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Fri, 16 Oct 2026 03:34:54 -0700 (PDT) From: Aveline Noir To: SJ Park Cc: 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 In-Reply-To: <20261016002130.113813-1-sj@kernel.org> References: <179207592479.3.11595453180868898703@gmail.com> <20261016002130.113813-1-sj@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Date: Fri, 16 Oct 2026 17:22:58 +0800 Message-ID: <179214257862.2.17756454862637555633@gmail.com> X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 703FF180007 X-Rspam-User: X-Stat-Signature: cm7msyg5ff6mk1zyatmgr9mjo1bnfnra X-HE-Tag: 1792146896-440764 X-HE-Meta: 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 viumGIxc jCavfj9rBQJ5wvCiO5FhgY+3B6e5zedglwBmL/pp5wyvKLN/JPN+YODK6XaMrdG6XMcqg9poW4zjFPrhHk3PH3Zlr59XzRwlCoHO54zVA9Hq4agEJK9wkNgeDOQrxVrvvF/EEE0Ey2NwCoP/kdTgJniiZENc913D9ZNSqp0sKxXxhmdcgEKQDA4onq03y1nn7RRBU9QeCi/LFbPvdDfbrCnvG49i+BnvfFtyXgNuKyhj9RQaIC0rM5DGVpIUzaMAahdSfqwVpGrNIg50o2cVeg5gSX+Y/QK2XqFqHIZz9ZaeuMyg6NXpsuQSy5z1mKV7bhfDCBVO7a8apJZ2/EX8z30EtXzEtTxkVXasVS3eJsovU7auOsnK0mLeSC9eb2h1M+UgmraP+XfrqvxVXGAJigJJxWEjTe6t0Df14qLOPxztqQKntwwry9iozfQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. Thanks, Aveline Noir