The seed filesystem was destroyed by the device replace, the reproduce
method is:
 # mkfs.btrfs -f <dev0>
 # btrfstune -S 1 <dev0>
 # mount <dev0> <mnt>
 # btrfs device add <dev1> <mnt>
 # umount <mnt>
 # mount <dev1> <mnt>
 # btrfs replace start -f <dev0> <dev2> <mnt>
 # umount <mnt>
 # mount <dev0> <mnt>

It is because we erase the super block on the seed device. It is wrong,
we should not change anything on the seed device.

Signed-off-by: Miao Xie <mi...@cn.fujitsu.com>
---
 fs/btrfs/volumes.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 2776070f..19188df 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1853,8 +1853,12 @@ void btrfs_rm_dev_replace_srcdev(struct btrfs_fs_info 
*fs_info,
        if (srcdev->bdev) {
                fs_info->fs_devices->open_devices--;
 
-               /* zero out the old super */
-               btrfs_scratch_superblock(srcdev);
+               /*
+                * zero out the old super if it is not writable
+                * (e.g. seed device)
+                */
+               if (srcdev->writeable)
+                       btrfs_scratch_superblock(srcdev);
        }
 
        call_rcu(&srcdev->rcu, free_device);
-- 
1.9.3

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