linux-kernel - Re: [PATCH v2 1/2] lib/sbitmap: convert shallow

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Message-ID: <12d82243-237c-d890-a975-392b151d55a9@huaweicloud.com>
Date: Fri, 1 Aug 2025 08:24:58 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Jan Kara <jack@...e.cz>, Yu Kuai <yukuai1@...weicloud.com>
Cc: yukuai@...nel.org, axboe@...nel.dk, akpm@...ux-foundation.org,
 yang.yang@...o.com, dlemoal@...nel.org, ming.lei@...hat.com,
 linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 yi.zhang@...wei.com, yangerkun@...wei.com, johnny.chenyi@...wei.com,
 Omar Sandoval <osandov@...com>, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v2 1/2] lib/sbitmap: convert shallow_depth from one word
 to the whole sbitmap

Hi,

在 2025/08/01 0:27, Jan Kara 写道:
> On Thu 31-07-25 10:38:58, Yu Kuai wrote:
>> Hi,
>>
>> 在 2025/07/31 2:24, Yu Kuai 写道:
>>> hi, Jan!
>>>
>>> 在 2025/7/30 21:03, Jan Kara 写道:
>>>> I think having two APIs will be even more confusing than the current
>>>> state.
>>>> But as I wrote I think you can have API to specify shallow depth in total
>>>> size and in sbitmap_queue_get_shallow() do:
>>>>
>>>> shallow_per_word = (shallow_depth << sb->shift) / sb->depth;
>> In order to consider the last word, I think we should use __map_depth()
>> here.
> 
> Right.
> 
>>>> rounding_index = shallow_depth - shallow_per_word * sb->depth;
>> And then it's not possible to calculate this rounding index easily. How
>> about following, although the reminder handling is not perfect.
>>
>>   static unsigned int __map_depth_with_shallow(const struct sbitmap *sb,
>>                                               int index,
>>                                               unsigned int shallow_depth)
>>   {
>>          unsigned int word_depth = __map_depth(sb, index);
>>          unsigned int shallow_word_depth = word_depth * shallow_depth;
>>          unsigned reminder = do_div(shallow_word_depth, sb->depth);
>>
>>          if (reminder && !(index & 0x1))
> 
> Well, why not:
> 	if (remainder > index)
Do you mean reminder > index * shallow_depth? This looks correct, and
with the consideration for the last word:

if (index == sb->map_nr - 1)
	shallow_word_depth = max(shallow_word_depth, 1);
else if (reminder > index * shallow_depth)
	shallow_word_depth++;

Thanks,
Kuai

> ?
> 
> That should accurately distribute the remainder across the remaining words,
> shouldn't it?
> 
>>                  shallow_word_depth++;
>>
>>          return shallow_word_depth;
>>   }
> 
> 								Honza
>