Re: [PATCH] mm, swap: Use prandom_u32_max()
From: Huang\, Ying
Date: Tue May 12 2020 - 03:38:19 EST
Michal Hocko <mhocko@xxxxxxxxxx> writes:
> On Tue 12-05-20 15:14:46, Huang, Ying wrote:
>> Michal Hocko <mhocko@xxxxxxxxxx> 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@xxxxxxxxx>
>> >> Cc: Michal Hocko <mhocko@xxxxxxxx>
>> >> Cc: Minchan Kim <minchan@xxxxxxxxxx>
>> >> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
>> >> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
>> >
>> > To the change itself
>> > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>>
>> 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