Re: [PATCH v6] memcg: Don't wait writeback completion when release memcg.

[Tejun Heo <tj@kernel.org>]

Hello,

On Thu, Sep 18, 2025 at 05:29:59AM +0800, Julian Sun wrote:
...
> The wb_wait_for_completion() here is probably only used to prevent
> use-after-free. Therefore, we manage 'done' separately and automatically
> free it.
> 
> This allows us to remove wb_wait_for_completion() while preventing
> the use-after-free issue.
> 
> Fixes: 97b27821b485 ("writeback, memcg: Implement foreign dirty flushing")
> Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
> Signed-off-by: Julian Sun <sunjunchao@bytedance.com>

  Acked-by: Tejun Heo <tj@kernel.org>

Minor comments below:

> +/*
> + * This structure exists to avoid waiting for writeback to finish on
> + * memcg release, which could lead to a hang task.
                                           ^
                                           hung

> + * @done.cnt is always > 0 before a memcg is released, so @wq_entry.func
> + * may only be invoked by finish_writeback_work() after memcg is freed.
> + * See mem_cgroup_css_free() for details.
> + */

I'm not sure this gives enough of a picture of what's going on. It'd be
better to expand a bit - briefly describe what the whole mechanism is for
and how hung tasks can happen.

Thanks.

-- 
tejun