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Subject: Re: [PATCH v7 1/3] bio: limit bio max size
To:     Changheun Lee <nanich.lee@samsung.com>, damien.lemoal@wdc.com,
        Johannes.Thumshirn@wdc.com, asml.silence@gmail.com,
        axboe@kernel.dk, gregkh@linuxfoundation.org, hch@infradead.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        ming.lei@redhat.com, osandov@fb.com, patchwork-bot@kernel.org,
        tj@kernel.org, tom.leiming@gmail.com
Cc:     jisoo2146.oh@samsung.com, junho89.kim@samsung.com,
        mj0123.lee@samsung.com, seunghwan.hyun@samsung.com,
        sookwan7.kim@samsung.com, woosung2.lee@samsung.com,
        yt0928.kim@samsung.com
References: <20210413025502.31579-1-nanich.lee@samsung.com>
 <CGME20210413031257epcas1p329f38effa71445de2464cee32002e618@epcas1p3.samsung.com>
 <20210413025502.31579-2-nanich.lee@samsung.com>
From:   Bart Van Assche <bvanassche@acm.org>
Message-ID: <2e54f27a-ae4c-af65-34ba-18b43bd4815d@acm.org>
Date:   Tue, 13 Apr 2021 09:18:00 -0700
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On 4/12/21 7:55 PM, Changheun Lee wrote:
> +unsigned int bio_max_size(struct bio *bio)
> +{
> +	struct request_queue *q = bio->bi_bdev->bd_disk->queue;
> +
> +	if (blk_queue_limit_bio_size(q))
> +		return blk_queue_get_max_sectors(q, bio_op(bio))
> +			<< SECTOR_SHIFT;
> +
> +	return UINT_MAX;
> +}

This patch adds an if-statement to the hot path and that may have a
slight negative performance impact. I recommend to follow the approach
of max_hw_sectors. That means removing QUEUE_FLAG_LIMIT_BIO_SIZE and to
initialize the maximum bio size to UINT_MAX in blk_set_default_limits().

Thanks,

Bart.