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

[Filipe Manana <fdmanana@kernel.org>]

On Fri, Mar 11, 2022 at 04:38:02PM +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.
> 
> Fixes: 5accdf82ba25 ("fs: Improve filesystem freezing handling")

This seems odd to me. I read the note you left on the cover letter about
this, but honestly I don't think it's fair to blame that commit. I see
it more as btrfs specific problem.

Plus it's a 10 years old commit, so instead of the Fixes tag, adding a
minimal kernel version to the CC stable tag below makes more sense.

> Cc: stable@vger.kernel.org
> Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
> ---
>  fs/btrfs/volumes.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 1be7cb2f955f..0d27d8d35c7a 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -4443,6 +4443,7 @@ static int balance_kthread(void *data)
>  	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;
>  }
> @@ -4463,6 +4464,7 @@ int btrfs_resume_balance_async(struct btrfs_fs_info *fs_info)
>  		return 0;
>  	}
>  
> +	sb_start_write(fs_info->sb);

I don't understand this.

We are doing the sb_start_write() here, in the task doing the mount, and then
we do the sb_end_write() at the kthread that runs balance_kthread().

Why not do the sb_start_write() in the kthread?

This is also buggy in the case the call below to kthread_run() fails, as
we end up never calling sb_end_write().

Thanks.

>  	spin_lock(&fs_info->super_lock);
>  	ASSERT(fs_info->exclusive_operation == BTRFS_EXCLOP_BALANCE_PAUSED);
>  	fs_info->exclusive_operation = BTRFS_EXCLOP_BALANCE;
> -- 
> 2.35.1
>