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

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Message-ID: <20200512073236.GQ29153@dhcp22.suse.cz>
Date:   Tue, 12 May 2020 09:32:36 +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 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.

> >> 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

-- 
Michal Hocko
SUSE Labs