Re: [PATCH 3/3] btrfs: don't init dev replace for bad dev root

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

On 12/3/21 12:23 am, Josef Bacik wrote:
> While helping Neal fix his broken file system I added a debug patch to
> catch if we were calling btrfs_search_slot with a NULL root, and this
> stack trace popped
> 
> we tried to search with a NULL root
> CPU: 0 PID: 1760 Comm: mount Not tainted 5.11.0-155.nealbtrfstest.1.fc34.x86_64 #1
> Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 07/22/2020
> Call Trace:
>   dump_stack+0x6b/0x83
>   btrfs_search_slot.cold+0x11/0x1b
>   ? btrfs_init_dev_replace+0x36/0x450
>   btrfs_init_dev_replace+0x71/0x450
>   open_ctree+0x1054/0x1610
>   btrfs_mount_root.cold+0x13/0xfa
>   legacy_get_tree+0x27/0x40
>   vfs_get_tree+0x25/0xb0
>   vfs_kern_mount.part.0+0x71/0xb0
>   btrfs_mount+0x131/0x3d0
>   ? legacy_get_tree+0x27/0x40
>   ? btrfs_show_options+0x640/0x640
>   legacy_get_tree+0x27/0x40
>   vfs_get_tree+0x25/0xb0
>   path_mount+0x441/0xa80
>   __x64_sys_mount+0xf4/0x130
>   do_syscall_64+0x33/0x40
>   entry_SYSCALL_64_after_hwframe+0x44/0xa9
> RIP: 0033:0x7f644730352e
> 
> Fix this by not starting the device replace stuff if we do not have a
> NULL dev root.
> 
> Reported-by: Neal Gompa <ngompa13@gmail.com>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>   fs/btrfs/dev-replace.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
> index 3a9c1e046ebe..d05f73530af7 100644
> --- a/fs/btrfs/dev-replace.c
> +++ b/fs/btrfs/dev-replace.c
> @@ -81,6 +81,9 @@ int btrfs_init_dev_replace(struct btrfs_fs_info *fs_info)
>   	struct btrfs_dev_replace_item *ptr;
>   	u64 src_devid;
>   
> +	if (!dev_root)
> +		return 0;
> +

  If we have a replace device target. fs_info->dev_replace->is_valid is
  0, and btrfs_dev_replace_is_ongoing() is false. And num_device count
  at various places does not include replace target device. And also
  this happens only in RO mount.

  Reviewed-by: Anand Jain <anand.jain@oracle.com>


>   	path = btrfs_alloc_path();
>   	if (!path) {
>   		ret = -ENOMEM;
>