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b=Qnp4nuoEfk8SdEnmrUkDHhXQUJXar6SX8Pi/DJVfadKNXU+/ctRAHnZp8dZaCRxAw fcYb6hZd1zRLPUMakrEKVr0sOc7ajeDE5Izdae3NY1zvrzLlN761lH3e4+fPxdsTRU CxkjDup95CmCwWKMw0/+xp3bICK8mdacJAN6HZJZCAVzm7Rc4vbgGBDPZagaqjZooQ nieDQzA6NqrKQXFK4LIprqJKLKd4AcDQn3nuLVlLQ0CjQFRJblDxFCxTP+eQ7meUWj zlD+6ugLz8L6ZAbPh5zuzDxjfOSL1v2LPhUQRMozZ7TzLTeP71yNW9B831St1pJ0na 9Wiu5/7EtuXlg== From: SJ Park To: Wang Guangke Cc: SJ Park , akpm@linux-foundation.org, damon@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/damon/stat: avoid overflow in estimated_memory_bandwidth calculation Date: Mon, 2 Nov 2026 06:25:28 -0800 Message-ID: <20261102142528.27792-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: References: X-Archived-By: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 7B55E100013 X-Rspam-User: X-Stat-Signature: ymsxiqt5a9e7ys7835wuno93qt54kt4s X-Spam-Status: No, score=-8.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam03 X-HE-Tag: 1793629536-759549 X-HE-Meta: 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 sLx8sQxp m+VX03S28nzE4ETichghWRZ8pP7leLRi4EbH6mYUun6oXWWFP0VJ868vFqiGjRxY/3udct3sOK/EeMfLPdUYlODnF+a7Hl7sLamqjWwdjipq9jcKJGkZ6HZ1oHce2CUSKxoXb11ZQ650GE9Hb5TWYFo5U/AiVY/E0WOqcsLQTu5Uj+VPoUN52srCnAMNHlHwu0rlsBEQ3n/AQWMwGpKKbFGeQX+D+WqaqPDkBLA7gtF1AeDd8pFoTywe04nhWwhIY/c4KP4VC+8X3WTkFqEvlWQSQZv0eG5fm6RtSZWJnoAVk8aepi3UYXho2tQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Mon, 2 Nov 2026 18:04:23 +0800 Wang Guangke wrote: > Hi SJ, > Thanks for your review. > > On Sun, Nov 01, 2026 at 11:13:43AM -0800, SJ Park wrote: > > Hello Wang, > > > > On Mon, 2 Nov 2026 02:00:01 +0800 Wang Guangke wrote: > > > > > The calculation of estimated_memory_bandwidth computes > > > access_bytes * USEC_PER_MSEC * MSEC_PER_SEC / aggr_interval directly. > > > On 32-bit systems, the intermediate product > > > access_bytes * USEC_PER_MSEC * MSEC_PER_SEC may overflow before the > > > division is performed. > > > > Do you find this issue from real usage? Or, jsut theoretical? Could you > > please clarify it, and user impacts of the issue? > > I found it by code inspection, not from real usage on a 32-bit system. Thank you for clarifying. [...] > > > + estimated_memory_bandwidth = > > > + bytes_per_interval * USEC_PER_MSEC * MSEC_PER_SEC + > > > + div64_u64(remainder * USEC_PER_MSEC * MSEC_PER_SEC, > > > + c->attrs.aggr_interval); > > > > Let's use USEC_PER_SEC instead of 'USEC_PER_MSEC * MSEC_PER_SEC'. > > Thanks, I will rename the variable to bytes_per_us, and use USEC_PER_SEC > instead of 'USEC_PER_MSEC * MSEC_PER_SEC'. Thank you for accepting my humble suggestions. Looking forward to v2. Thanks, SJ [...]