[PATCH 1/3] btrfs: introduce BTRFS_EXCLOP_BALANCE_PAUSED exclusive state

[Nikolay Borisov <nborisov@suse.com>]

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>
---
 fs/btrfs/ctree.h   |  1 +
 fs/btrfs/ioctl.c   | 20 +++++++++++++++++---
 fs/btrfs/volumes.c | 23 +++++++++++++++++++----
 fs/btrfs/volumes.h |  2 +-
 4 files changed, 38 insertions(+), 8 deletions(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 7553e9dc5f93..b1916fa548b6 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -613,6 +613,7 @@ enum {
  */
 enum btrfs_exclusive_operation {
 	BTRFS_EXCLOP_NONE,
+	BTRFS_EXCLOP_BALANCE_PAUSED,
 	BTRFS_EXCLOP_BALANCE,
 	BTRFS_EXCLOP_DEV_ADD,
 	BTRFS_EXCLOP_DEV_REMOVE,
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index c474f7e24163..4aa6191889a3 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -3982,6 +3982,7 @@ static long btrfs_ioctl_balance(struct file *file, void __user *arg)
 	struct btrfs_fs_info *fs_info = root->fs_info;
 	struct btrfs_ioctl_balance_args *bargs;
 	struct btrfs_balance_control *bctl;
+	bool paused = false;
 	bool need_unlock; /* for mut. excl. ops lock */
 	int ret;
 
@@ -4019,6 +4020,10 @@ static long btrfs_ioctl_balance(struct file *file, void __user *arg)
 			if (fs_info->balance_ctl &&
 			    !test_bit(BTRFS_FS_BALANCE_RUNNING, &fs_info->flags)) {
 				/* this is (3) */
+				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);
 				need_unlock = false;
 				goto locked;
 			}
@@ -4100,7 +4105,7 @@ static long btrfs_ioctl_balance(struct file *file, void __user *arg)
 	 */
 	need_unlock = false;
 
-	ret = btrfs_balance(fs_info, bctl, bargs);
+	ret = btrfs_balance(fs_info, bctl, bargs, &paused);
 	bctl = NULL;
 
 	if ((ret == 0 || ret == -ECANCELED) && arg) {
@@ -4114,8 +4119,17 @@ static long btrfs_ioctl_balance(struct file *file, void __user *arg)
 	kfree(bargs);
 out_unlock:
 	mutex_unlock(&fs_info->balance_mutex);
-	if (need_unlock)
-		btrfs_exclop_finish(fs_info);
+	if (need_unlock) {
+		if (paused) {
+			/* Transition directly to BALANCE_PAUSED */
+			spin_lock(&fs_info->super_lock);
+			ASSERT(fs_info->exclusive_operation == BTRFS_EXCLOP_BALANCE);
+			fs_info->exclusive_operation = BTRFS_EXCLOP_BALANCE_PAUSED;
+			spin_unlock(&fs_info->super_lock);
+		} else {
+			btrfs_exclop_finish(fs_info);
+		}
+	}
 out:
 	mnt_drop_write_file(file);
 	return ret;
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 546bf1146b2d..593583307f55 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4219,7 +4219,7 @@ static void describe_balance_start_or_resume(struct btrfs_fs_info *fs_info)
  */
 int btrfs_balance(struct btrfs_fs_info *fs_info,
 		  struct btrfs_balance_control *bctl,
-		  struct btrfs_ioctl_balance_args *bargs)
+		  struct btrfs_ioctl_balance_args *bargs, bool *paused)
 {
 	u64 meta_target, data_target;
 	u64 allowed;
@@ -4355,8 +4355,11 @@ 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");
+		if (paused)
+			*paused = true;
+	}
 	/*
 	 * Balance can be canceled by:
 	 *
@@ -4406,13 +4409,21 @@ int btrfs_balance(struct btrfs_fs_info *fs_info,
 static int balance_kthread(void *data)
 {
 	struct btrfs_fs_info *fs_info = data;
+	bool paused = false;
 	int ret = 0;
 
 	mutex_lock(&fs_info->balance_mutex);
 	if (fs_info->balance_ctl)
-		ret = btrfs_balance(fs_info, fs_info->balance_ctl, NULL);
+		ret = btrfs_balance(fs_info, fs_info->balance_ctl, NULL, &paused);
 	mutex_unlock(&fs_info->balance_mutex);
 
+	if (paused) {
+		spin_lock(&fs_info->super_lock);
+		ASSERT(fs_info->exclusive_operation == BTRFS_EXCLOP_BALANCE);
+		fs_info->exclusive_operation = BTRFS_EXCLOP_BALANCE_PAUSED;
+		spin_unlock(&fs_info->super_lock);
+	}
+
 	return ret;
 }
 
@@ -4432,6 +4443,10 @@ 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
@@ -4500,7 +4515,7 @@ 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");
 
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 3b8130680749..babcf447104b 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -541,7 +541,7 @@ int btrfs_shrink_device(struct btrfs_device *device, u64 new_size);
 int btrfs_init_new_device(struct btrfs_fs_info *fs_info, const char *path);
 int btrfs_balance(struct btrfs_fs_info *fs_info,
 		  struct btrfs_balance_control *bctl,
-		  struct btrfs_ioctl_balance_args *bargs);
+		  struct btrfs_ioctl_balance_args *bargs, bool *paused);
 void btrfs_describe_block_groups(u64 flags, char *buf, u32 size_buf);
 int btrfs_resume_balance_async(struct btrfs_fs_info *fs_info);
 int btrfs_recover_balance(struct btrfs_fs_info *fs_info);
-- 
2.25.1