From ff61d17c6324d1b483fbbc5144f09668c24ff60c Mon Sep 17 00:00:00 2001
From: Miao Xie <miaox@cn.fujitsu.com>
Date: Thu, 24 Jul 2014 11:37:06 +0800
Subject: [PATCH] Btrfs: Fix the problem that the replace destroys the seed
 filesystem

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 <miaox@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.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 da0e632a21fc..00c8efdcd1e5 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1848,8 +1848,12 @@ 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);