btrfs: add helper to obtain number of devices with ongoing dev-replace

When the replace is running the fs_devices::num_devices also includes
the replaced device, however in some operations like device delete and
balance it needs the actual num_devices without the repalced devices.
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 unmount or system power-cycle. During following mount, the
open_ctree() first restarts the balance so it must check for the device
replace otherwise our num_devices calculation will be wrong.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 3946fe951b60ce4bd85b96c86b08d1c5e7e57636..ea9845a5e6012b71893ae35d1a03cd0ec4838bee 100644 (file)
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1854,6 +1854,24 @@ void btrfs_assign_next_active_device(struct btrfs_device *device,
                fs_info->fs_devices->latest_bdev = next_device->bdev;
 }
 
+/*
+ * Return btrfs_fs_devices::num_devices excluding the device that's being
+ * currently replaced.
+ */
+static u64 btrfs_num_devices(struct btrfs_fs_info *fs_info)
+{
+       u64 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)) {
+               ASSERT(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 +1883,7 @@ 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)) {
-               ASSERT(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 +3752,8 @@ 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)) {
-               ASSERT(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);