btrfs: Add noautodefrag mount option.

Btrfs has autodefrag mount option but no pairing noautodefrag option,
which makes it impossible to disable autodefrag without umount.

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

diff --git a/Documentation/filesystems/btrfs.txt b/Documentation/filesystems/btrfs.txt
index ce487a2..e87609a 100644
--- a/Documentation/filesystems/btrfs.txt
+++ b/Documentation/filesystems/btrfs.txt
@@ -46,10 +46,12 @@
 	bytes, optionally with a K, M, or G suffix, case insensitive.
 	Default is 1MB.
 
+  noautodefrag(*)
   autodefrag
-	Detect small random writes into files and queue them up for the
-	defrag process.  Works best for small files; Not well suited for
-	large database workloads.
+	Disable/enable auto defragmentation.
+	Auto defragmentation detects small random writes into files and queue
+	them up for the defrag process.  Works best for small files;
+	Not well suited for large database workloads.
 
   check_int
   check_int_data

fs/btrfs/super.c

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index b02d25a..44513f3 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -323,7 +323,7 @@
 	Opt_no_space_cache, Opt_recovery, Opt_skip_balance,
 	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_commit_interval, Opt_barrier, Opt_nodefrag,
 	Opt_err,
 };
 
@@ -357,6 +357,7 @@
 	{Opt_enospc_debug, "enospc_debug"},
 	{Opt_subvolrootid, "subvolrootid=%d"},
 	{Opt_defrag, "autodefrag"},
+	{Opt_nodefrag, "noautodefrag"},
 	{Opt_inode_cache, "inode_cache"},
 	{Opt_no_space_cache, "nospace_cache"},
 	{Opt_recovery, "recovery"},
@@ -602,6 +603,11 @@
 			btrfs_info(root->fs_info, "enabling auto defrag");
 			btrfs_set_opt(info->mount_opt, AUTO_DEFRAG);
 			break;
+		case Opt_nodefrag:
+			if (btrfs_test_opt(root, AUTO_DEFRAG))
+				btrfs_info(root->fs_info, "disabling auto defrag");
+			btrfs_clear_opt(info->mount_opt, AUTO_DEFRAG);
+			break;
 		case Opt_recovery:
 			btrfs_info(root->fs_info, "enabling auto recovery");
 			btrfs_set_opt(info->mount_opt, RECOVERY);