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b=l0YSS5Dh6e/G7+zzbyorV8r97fLmf6IUEwf9ogcwCELprQUPx6gmVUJBhK/HXRoY9 LKPQPCvnZz8IsJgmWJBIMS6Fcy9C6R3oOu2zQ1v7xwOgL64Xu7Ito4qX+U0S195aB5 6X1juTWcf3xtoFh6B5GWEDcZRmoZz6fvC8kMuzoN5hlfpT6XFawpT3EzVGqxIAxGFd w7REcOpnrH9uc/RNYlfP7MLc7SEi7Ncl/MOQ3oWmP3BXN4EfMKHU6+XWmXERQgWcny ogQRLZzMMUgQkZZw0+bBoBjC5jf62bDT2uWZLlehFyQ/HmK3xDBAZ1X1sdN24e9MFW KI3Z+FScSoCJw== From: SeongJae Park To: SeongJae Park Cc: Andrew Morton , "# 5 . 16 . x" , damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] mm/damon/core: handle zero intervals in damon_max_nr_accesses() Date: Sun, 21 Jun 2026 11:03:47 -0700 Message-ID: <20260621180348.92118-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260621154808.86431-1-sj@kernel.org> References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Stat-Signature: 5uchouh5bwubonjyhhcu8adce6n53ufd X-Rspamd-Queue-Id: 47C0620003 X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1782065041-923951 X-HE-Meta: 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 cWTyg+FI WTsHeWnrcOZZHJa2tl7yrmHvst2FB4TUWYqXor3J5LlA+sZWlP+26lyuIOcsuxqk+u/AF/tNB61wf0EdbIkoYeK9CuVdt5B9aTKC6CknJNNRdiOw+3yUsNXdOWHGShgMp+vJ4F0HGG/PjcX9c8wMR4v2H/ByhMF2U756DVsPkmoQXk13KwN/JlHRbZlVx0dW/dxudGD+o77fazLi3lyUuQi1a+YRU+/Eo+6LHw+hHpKH8OHYiMCJ7eSGExBQh0ZR6fL7z0gVFdNStSzpwN39p8lzrtg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sun, 21 Jun 2026 08:48:06 -0700 SeongJae Park wrote: > damon_max_nr_accesses() causes a divide-by-zero if the sampling interval > is set to zero by the user. If the aggregation interval is set to zero, > the function returns zero. It is wrong, since the real maximum > nr_acceses in the setup should be one. Worse yet, it can cause another > divide-by-zero from its caller, damon_hot_score(), since it uses > damon_max_nr_accesses() return value as a denominator. > > Fix the problem by setting the denominator in the function as 1 when the > sampling interval is zero. Also ensure the return value is always 1 or > greater. > > The issue was discovered [1] by Sashiko. > > [1] https://lore.kernel.org/20260619202459.145010-1-sj@kernel.org > > Fixes: 198f0f4c58b9 ("mm/damon/vaddr,paddr: support pageout prioritization") Sashiko found [1] another bug that was introduced by another commit. I will repost this patch with a fix for the another bug. [1] https://lore.kernel.org/20260621175849.91990-1-sj@kernel.org Thanks, SJ [...]