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

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Message-ID: <ZpC9le_FsIO0FCJQ@infradead.org>
Date: Thu, 11 Jul 2024 22:22:29 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Yu Kuai <yukuai1@...weicloud.com>
Cc: Bart Van Assche <bvanassche@....org>, tj@...nel.org,
	josef@...icpanda.com, jack@...e.cz, axboe@...nel.dk,
	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

On Fri, Jul 12, 2024 at 09:10:30AM +0800, Yu Kuai wrote:
> > 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.

Can you throw in a patch documenting this?  Any maybe add a check
that pd_alloc_fn / pd_free_fn exist in blkcg_activate_policy and
WARN and return an error otherwise?