btrfs: introduce exclusive operation BALANCE_PAUSED state

Current set of exclusive operation states is not sufficient to handle
all practical use cases. In particular there is a need to be able to add
a device to a filesystem that have paused balance. Currently there is no
way to distinguish between a running and a paused balance. Fix this by
introducing BTRFS_EXCLOP_BALANCE_PAUSED which is going to be set in 2
occasions:

1. When a filesystem is mounted with skip_balance and there is an
   unfinished balance it will now be into BALANCE_PAUSED instead of
   simply BALANCE state.

2. When a running balance is paused.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -611,6 +611,7 @@ enum {
  */
 enum btrfs_exclusive_operation {
 	BTRFS_EXCLOP_NONE,
+	BTRFS_EXCLOP_BALANCE_PAUSED,
 	BTRFS_EXCLOP_BALANCE,
 	BTRFS_EXCLOP_DEV_ADD,
 	BTRFS_EXCLOP_DEV_REMOVE,
@@ -3321,6 +3322,9 @@ bool btrfs_exclop_start_try_lock(struct btrfs_fs_info *fs_info,
 				 enum btrfs_exclusive_operation type);
 void btrfs_exclop_start_unlock(struct btrfs_fs_info *fs_info);
 void btrfs_exclop_finish(struct btrfs_fs_info *fs_info);
+void btrfs_exclop_balance(struct btrfs_fs_info *fs_info,
+			  enum btrfs_exclusive_operation op);
+
 
 /* file.c */
 int __init btrfs_auto_defrag_init(void);

fs/btrfs/ioctl.c

@@ -414,6 +414,29 @@ void btrfs_exclop_finish(struct btrfs_fs_info *fs_info)
 	sysfs_notify(&fs_info->fs_devices->fsid_kobj, NULL, "exclusive_operation");
 }
 
+void btrfs_exclop_balance(struct btrfs_fs_info *fs_info,
+			  enum btrfs_exclusive_operation op)
+{
+	switch (op) {
+	case BTRFS_EXCLOP_BALANCE_PAUSED:
+		spin_lock(&fs_info->super_lock);
+		ASSERT(fs_info->exclusive_operation == BTRFS_EXCLOP_BALANCE ||
+		       fs_info->exclusive_operation == BTRFS_EXCLOP_DEV_ADD);
+		fs_info->exclusive_operation = BTRFS_EXCLOP_BALANCE_PAUSED;
+		spin_unlock(&fs_info->super_lock);
+		break;
+	case BTRFS_EXCLOP_BALANCE:
+		spin_lock(&fs_info->super_lock);
+		ASSERT(fs_info->exclusive_operation == BTRFS_EXCLOP_BALANCE_PAUSED);
+		fs_info->exclusive_operation = BTRFS_EXCLOP_BALANCE;
+		spin_unlock(&fs_info->super_lock);
+		break;
+	default:
+		btrfs_warn(fs_info,
+			"invalid exclop balance operation %d requested", op);
+	}
+}
+
 static int btrfs_ioctl_getversion(struct file *file, int __user *arg)
 {
 	struct inode *inode = file_inode(file);
@@ -4056,6 +4079,7 @@ static long btrfs_ioctl_balance(struct file *file, void __user *arg)
 			spin_lock(&fs_info->balance_lock);
 			bctl->flags |= BTRFS_BALANCE_RESUME;
 			spin_unlock(&fs_info->balance_lock);
+			btrfs_exclop_balance(fs_info, BTRFS_EXCLOP_BALANCE);
 
 			goto do_balance;
 		}

fs/btrfs/volumes.c

@@ -4394,8 +4394,10 @@ int btrfs_balance(struct btrfs_fs_info *fs_info,
 	ret = __btrfs_balance(fs_info);
 
 	mutex_lock(&fs_info->balance_mutex);
-	if (ret == -ECANCELED && atomic_read(&fs_info->balance_pause_req))
+	if (ret == -ECANCELED && atomic_read(&fs_info->balance_pause_req)) {
 		btrfs_info(fs_info, "balance: paused");
+		btrfs_exclop_balance(fs_info, BTRFS_EXCLOP_BALANCE_PAUSED);
+	}
 	/*
 	 * Balance can be canceled by:
 	 *
@@ -4471,6 +4473,10 @@ int btrfs_resume_balance_async(struct btrfs_fs_info *fs_info)
 		return 0;
 	}
 
+	spin_lock(&fs_info->super_lock);
+	ASSERT(fs_info->exclusive_operation == BTRFS_EXCLOP_BALANCE_PAUSED);
+	fs_info->exclusive_operation = BTRFS_EXCLOP_BALANCE;
+	spin_unlock(&fs_info->super_lock);
 	/*
 	 * A ro->rw remount sequence should continue with the paused balance
 	 * regardless of who pauses it, system or the user as of now, so set
@@ -4539,7 +4545,7 @@ int btrfs_recover_balance(struct btrfs_fs_info *fs_info)
 	 * is in a paused state and must have fs_info::balance_ctl properly
 	 * set up.
 	 */
-	if (!btrfs_exclop_start(fs_info, BTRFS_EXCLOP_BALANCE))
+	if (!btrfs_exclop_start(fs_info, BTRFS_EXCLOP_BALANCE_PAUSED))
 		btrfs_warn(fs_info,
 	"balance: cannot set exclusive op status, resume manually");