btrfs: Add datacow mount option.

Add datacow mount option to enable copy-on-write with
remount option.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>

Documentation/filesystems/btrfs.txt

@@ -154,9 +154,10 @@ Options with (*) are default options and will not show in the mount options.
 	(non-battery-backed) write-back cache, nobarrier option will lead to
 	filesystem corruption on a system crash or power loss.
 
+  datacow(*)
   nodatacow
-	Disable data copy-on-write for newly created files.  Implies nodatasum,
-	and disables all compression.
+	Enable/disable data copy-on-write for newly created files.
+	Nodatacow implies nodatasum, and disables all compression.
 
   nodatasum
 	Disable data checksumming for newly created files.

fs/btrfs/super.c

@@ -324,7 +324,7 @@ enum {
 	Opt_check_integrity, Opt_check_integrity_including_extent_data,
 	Opt_check_integrity_print_mask, Opt_fatal_errors, Opt_rescan_uuid_tree,
 	Opt_commit_interval, Opt_barrier, Opt_nodefrag, Opt_nodiscard,
-	Opt_noenospc_debug, Opt_noflushoncommit, Opt_acl,
+	Opt_noenospc_debug, Opt_noflushoncommit, Opt_acl, Opt_datacow,
 	Opt_err,
 };
 
@@ -335,6 +335,7 @@ static match_table_t tokens = {
 	{Opt_device, "device=%s"},
 	{Opt_nodatasum, "nodatasum"},
 	{Opt_nodatacow, "nodatacow"},
+	{Opt_datacow, "datacow"},
 	{Opt_nobarrier, "nobarrier"},
 	{Opt_barrier, "barrier"},
 	{Opt_max_inline, "max_inline=%s"},
@@ -446,6 +447,11 @@ int btrfs_parse_options(struct btrfs_root *root, char *options)
 			btrfs_set_opt(info->mount_opt, NODATACOW);
 			btrfs_set_opt(info->mount_opt, NODATASUM);
 			break;
+		case Opt_datacow:
+			if (btrfs_test_opt(root, NODATACOW))
+				btrfs_info(root->fs_info, "setting datacow");
+			btrfs_clear_opt(info->mount_opt, NODATACOW);
+			break;
 		case Opt_compress_force:
 		case Opt_compress_force_type:
 			compress_force = true;