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bh=uNO+RQmUCKpyh3OJUwnU0KKiT0g7xqCEn/1y3Eui4M4=; b=C9jfdbl3CMirTaYjhJ060Q9nXYOsYXmZ40THrJ41lhshSUX9i6ZEfWNXrhm3xlwleqe/lE hjDHXRGKNUDbCl+h98p0ZVlHiqudzp/CI6LUUrj/bVmZxUVvS1Ck7a2C69owf+K7oeymUi WWKajamyWpbfGZVrR8SR2iyKOBp1vfc= ARC-Authentication-Results: i=1; imf26.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b="eV/LLi9a"; spf=pass (imf26.hostedemail.com: domain of sj@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=sj@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 8DD31406B0; Tue, 23 Jun 2026 01:17:03 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 84B141F000E9; Tue, 23 Jun 2026 01:17:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782177423; bh=uNO+RQmUCKpyh3OJUwnU0KKiT0g7xqCEn/1y3Eui4M4=; h=From:To:Cc:Subject:Date; b=eV/LLi9aYqGVT3NMiY/2Sw2taMBtu2M9N8g7QZ10iOebp3xP2JdyJAZhQsvvjVDEf gHce/8tStbVSjqGLUjJ/tpFZhu9hgWr5ucijcRBCxK2Qevkjt3dHZlozXuzAdItLDx UJLrN0u/pWSRqCXvQDu5JVKNNP3V91HgGZXCfyD1BN7lmOaP7zhauhfZHGEVCnZ9Eb teiX5rlJIMiYa8iZA/a5DnnziqAenk1EkYJntrkY/wVd4OtM8+wxYMw5SZcyb+An1c vjgRu7ifY3fKkU8OZTti41WbpgzWZ52YL0cm8ZAjwEAPvsqikXhuSUuvLY+wgUYuaL oa+vRpYI18u2Q== From: SeongJae Park To: Cc: SeongJae Park , "# 5 . 16 . x" , Andrew Morton , damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [RFC PATCH v1.3] mm/damon/ops-common: handle extreme intervals in damon_hot_score() Date: Mon, 22 Jun 2026 18:16:51 -0700 Message-ID: <20260623011652.1354-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: ADC60140007 X-Stat-Signature: s74afz3y3493omj7o7jdcde1tboyhymh X-Rspam-User: X-Rspamd-Server: rspam03 X-HE-Tag: 1782177424-776044 X-HE-Meta: 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 IfAXs9ng YQqQ0HFgRaZhdOy4RQP8RNdz2USSOwftxq6EsPXrxJ6NptAc1/88TquDHXbNnngp+rZXFFoD48WstWrskxn8e8+TlUBXooVkEwpdNUo/FOT3mB63RODIW4ex5N8CNvqSPCl4eqVIG49R756bv43kKEkVk71AiQ5xK5EApLCzAH9TIFnt46IOQkH9xU3jobrSAQ/ISSfhpahGiUOgl7FLRCP9CDtnkIZp56Gt63JTRUEbGj0gKjw+ULAlNm+ISY2oZtbz1wh9Z3yKIAFtrf2GIWjS/HQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Fix three issues in damon_hot_score() that comes from wrong handling of extreme (zero or too high) monitoring intervals user setup. When the user sets sampling interval zero, damon_max_nr_accesses(), which is called from damon_hot_score(), causes a divide-by-zero. Needless to say, it is a problem. When the user sets the aggregation interval 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. When the user sets the aggregation interval very high, damon_hot_score() could return a value out of [0, DAMOS_MAX_SCORE] range. Since the return value is used as an index to the regions_score_histogram array, which is DAMOS_MAX_SCORE+1 size, it causes out of bounds array access. The issues can be relatively easily reproduced like below. The sysfs write permission is required, though. # ./damo start --damos_action lru_prio --damos_quota_space 100M \ --damos_quota_interval 1s # cd /sys/kernel/mm/damon/admin/kdamonds/0 # echo 0 > contexts/0/monitoring_attrs/intervals/sample_us # echo 0 > contexts/0/monitoring_attrs/intervals/aggr_us # echo commit > state # dmesg [...] [ 131.329762] Oops: divide error: 0000 [#1] SMP NOPTI [...] [ 131.336089] RIP: 0010:damon_hot_score+0x27/0xd0 [...] Fix the divide-by-zero intervals problems by explicitly handling the zero intervals in damon_max_nr_accesses(). Fix the out-of-bound array access by applying [0, DAMOS_MAX_SCORE] bounds before returning from damon_hot_score(). 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") Cc: # 5.16.x Signed-off-by: SeongJae Park --- Changes from RFC v1.2 - RFC v1.2: https://lore.kernel.org/20260622141027.29145-1-sj@kernel.org - Drop patch 2 and make patch 1 fixes all damon_hot_score() problems. Changes from v1 - v1: https://lore.kernel.org/20260621154808.86431-1-sj@kernel.org - Add out-of-bound array access bug fix as patch 2. - Add the RFC tag again. Changes from RFC v1.1 - RFC v1.1: https://lore.kernel.org/20260620171413.89555-1-sj@kernel.org - Wordsmith commit message. - Drop RFC tag. Changes from RFC v1 - RFC v1: https://lore.kernel.org/20260619205144.150664-1-sj@kernel.org - Handle zero aggr_interval case. include/linux/damon.h | 8 ++++++-- mm/damon/ops-common.c | 1 + 2 files changed, 7 insertions(+), 2 deletions(-) diff --git a/include/linux/damon.h b/include/linux/damon.h index 64d75c78f4df4..02ac34537df9a 100644 --- a/include/linux/damon.h +++ b/include/linux/damon.h @@ -1066,9 +1066,13 @@ static inline bool damon_target_has_pid(const struct damon_ctx *ctx) static inline unsigned int damon_max_nr_accesses(const struct damon_attrs *attrs) { - /* {aggr,sample}_interval are unsigned long, hence could overflow */ - return min(attrs->aggr_interval / attrs->sample_interval, + unsigned long sample_interval; + unsigned long max_nr_accesses; + + sample_interval = attrs->sample_interval ? : 1; + max_nr_accesses = min(attrs->aggr_interval / sample_interval, (unsigned long)UINT_MAX); + return max_nr_accesses ? : 1; } diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c index 5c93ef2bb8a97..d1842e2b00ef8 100644 --- a/mm/damon/ops-common.c +++ b/mm/damon/ops-common.c @@ -143,6 +143,7 @@ int damon_hot_score(struct damon_ctx *c, struct damon_region *r, * Transform it to fit in [0, DAMOS_MAX_SCORE] */ hotness = hotness * DAMOS_MAX_SCORE / DAMON_MAX_SUBSCORE; + hotness = max(min(hotness, DAMOS_MAX_SCORE), 0); return hotness; } base-commit: 0f0046f6171e76f3dba376cc13ace04d654bf372 -- 2.47.3