2008, 2021, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -30,6 +30,10 @@
 #include <unistd.h>
 #include <errno.h>
 
+#if defined(__linux__)
+#include <sys/sendfile.h>
+#endif
+
 #include "sun_nio_fs_UnixCopyFile.h"
 
 #define RESTARTABLE(_cmd, _result) do { \
@@ -47,15 +51,35 @@ static void throwUnixException(JNIEnv* env, int errnum) {
 }
 
 /**
- * Transfer all bytes from src to dst via user-space buffers
+ * Transfer all bytes from src to dst within the kernel if possible (Linux),
+ * otherwise via user-space buffers
  */
 JNIEXPORT void JNICALL
 Java_sun_nio_fs_UnixCopyFile_transfer
     (JNIEnv* env, jclass this, jint dst, jint src, jlong cancelAddress)
 {
-    char buf[8192];
     volatile jint* cancel = (jint*)jlong_to_ptr(cancelAddress);
 
+#if defined(__linux__)
+    // Transfer within the kernel
+    const size_t count = 1048576; // 1 MB to give cancellation a chance
+    ssize_t bytes_sent;
+    do {
+        // sendfile() can transfer at most 0x7ffff000 bytes
+        RESTARTABLE(sendfile64(dst, src, NULL, count), bytes_sent);
+        if (bytes_sent == -1) {
+            throwUnixException(env, errno);
+            return;
+        }
+        if (cancel != NULL && *cancel != 0) {
+            throwUnixException(env, ECANCELED);
+            return;
+        }
+    } while (bytes_sent > 0);
+#else
+    // Transfer via user-space buffers
+    char buf[8192];
+
     for (;;) {
         ssize_t n, pos, len;
         RESTARTABLE(read((int)src, &buf, sizeof(buf)), n);
@@ -82,4 +106,5 @@ Java_sun_nio_fs_UnixCopyFile_transfer
             len -= n;
         } while (len > 0);
     }
+#endif
 }