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[37.188.140.86]) by smtp.gmail.com with ESMTPSA id m3sm5751648wrn.96.2020.05.12.00.32.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 12 May 2020 00:32:37 -0700 (PDT) Date: Tue, 12 May 2020 09:32:36 +0200 From: Michal Hocko To: "Huang, Ying" Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Minchan Kim , Tim Chen , Hugh Dickins Subject: Re: [PATCH] mm, swap: Use prandom_u32_max() Message-ID: <20200512073236.GQ29153@dhcp22.suse.cz> References: <20200512064147.514493-1-ying.huang@intel.com> <20200512065049.GN29153@dhcp22.suse.cz> <875zd1pqu1.fsf@yhuang-dev.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <875zd1pqu1.fsf@yhuang-dev.intel.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 12-05-20 15:14:46, Huang, Ying wrote: > Michal Hocko 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" > >> Cc: Michal Hocko > >> Cc: Minchan Kim > >> Cc: Tim Chen > >> Cc: Hugh Dickins > > > > To the change itself > > Acked-by: Michal Hocko > > 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