Re: [PATCH v2 1/4] btrfs: mark resumed async balance as writing

[Filipe Manana <fdmanana@kernel.org>]

On Wed, Mar 16, 2022 at 10:22:37PM +0900, Naohiro Aota wrote:
> When btrfs balance is interrupted with umount, the background balance
> resumes on the next mount. There is a potential deadlock with FS freezing
> here like as described in commit 26559780b953 ("btrfs: zoned: mark
> relocation as writing").
> 
> Mark the process as sb_writing. To preserve the order of sb_start_write()
> (or mnt_want_write_file()) and btrfs_exclop_start(), call sb_start_write()
> at btrfs_resume_balance_async() before taking fs_info->super_lock.

This paragraph is now outdated, it should go away as it applied only to v1.
The ordering problem is no longer relevant and we don't do anything at
btrfs_resume_balance_async() anymore.

> 
> Cc: stable@vger.kernel.org # 4.9+
> Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>

Other than that, it looks good.

Reviewed-by: Filipe Manana <fdmanana@suse.com>

Thanks.

> ---
>  fs/btrfs/volumes.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 3fd17e87815a..3471698fd831 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -4430,10 +4430,12 @@ static int balance_kthread(void *data)
>  	struct btrfs_fs_info *fs_info = data;
>  	int ret = 0;
>  
> +	sb_start_write(fs_info->sb);
>  	mutex_lock(&fs_info->balance_mutex);
>  	if (fs_info->balance_ctl)
>  		ret = btrfs_balance(fs_info, fs_info->balance_ctl, NULL);
>  	mutex_unlock(&fs_info->balance_mutex);
> +	sb_end_write(fs_info->sb);
>  
>  	return ret;
>  }
> -- 
> 2.35.1
>