linux-kernel - Re: [PATCH] quota: code cleanup for hash bits calculation

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Message-ID: <20190923100259.GD20367@quack2.suse.cz>
Date:   Mon, 23 Sep 2019 12:02:59 +0200
From:   Jan Kara <jack@...e.cz>
To:     Chengguang Xu <cgxu519@...o.com.cn>
Cc:     Jan Kara <jack@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] quota: code cleanup for hash bits calculation

On Sat 21-09-19 09:56:28, Chengguang Xu wrote:
> Code cleanup for hash bits calculation by
> calling rounddown_pow_of_two() and ilog2()
> 
> Signed-off-by: Chengguang Xu <cgxu519@...o.com.cn>

Thanks for the patch! One comment below:

> diff --git a/fs/quota/dquot.c b/fs/quota/dquot.c
> index 6e826b454082..679dd3b5db70 100644
> --- a/fs/quota/dquot.c
> +++ b/fs/quota/dquot.c
> @@ -2983,13 +2983,9 @@ static int __init dquot_init(void)
>  
>  	/* Find power-of-two hlist_heads which can fit into allocation */
>  	nr_hash = (1UL << order) * PAGE_SIZE / sizeof(struct hlist_head);
> -	dq_hash_bits = 0;
> -	do {
> -		dq_hash_bits++;
> -	} while (nr_hash >> dq_hash_bits);
> -	dq_hash_bits--;
> +	nr_hash = rounddown_pow_of_two(nr_hash);
> +	dq_hash_bits = ilog2(nr_hash);
>  
> -	nr_hash = 1UL << dq_hash_bits;

Why not just:
	dq_hash_bits = ilog2(nr_hash);
	nr_hash = 1UL << dq_hash_bits;

That way we need to compute fls() only once...

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