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

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Date:   Tue, 12 May 2020 15:38:15 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
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()

Michal Hocko <mhocko@...nel.org> writes:

> On Tue 12-05-20 15:14:46, Huang, Ying wrote:
>> Michal Hocko <mhocko@...nel.org> writes:
>> 
>> > 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?
>> 
>> I am not expert on random number generator.  I have heard about that the
>> randomness of the low order bits of some random number generator isn't
>> good enough.  Anyway, by using the common implementation, the real
>> random number generator expert can fix the possible issue once for all
>> users.
>
> Please drop the quality argument if you cannot really justify it. This
> will likely just confuse future readers the same way it confused me
> here. Because prandom_u32_max uses the same source of randomness the
> only difference is the way how modulo vs. u64 overflow arithmetic is
> used for distributing values. I am not aware the later would be
> a way to achieve a higher quality randomness. If the interval
> distribution is better with the later then it would be great to have it
> documented.

OK. Fair enough.

Best Regards,
Huang, Ying

>> >> 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>
>> 
>> Thanks!
>> 
>> Best Regards,
>> Huang, Ying
>> 
>> >> ---
>> >>  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