[PATCH v4 4/4] btrfs: add helper btrfs_num_devices() to deduce num_devices

[Anand Jain <anand.jain@oracle.com>]

When the replace is running the fs_devices::num_devices also includes
the replace device, however in some operations like device delete and
balance it needs the actual num_devices without the repalce devices, so
now the function btrfs_num_devices() just provides that.

And here is a scenario how balance and repalce items could co-exist.
Consider balance is started and paused, now start the replace
followed by a power-recycle of the system. During following mount,
the open_ctree() first restarts the balance so it must check for the
replace device otherwise our num_devices calculation will be wrong.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
v3->v4: add comment and drop the inline (sorry missed it before)
v2->v3: update changelog with not so obvious balance and repalce
co-existance secnario
v1->v2: add comments

 fs/btrfs/volumes.c | 38 ++++++++++++++++++++++++--------------
 1 file changed, 24 insertions(+), 14 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 6dad9257d4ba..7359596ac8eb 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1854,6 +1854,27 @@ void btrfs_assign_next_active_device(struct btrfs_device *device,
 		fs_info->fs_devices->latest_bdev = next_device->bdev;
 }
 
+/* Returns btrfs_fs_devices::num_devices minus replace device if any */
+static u64 btrfs_num_devices(struct btrfs_fs_info *fs_info)
+{
+	u64 num_devices = fs_info->fs_devices->num_devices;
+
+	/*
+	 * balance and replace co-exists in a scenario as below..
+	 * start balance and pause, start replace and reboot.
+	 * now open_ctree() restarts the balance first and checks for the
+	 * num_devices which should exclude the replace target.
+	 */
+	btrfs_dev_replace_read_lock(&fs_info->dev_replace);
+	if (btrfs_dev_replace_is_ongoing(&fs_info->dev_replace)) {
+		BUG_ON(num_devices < 1);
+		num_devices--;
+	}
+	btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
+
+	return num_devices;
+}
+
 int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
 		u64 devid)
 {
@@ -1865,13 +1886,7 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
 
 	mutex_lock(&uuid_mutex);
 
-	num_devices = fs_devices->num_devices;
-	btrfs_dev_replace_read_lock(&fs_info->dev_replace);
-	if (btrfs_dev_replace_is_ongoing(&fs_info->dev_replace)) {
-		BUG_ON(num_devices < 1);
-		num_devices--;
-	}
-	btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
+	num_devices = btrfs_num_devices(fs_info);
 
 	ret = btrfs_check_raid_min_devices(fs_info, num_devices - 1);
 	if (ret)
@@ -3740,13 +3755,8 @@ int btrfs_balance(struct btrfs_fs_info *fs_info,
 		}
 	}
 
-	num_devices = fs_info->fs_devices->num_devices;
-	btrfs_dev_replace_read_lock(&fs_info->dev_replace);
-	if (btrfs_dev_replace_is_ongoing(&fs_info->dev_replace)) {
-		BUG_ON(num_devices < 1);
-		num_devices--;
-	}
-	btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
+	num_devices = btrfs_num_devices(fs_info);
+
 	allowed = BTRFS_AVAIL_ALLOC_BIT_SINGLE | BTRFS_BLOCK_GROUP_DUP;
 	if (num_devices > 1)
 		allowed |= (BTRFS_BLOCK_GROUP_RAID0 | BTRFS_BLOCK_GROUP_RAID1);
-- 
2.7.0