linux-kernel - Re: [PATCH] mm, swap: Use prandom_u32

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Message-ID: <20200512065049.GN29153@dhcp22.suse.cz>
Date:   Tue, 12 May 2020 08:50:49 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Huang Ying <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Minchan Kim <minchan@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH] mm, swap: Use prandom_u32_max()

On Tue 12-05-20 14:41:46, Huang Ying wrote:
> To improve the code readability and get random number with higher
> quality.

I understand the readability argument but why should prandom_u32_max
(which I was not aware of) provide a higher quality randomness?

> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: Tim Chen <tim.c.chen@...ux.intel.com>
> Cc: Hugh Dickins <hughd@...gle.com>

To the change itself
Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/swapfile.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index a0a123e59ce6..2ec8b21201d6 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -3220,7 +3220,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>  		 * select a random position to start with to help wear leveling
>  		 * SSD
>  		 */
> -		p->cluster_next = 1 + (prandom_u32() % p->highest_bit);
> +		p->cluster_next = 1 + prandom_u32_max(p->highest_bit);
>  		nr_cluster = DIV_ROUND_UP(maxpages, SWAPFILE_CLUSTER);
>  
>  		cluster_info = kvcalloc(nr_cluster, sizeof(*cluster_info),
> -- 
> 2.26.2

-- 
Michal Hocko
SUSE Labs