linux-kernel - Re: [PATCH v2] block: simplify blksize

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Message-ID: <bc8cf710-c0ad-24e8-7849-6282c8e3bcff@acm.org>
Date:   Sat, 29 Oct 2022 20:33:22 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Dawei Li <set_pte_at@...look.com>
Cc:     axboe@...nel.dk, hch@....de, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] block: simplify blksize_bits() implementation

On 10/29/22 20:25, Dawei Li wrote:
> On Sat, Oct 29, 2022 at 08:00:58PM -0700, Bart Van Assche wrote:
>> On 10/29/22 19:17, Dawei Li wrote:
>>> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
>>> index 57ed49f20d2e..7b537afe8b38 100644
>>> --- a/include/linux/blkdev.h
>>> +++ b/include/linux/blkdev.h
>>> @@ -1349,12 +1349,7 @@ static inline int blk_rq_aligned(struct request_queue *q, unsigned long addr,
>>>    /* assumes size > 256 */
>>>    static inline unsigned int blksize_bits(unsigned int size)
>>>    {
>>> -	unsigned int bits = 8;
>>> -	do {
>>> -		bits++;
>>> -		size >>= 1;
>>> -	} while (size > 256);
>>> -	return bits;
>>> +	return order_base_2((size + SECTOR_SIZE - 1) >> SECTOR_SHIFT) + SECTOR_SHIFT;
>>>    }
>>
>> Why the rounding ("+ SECTOR_SIZE - 1")? The blksize_bits() argument should
>> be an argument of two.
> 
> Yeah, that's what's supposed to be.
> But I thought maybe a "just in case" is more robust?
> Should we consider these corner cases(!is_power_of_2())?

I don't think that the Linux kernel supports block sizes that are not a 
power of two. Hence my request to leave out the rounding code. Keeping 
that code would be misleading because it would suggest that the 
blksize_bits() argument can be something else than a power of two.

Thanks,

Bart.