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Date:   Mon, 12 Dec 2022 00:53:26 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     Tejun Heo <tj@kernel.org>
Cc:     Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, darklight2357@icloud.com,
        Josef Bacik <josef@toxicpanda.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        cgroups@vger.kernel.org
Subject: Re: [PATCH 2/2 block/for-6.2] blk-iolatency: Make initialization lazy
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> +static int blk_iolatency_try_init(char *input)
> +{
> +	static DEFINE_MUTEX(init_mutex);
> +	struct block_device *bdev;
> +	int ret = 0;
> +
> +	bdev = blkcg_conf_open_bdev(&input);
> +	if (IS_ERR(bdev))
> +		return PTR_ERR(bdev);

> +retry:
>  	ret = blkg_conf_prep(blkcg, &blkcg_policy_iolatency, buf, &ctx);
> +	if (ret == -EOPNOTSUPP) {
> +		ret = blk_iolatency_try_init(buf);

It's a little sad to do two block device lookups here (even if it
obviously doesn't matter for performance).  I wonder if it would
make sense to explicitly support the lazy init pattern
in blkg_conf_prep somehow.

Otherwise I'm all for the lazy init.