btrfs: Add nodiscard mount option.

Add nodiscard mount option to disable discard 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

@@ -98,9 +98,12 @@ Options with (*) are default options and will not show in the mount options.
 	can be avoided.  Especially useful when trying to mount a multi-device
 	setup as root.  May be specified multiple times for multiple devices.
 
+  nodiscard(*)
   discard
-	Issue frequent commands to let the block device reclaim space freed by
-	the filesystem.  This is useful for SSD devices, thinly provisioned
+	Disable/enable discard mount option.
+	Discard issues frequent commands to let the block device reclaim space
+	freed by the filesystem.
+	This is useful for SSD devices, thinly provisioned
 	LUNs and virtual machine images, but may have a significant
 	performance impact.  (The fstrim command is also available to
 	initiate batch trims from userspace).

fs/btrfs/super.c

@@ -323,7 +323,7 @@ enum {
 	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_nodefrag,
+	Opt_commit_interval, Opt_barrier, Opt_nodefrag, Opt_nodiscard,
 	Opt_err,
 };
 
@@ -351,6 +351,7 @@ static match_table_t tokens = {
 	{Opt_flushoncommit, "flushoncommit"},
 	{Opt_ratio, "metadata_ratio=%d"},
 	{Opt_discard, "discard"},
+	{Opt_nodiscard, "nodiscard"},
 	{Opt_space_cache, "space_cache"},
 	{Opt_clear_cache, "clear_cache"},
 	{Opt_user_subvol_rm_allowed, "user_subvol_rm_allowed"},
@@ -575,6 +576,9 @@ int btrfs_parse_options(struct btrfs_root *root, char *options)
 		case Opt_discard:
 			btrfs_set_opt(info->mount_opt, DISCARD);
 			break;
+		case Opt_nodiscard:
+			btrfs_clear_opt(info->mount_opt, DISCARD);
+			break;
 		case Opt_space_cache:
 			btrfs_set_opt(info->mount_opt, SPACE_CACHE);
 			break;