From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH] block,iocost: fix potential kernel NULL
Date: Fri, 13 May 2022 06:28:41 -1000
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Sender: Tejun Heo <htejun@gmail.com>
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To: Yahu Gao <gaoyahu19@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>, cgroups@vger.kernel.org, linux-block@vger.kernel.org, Yahu Gao <yahugao@didiglobal.com>, Kunhai Dai <daikunhai@didiglobal.com>

Hello,

On Fri, May 13, 2022 at 10:59:28PM +0800, Yahu Gao wrote:
> From: Yahu Gao <yahugao@didiglobal.com>
> 
> Some inode pinned dying memory cgroup and its parent destroyed at first.
> The parent's pd of iocost won't be allocated during function
> blkcg_activate_policy.
> Ignore the DYING CSS to avoid kernel NULL during iocost policy data init.

Thanks for the analysis and patch but I'm not quite sure the analysis is
correct. When a cgroup goes down, its blkgs are destroyed
blkcg_destroy_blkgs() which is invoked by blkcg_unpin_online() when its
online_pin reaches zero which is incremented and decremented recursively, so
an ancestor's blkgs should be destroyed before a descendant's if the code is
working as intended. Can you guys dig a bit deeper and why we're losing
ancestor blkgs before descendants?

Thanks.

-- 
tejun