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

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Message-ID: <imit4dkxy55c6a6tfqzaa6hf67nrawedotc3rzltlmmgmf3b3t@nde6h6cy3agw>
Date: Thu, 31 Jul 2025 18:27:37 +0200
From: Jan Kara <jack@...e.cz>
To: Yu Kuai <yukuai1@...weicloud.com>
Cc: yukuai@...nel.org, Jan Kara <jack@...e.cz>, 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

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)
?

That should accurately distribute the remainder across the remaining words,
shouldn't it?

>                 shallow_word_depth++;
> 
>         return shallow_word_depth;
>  }

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR