linux-kernel - Re: [PATCH RESEND v2 1/5] sbitmap: remove unnecessary calculation of alloc_hint in __sbitmap_get

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Message-ID: <20221222110116.mpxvoiiok24yxsgd@quack3>
Date:   Thu, 22 Dec 2022 12:01:16 +0100
From:   Jan Kara <jack@...e.cz>
To:     Kemeng Shi <shikemeng@...weicloud.com>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, jack@...e.cz, kbusch@...nel.org
Subject: Re: [PATCH RESEND v2 1/5] sbitmap: remove unnecessary calculation of
 alloc_hint in __sbitmap_get_shallow

On Thu 22-12-22 22:33:49, Kemeng Shi wrote:
> We calculate allow_hint in next word as following:
> 
> /* low sb->shift bit of alloc_hint will be 0 after this shift */
> alloc_hint = index << sb->shift;
> 
> /* get low sb->shift bit of alloc_hit */
> SB_NR_TO_BIT(sb, alloc_hint)
> 
> So alloc_hit in next word will always be zero. Simpfy alloc_hit calculation
> in __sbitmap_get_shallow according to the alloc_hit calculation in
> __sbitmap_get.
> 
> Signed-off-by: Kemeng Shi <shikemeng@...weicloud.com>

Looks good to me, nice cleanup. I'd just somewhat rephrase the changelog
to:

Updates to alloc_hint in the loop in __sbitmap_get_shallow() are mostly
pointless and equivalent to setting alloc_hint to zero (because
SB_NR_TO_BIT() considers only low sb->shift bits from alloc_hint). So
simplify the logic.

Anyway, feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  lib/sbitmap.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 586deb333237..cb5e03a2d65b 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -243,6 +243,7 @@ static int __sbitmap_get_shallow(struct sbitmap *sb,
>  	int nr = -1;
>  
>  	index = SB_NR_TO_INDEX(sb, alloc_hint);
> +	alloc_hint = SB_NR_TO_BIT(sb, alloc_hint);
>  
>  	for (i = 0; i < sb->map_nr; i++) {
>  again:
> @@ -250,7 +251,7 @@ static int __sbitmap_get_shallow(struct sbitmap *sb,
>  					min_t(unsigned int,
>  					      __map_depth(sb, index),
>  					      shallow_depth),
> -					SB_NR_TO_BIT(sb, alloc_hint), true);
> +					alloc_hint, true);
>  		if (nr != -1) {
>  			nr += index << sb->shift;
>  			break;
> @@ -260,13 +261,9 @@ static int __sbitmap_get_shallow(struct sbitmap *sb,
>  			goto again;
>  
>  		/* Jump to next index. */
> -		index++;
> -		alloc_hint = index << sb->shift;
> -
> -		if (index >= sb->map_nr) {
> +		alloc_hint = 0;
> +		if (++index >= sb->map_nr)
>  			index = 0;
> -			alloc_hint = 0;
> -		}
>  	}
>  
>  	return nr;
> -- 
> 2.30.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR