Re: [PATCH v2] btrfs: fix rw device counting in __btrfs_free_extra_devids

[David Sterba]

On Tue, Jul 27, 2021 at 03:13:03PM +0800, Desmond Cheong Zhi Xi wrote:
> When removing a writeable device in __btrfs_free_extra_devids, the rw
> device count should be decremented.
> 
> This error was caught by Syzbot which reported a warning in
> close_fs_devices because fs_devices->rw_devices was not 0 after
> closing all devices. Here is the call trace that was observed:
> 
>   btrfs_mount_root():
>     btrfs_scan_one_device():
>       device_list_add();   <---------------- device added
>     btrfs_open_devices():
>       open_fs_devices():
>         btrfs_open_one_device();   <-------- writable device opened,
> 	                                     rw device count ++
>     btrfs_fill_super():
>       open_ctree():
>         btrfs_free_extra_devids():
> 	  __btrfs_free_extra_devids();  <--- writable device removed,
> 	                              rw device count not decremented
> 	  fail_tree_roots:
> 	    btrfs_close_devices():
> 	      close_fs_devices();   <------- rw device count off by 1
> 
> As a note, prior to commit cf89af146b7e ("btrfs: dev-replace: fail
> mount if we don't have replace item with target device"), rw_devices
> was decremented on removing a writable device in
> __btrfs_free_extra_devids only if the BTRFS_DEV_STATE_REPLACE_TGT bit
> was not set for the device. However, this check does not need to be
> reinstated as it is now redundant and incorrect.
> 
> In __btrfs_free_extra_devids, we skip removing the device if it is the
> target for replacement. This is done by checking whether device->devid
> == BTRFS_DEV_REPLACE_DEVID. Since BTRFS_DEV_STATE_REPLACE_TGT is set
> only on the device with devid BTRFS_DEV_REPLACE_DEVID, no devices
> should have the BTRFS_DEV_STATE_REPLACE_TGT bit set after the check,
> and so it's redundant to test for that bit.
> 
> Additionally, following commit 82372bc816d7 ("Btrfs: make
> the logic of source device removing more clear"), rw_devices is
> incremented whenever a writeable device is added to the alloc
> list (including the target device in btrfs_dev_replace_finishing), so
> all removals of writable devices from the alloc list should also be
> accompanied by a decrement to rw_devices.
> 
> Fixes: cf89af146b7e ("btrfs: dev-replace: fail mount if we don't have replace item with target device")
> Reported-by: syzbot+a70e2ad0879f160b9217@xxxxxxxxxxxxxxxxxxxxxxxxx
> Tested-by: syzbot+a70e2ad0879f160b9217@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
> Reviewed-by: Anand Jain <anand.jain@xxxxxxxxxx>

Added to misc-next, thanks.