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bh=/rY+da4gZ/gT6x5YDiZjxmuF5eCELrbiaY4rx2johDE=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=UGe+v27p90GzDxWWTa5dWIjd7PcDB2ayLWfH/O8Pnfw2ZQl1W6mNxyXCSMa9WOhyX nbSo/WYdZbmdS02R3+6eiI8Uw5iYDt5WynvXdf7qVpVFMMjvTlqecHl8Yz5pMYFM8l DeVjAvJ3YnE+W/u4AkDZkIB3IEaXeeslVPTczncGkMjsrHqkKqYySJuA0M/myrVNRM p7u/1JzWgh39EI7wWAwWtVKAKXnlJHjhMLx+HrCcZm4iR39ddN1kBEwd3Xqnie5TyC Oto7KE3HDp/jXN9a+4EDGbxN8eMxI8HFISKckDQEWXKmVy9qo5UwS/hQj3laK/G6QM szuTh7q0wRHvQ== From: SJ Park To: Aveline Noir Cc: SJ Park , Andrew Morton , damon@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/damon/stat: avoid 32-bit overflow in idle percentiles Date: Fri, 16 Oct 2026 07:09:00 -0700 Message-ID: <20261016140900.125937-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <179214257862.2.17756454862637555633@gmail.com> References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 131024000B X-Stat-Signature: r8ae7pqdpwh3w7frmepd6nhbqrm6nqn3 X-HE-Tag: 1792159748-700731 X-HE-Meta: U2FsdGVkX1+ByY6Vmluyjrpxn/EEVzFb3zC28ccgqvDhBa/UVXIU4k4AhjzfRTuL5Zhm0iyVnFYN94KL+aDZdNXvjKvAQ/rrFfn0YKdreB2sl00teAC00FCLCUFtTbYsf84T70/+L8hYCeS08+Dh6VXSDsXw7wFYkGIxiylitSJ55PyRmuEZaYRnmJl0dLl4ekrddd25NGZpzhtmRFhhikPMXkt1heMA90EhiEIpdmaDuZ7eksDKaCV+0aF8o61x6JJk7VRRDLTgzxJlULzxXnIuMhJ7ZB5vi2Ukews5cVTJk7fTRjqS5Jihjy8dLdpuqPEYS/izYBHpL9AKVys8Dh6UY+7cZT1a++vqeQ/2klDR3jALJLudVrXbhegsN3NEO6E9YdHlGlMXVVZ97eHpVxwj5vWlCz0gsdws/6lCCPWHgvLGvJc8i99mYDpVxQCrIBXuhdb82EhASg87u4Mol7msq352eLqc4YcH0Gqp/bvJADf+FgIia9bhSVyR8sxqXibdM6yfUS6cBoYgfp/HZk5kg4PWiqRLaIgUvBIQmUyEZq+fjNSXSAFFFCLxRkczj7cm5UlLwmBGiUD1TFpWXVH5Q2b5drgEPWfg3cde63Kfb1FoSevGZF8Lv8xkihKBIVzVe5WPWlJBfUYZW7CnBOuilA/89c4WUD/NSKpGtXhmAbauebf1Q3GGRYrxTYkPD5n4T51Gut+xJPaE31nZ+36/W3dcpUoichYqJAixTvBv1M38iGAUTfj8rVjGtenxoSpMKPabH2b4K2PfBFzQeEMPY+Z5vHluEobFyHgctl10O4nVVYL6g6UMEWZ5J6DN5PTo6uVroLdN4zshAa2tvMVCi4PBmEJ62qQXvNUMWSSS+2zK4Z0WLbm24qyg3ZKGWDRJb/IB8w7IOpYu8+kOpJhzfGIChlKLNO+h7LqHLjeh6L9FwIg/vue4hOJVxkurG8AOpurDqgI7Dxr7SFy b45csLpY nitnzcJ2hv3luI9x2YjrViZxxOsp0w3S/5Fiz8mDYGsXjPaVTr2r/QNNzjjp7wlVprYbOMXXrt88QWXm72uGJc2i5LL6nLkDC/0lyORPcpEyxlrdB92o2aAA3fQIzIqWipEsjvA+rIFUOP9SMmPNxM1xOyEIuSwFpolOnvU/iVHAtGXb9VRPKVbp0iVal3tblWt3RV8aDq0iCZBl8K3WmaKY5SJ2j6UCvMEfcmeWVtN8mfh0= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, 16 Oct 2026 17:22:58 +0800 Aveline Noir wrote: > Hi SJ, > > On Thu, Oct 15, 2026 at 05:21:29PM -0700, SJ Park wrote: > > This patch is sent as a followup of your previous bug report, right? > > Unless it is very urgent, please make sure the previous discussion is > > completed before making a followup action like sending this patch. As a > > rule of thumb for DAMON, I'd suggest waiting for ~10 hours after the last > > comment on the thread. > > The patch was already prepared and additional tests were underway when > your reply to the bug report arrived, so I went ahead and sent it. > I'll leave more time for discussion before sending follow-up patches. > > > What is 38? > > 38 individual VM runs, forming 19 before/after pairs. The 20 same-image > runs formed 10 control pairs. > > > Do you have a plan to run more tests to be able to tell that? > > I've run longer VM tests, but even same-kernel runs vary substantially, > so the overall patch effect remains unclear. In a fixed-input 32-bit > userspace test with 10 regions over 256 MiB, the median increase was > 227 ns/call (+42%). The old code updated 15 entries, versus 101 with the > fix. I haven't separated the extra updates from arithmetic cost. > Maybe we could record region counts and sizes alongside VM timings to > check whether both versions are doing comparable work. > > > Any reason to use div64_ul() instead of div_u64()? > > total_sz is unsigned long. div_u64() would truncate its divisor on 64-bit > systems if it exceeds U32_MAX. div64_ul() preserves that width and maps > to div_u64() on 32-bit. All answers make sense. Thank you. Thanks, SJ [...]