From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 709F137F8C3; Tue, 23 Jun 2026 13:58:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782223128; cv=none; b=QPXrUjPYYTCsY3qq2/tyb0Due8t0VJJN0qUdM/PhmC9s7YmSrCVrnr0PsRxqj/1pEqQz18W60C39qiXNvvuqFfOS7icfrWeFvQt/sZZkDCd6aMXnFl312fBX/SfywIAwoFngXjR6oFKI4VrPedRj6VSqnhcUWJM1QZ1n5t56Yvc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782223128; c=relaxed/simple; bh=ev2VrbjCP3fdzkAcWElGwDW6j0kryEOXvcL+txeyncs=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=IFjejdO3DB6mqSbWe6nL5lypkQ25qZGpfMrDMaeQ68LWOrt+AfMSZAkgulUniuYcgszisQnJpRTz4AGm3SdOxwq6gavXwUjvTEWW9WincrzvK4/vuqeNltgo6QRzxyVxnszy5eq9m8N+nkna5cs+amEAAqqgWciOgrKAW483Ud4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=XfKOJ6+D; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="XfKOJ6+D" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 637FE1F000E9; Tue, 23 Jun 2026 13:58:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782223127; bh=qPJB5FWFfqGea7PI1Ch17WntCkpS3wYF0Wfk6C5GHV0=; h=From:To:Cc:Subject:Date; b=XfKOJ6+DecxCNelskMTyAqIzRcKsZKS/J/idHWt196ogpNgr8yfmxTKB3WG5qk41u 4oNhZyKcT2ztSlGEzecagYQddoGB7qhQ4+zbvstJfIezdbx/qyNIr5s6kLpw74Ui/W /R+ATITjJ1q5RD8XDtcoD5cGaplDWh/rjpQW856c2k+5e1PD0uGo0K2i8m0PwYP8qI RTwnIic+YghWruvA0y+Cua8nYI5IYzR/TlosREefjUAlT3Gn4z7/62nnAAswSQ6nha tH/7IQjycQ3RUrA1Uf2hNPz2k+26saBel+QBEOKKLD75Yoa2LRN7tKDeJ8NHFT6V02 UvpfoVk6UO/9g== From: SeongJae Park To: Andrew Morton Cc: SeongJae Park , "# 5 . 16 . x" , damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH v2] mm/damon/ops-common: handle extreme intervals in damon_hot_score() Date: Tue, 23 Jun 2026 06:58:31 -0700 Message-ID: <20260623135834.67189-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 v1.3 - RFC v1.3: https://lore.kernel.org/20260623011652.1354-1-sj@kernel.org - Drop RFC again. 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 6f7edb3590ef9..888570f55b416 100644 --- a/include/linux/damon.h +++ b/include/linux/damon.h @@ -1065,9 +1065,13 @@ 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: c12377ad97c98e0bee10870abf4ab1101a946b4c -- 2.47.3