linux-kernel - Re: [PATCH 2/2] blk-ioprio: remove per-disk structure

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Message-ID: <520b9c10-c152-77f3-bd5a-b86a1f5ac8ea@huaweicloud.com>
Date: Fri, 12 Jul 2024 09:10:30 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Bart Van Assche <bvanassche@....org>, Yu Kuai <yukuai1@...weicloud.com>,
 tj@...nel.org, josef@...icpanda.com, jack@...e.cz, axboe@...nel.dk
Cc: cgroups@...r.kernel.org, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org, yi.zhang@...wei.com, yangerkun@...wei.com,
 "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH 2/2] blk-ioprio: remove per-disk structure

Hi,

在 2024/07/12 2:03, Bart Van Assche 写道:
> On 7/11/24 2:00 AM, Yu Kuai wrote:
>> ioprio works on the blk-cgroup level, all disks in the same cgroup
>> are the same, and the struct ioprio_blkg doesn't have anything in it.
>> Hence register the policy is enough, because cpd_alloc/free_fn will
>> be handled for each blk-cgroup, and there is no need to activate the
>> policy for disk.
> 
> As one can see in the output of git grep -nHEB1 '>pd_(alloc|free)_fn\(',
> none of the pd_alloc_fn / pd_free_fn callers checks whether or not these
> pointers are NULL. Hence my question why this patch does not trigger any
> NULL pointer dereferences?

Because the blkcg_deactivate_policy() is removed as well, there are no
callers now... blkcg_policy_register() is still called to make sure
cpd_(alloc|free)_fn will still be called.

Thanks,
Kuai

> 
> Thanks,
> 
> Bart.
> .
>